One problem with Perl it is impossible to remember all the language, it is just too big. For example, the details for printf,
exotic futures of regular expression engine and command line switches. You need some help from sheet sheets.
There are many on the Web and you can create your own combining best features of each into one the is more suitable for your needs.
Among possible sources:
Here are some of the most useful Perl tips and snippets that I collected:
If you use OFMs it is easy to save stokes in checking Perl scripts.
Configure extension pl or usemenu item p to invoke perl -cw !.! (!.! is FAR idiom, other OFMs like
Midnight Commander use different macros).
Special variable $^O contains the name of your operating system
in the format provided by uname.
No need for something like:
$OS=`uname`; chomp $OS;
Some additional scalars that Perl defines for you:
$^T contains the time at which your program began.
$0 contains the name of your program
Create a log file and write important messages to the log file
In more or less complex script control printing of debugging information
using some variable (for example$debug ). Design and maintain
your own system of diagnostic output from various subroutines of the program
For more or less complex program diagnostic output using special print
statements is the most efficient debugging method. It should be controlled
by special variable, for example $debug, which can be integer or bit value.
For example:
($debug) && print "text=$test";
You can also use binary numbers and & operator which permits you
operating with small sets of debug flags one for each section of a program
The following code snippet demonstrates this:
Dynamic activation of the debugger (from "Perl debugged" book):
while (<INPUT>) {
$DB::trace = 1, next if /debug/;
$DB::trace = 0, next if /nodebug/;
# more code
}
When run under the debugger, this enables tracing when the loop encounters
an input line containing "debug" and ceases tracing upon reading one containing
"nodebug".
You can switch to interactive debugging by using:
$DB::single = 1
instead. That also provide a way you can debug code in BEGIN blocks (which
otherwise are executed before control is given to the debugger).
Sometimes it makes sense to use regular expressions instead of substr.
One such task is extraction of component of date, for example:
Getting Perl cross-reference reports. The B::Xref module
can be used to generate cross-reference reports for
Perl programs.
perl -MO=Xref[,OPTIONS] scriptname.plx
Setting a value of parameter to default value:
# --- process the second parameter
$msglevel=($ARGV[1]) ? $ARGV[1] : $msglevel; # defaults is the three digit constant(see below)
($msglevel1, $msglevel2, $testing) = split(//,$msglevel); # get one byte flags
Creating timestamp
# Timestamp
#
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)=localtime(time);
$year+=1900;
$mon++;
for ($mon, $mday, $hour, $min, $sec) {
if (length($_)==1) {
$_="0$_";
}
}
Move via link/unlink (should be the same filesystem):
link($_[0], $target);
if (-e $target) {
unlink($_[0]);
} else {
logger("SFail to move the file '$_[0]' to '$home/$_[1]/$target' \n");
return;
}
Removing duplicates: here the second part will be executed only if $new{$match}
is still undefined:
Anything after the __DATA__ line is not part of the program but is available to
the program through the special DATA filehandle:
#!/usr/bin/perl
print "---Outputting DATA\n", <DATA>, "---Done\n";
__DATA__
Dog
Cat
Bird
The output shows each line after __DATA__ :
---Outputting DATA
Dog
Cat
Bird
---Done
I typically go the other way by starting with a data file and adding a program to the top of
it:
#!/usr/bin/perl
use v5.26;
use Text::CSV_XS;
my $csv = Text::CSV_XS->new;
while( my $row = $csv->getline(*DATA) ) {
say join ':', $row->@[3,7];
}
__DATA__
...many CSV lines...
This is the end, my friend, the END
You probably also know that you can use __END__ instead. I'm used to using that
because it's a holdover from Perl 4 and that's where I first learned this:
#!/usr/bin/perl
print "---Outputting DATA\n", <DATA>, "---Done\n";
__END__
Dog
Cat
Bird
We've had fun with the perl interpreter and the shebang, but perl
has a -x which is already fun by design. This option tells Perl that the program
to execute is actually embedded in a larger chunk of unrelated text to ignore. Perhaps the Perl
program is in the middle of an email message:
"I do not know if it is what you want, but it is what you get.
-- Larry Wall"
#!/usr/bin/env perl
print "perl -x ignores everything before shebang\n";
print <DATA>;
__END__
"Fortunately, it is easier to keep an old interpreter around than an
old computer.
-- Larry Wall"
Executing this as a program is a syntax error because the Larry Wall quote before the
shebang is not valid Perl. When we execute this code with perl -x , everything
before the shebang is ignored and it works:
$ perl -x email.txt
perl -x ignores everything before shebang
"Fortunately, it is easier to keep an old interpreter around than an
old computer.
-- Larry Wall"
Out of curiosity, what if we tried to go one step further? How about multiple shebangs in a
file, where one of them has a -x :
#!/usr/bin/perl -x
#!/usr/bin/perl
But it only produces an error:
Can't emulate -x on #! line.
There is however a trick to achieve this, by using shell eval . That perl
-x is now executed in a shell process and not interpreted by perl binary like
previously.:
This article would not be complete without discussing a bit about the config variable
$Config{startperl} . This variable comes from Config.pm that provides
information about configuration environment (which you also see with perl -V
):
This is actually built during compilation from defaults or user/vendor provided configs.
What if we want a different value? Simply specify the value of this during the
./Configure step, the configure option is -Dstartperl='...' . We then
need to rebuild perl :
$ ./Configure -des -Dstartperl='#!/my/shebang'
$ make test install
Take care to use an interpreter or a program that behaves like a perl
interpreter! Some CPAN modules use startperl to write first line of generated perl
tests. The /usr/bin/env limitation still apply here.
If I give open a filename of an explicit undef and the read-write
mode ( +> or +< ), Perl opens an anonymous temporary file:
open my $fh, '+>', undef;
Perl actually creates a named file and opens it, but immediately unlinks the name. No one
else will be able to get to that file because no one else has the name for it. If I had used
File::Temp , I might leave
the temporary file there, or something else might be able to see it while I'm working with
it.
Print to a string
If my perl is compiled with PerlIO (it probably is), I can open a filehandle on a scalar
variable if the filename argument is a reference to that variable.
open my $fh, '>', \ my $string;
This is handy when I want to capture output for an interface that expects a filehandle:
something_that_prints( $fh );
Now $string contains whatever was printed by the function. I can inspect it by
printing it:
say "I captured:\n$string";
Read lines from a string
I can also read from a scalar variable by opening a filehandle on it.
open my $fh, '<', \ $string;
Now I can play with the string line-by-line without messing around with regex anchors or
line endings:
Most Unix programmers probably already know that they can read the output from a command as
the input for another command. I can do that with Perl's open too:
use v5.10;
open my $pipe, '-|', 'date';
while( <$pipe> ) {
say "$_";
}
This reads the output of the date system command and prints it. But, I can have
more than one command in that pipeline. I have to abandon the three-argument form which
purposely prevents this nonsense:
open my $pipe, qq(cat '$0' | sort |);
while( <$pipe> ) {
print "$.: $_";
}
This captures the text of the current program, sorts each line alphabetically and prints the
output with numbered lines. I might get a Useless Use of cat Award for that program that sorts
the lines of the program, but it's still a feature.
gzip on the fly
In Gzipping data
directly from Perl , I showed how I could compress data on the fly by using Perl's gzip IO
layer. This is handy when I have limited disk space:
open my $fh, '>:gzip', $filename
or die "Could not write to $filename: $!";
while( $_ = something_interesting() ) {
print { $fh } $_;
}
I can go the other direction as well, reading directly from compressed files when I don't
have enough space to uncompress them first:
open my $fh, '<:gzip', $filename
or die "Could not read from $filename: $!";
while( <$fh> ) {
print;
}
Change STDOUT
I can change the default output filehandle with select if I don't like standard
output, but I can do that in another way. I can change STDOUT for the times when
the easy way isn't fun enough. David Farrell showed some of this in How to
redirect and restore STDOUT .
First I can say the "dupe" the standard output filehandle with the special
& mode:
use v5.10;
open my $STDOLD, '>&', STDOUT;
Any of the file modes will work there as long as I append the & to it.
I can then re-open STDOUT :
open STDOUT, '>>', 'log.txt';
say 'This should be logged to log.txt.';
When I'm ready to change it back, I do the same thing:
open STDOUT, '>&', $STDOLD;
say 'This should show in the terminal';
If I only have the file descriptor, perhaps because I'm working with an old Unix programmer
who thinks vi is a crutch, I can use that:
open my $fh, "<&=$fd"
or die "Could not open filehandle on $fd\n";
This file descriptor has a three-argument form too:
open my $fh, '<&=', $fd
or die "Could not open filehandle on $fd\n";
I can have multiple filehandles that go to the same place since they are different names for
the same file descriptor:
use v5.10;
open my $fh, '>>&=', fileno(STDOUT);
say 'Going to default';
say $fh 'Going to duped version. fileno ' . fileno($fh);
say STDOUT 'Going to STDOUT. fileno ' . fileno($fh);
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Chris Madden ,
172 38
I have Perl script and need to determine the full path and filename of the script during
execution. I discovered that depending on how you call the script $0 varies and
sometimes contains the fullpath+filename and sometimes just
filename . Because the working directory can vary as well I can't think of a way
to reliably get the fullpath+filename of the script.
I know this was a long time ago but I was just looking for a perl windows way of doing this
and am quite happy with my solution #!/usr/bin/perl -w my @catalog= dir ;
$myHome = substr($catalog[3],14); $myHome = &rtrim($myHome); print qq(<$myHome>\n);
# Right trim function to remove trailing whitespace sub rtrim { my $string = shift; $string
=~ s/\s+$//; return $string; } just thought I'd share – user1210923 Dec 4 '20 at 17:42
Drew Stephens , 2008-09-18 07:30:54
259
There are a few ways:
$0
is the currently executing script as provided by POSIX, relative to the current working
directory if the script is at or below the CWD
Additionally, cwd() , getcwd() and abs_path()
are provided by the Cwd module and tell you where the
script is being run from
The module FindBin provides the
$Bin & $RealBin variables that usually are the path to
the executing script; this module also provides $Script &
$RealScript that are the name of the script
__FILE__ is the
actual file that the Perl interpreter deals with during compilation, including its full
path.
I've seen the first three ( $0 , the Cwd module and the FindBin module) fail under
mod_perl spectacularly, producing worthless output such as '.' or
an empty string. In such environments, I use __FILE__ and get
the path from that using the File::Basename module:
use File::Basename;
my $dirname = dirname(__FILE__);
This is really the best solution, especially if you already have a modified $0 –
Caterham Jan 8 '12
at 1:04
Ovid , 2008-09-17 16:19:48
148
$0 is typically the name of your program, so how about this?
use Cwd 'abs_path';
print abs_path($0);
Seems to me that this should work as abs_path knows if you are using a relative or
absolute path.
Update For anyone reading this years later, you should read Drew's answer . It's much better than mine.
Share Improve this answer
Follow edited Jul 4
'19 at 2:47 cxw 15.4k 2 2 gold badges 37 37 silver badges 69 69 bronze badges answered
Sep 17 '08 at 16:19 Ovid 11.1k 7 7 gold badges 41 41 silver
badges 75 75 bronze badges
GreenGiant ,
Small comment, on activestate perl on windows $0 typically contains backslashes and abs_path
returned forward slashes, so a quick "tr /\//\\/;" was needed to fix it. – Chris Madden Sep 17 '08 at
17:03
@bmdhacks, you're right. Presumption is, you didn't change 0$. For example you do work above
as soon as script starts (in initialization block), or elsewhere when you don't change $0.
But $0 is excellent way to change process description visible under 'ps' unix tool :) This
can show curren process status, etc. This is depended on programmer purpose :) –
Znik Mar 3 '14 at
12:24
Eric Wilhelm ,
9
Getting the absolute path to $0 or __FILE__ is what you want.
The only trouble is if someone did a chdir() and the $0 was
relative -- then you need to get the absolute path in a BEGIN{} to prevent any
surprises.
FindBin tries to go one better and grovel around in the $PATH
for something matching the basename($0) , but there are times when that does
far-too-surprising things (specifically: when the file is "right in front of you" in the
cwd.)
Is it really likely that anyone would be so foolish as to (permanently) chdir()
at compile time? – SamB Feb 12 '12 at 21:23
wnoise , 2008-09-17 16:52:24
7
Some short background:
Unfortunately the Unix API doesn't provide a running program with the full path to the
executable. In fact, the program executing yours can provide whatever it wants in the field
that normally tells your program what it is. There are, as all the answers point out, various
heuristics for finding likely candidates. But nothing short of searching the entire
filesystem will always work, and even that will fail if the executable is moved or
removed.
But you don't want the Perl executable, which is what's actually running, but the script
it is executing. And Perl needs to know where the script is to find it. It stores this in
__FILE__ , while $0 is from the Unix API. This can still be a
relative path, so take Mark's suggestion and canonize it with File::Spec->rel2abs(
__FILE__ );Share Improve
this answer Follow edited Aug 19 '13 at 21:30 the Tin Man
151k 39 39 gold badges 197 197 silver badges 279 279 bronze badges answered Sep 17 '08 at
16:52 wnoise 9,310
32 32 silver badges 46 46 bronze badges
use FindBin '$Bin';
print "The script is located in $Bin.\n";
It really depends on how it's being called and if it's CGI or being run from a normal
shell, etc. Share Improve this
answer Follow answered Sep 17 '08 at 16:21 Sean 4,433 1 1 gold badge 17 17 silver badges
17 17 bronze badges
perlfaq8 answers a
very similar question with using the rel2abs() function on $0 .
That function can be found in File::Spec. Share Improve this answer Follow edited Aug 19 '13 at 21:31 the Tin
Man 151k 39 39 gold badges 197 197 silver badges 279 279 bronze badges answered Sep 17
'08 at 16:34 moritz 12.3k 1 1 gold badge 36 36 silver
badges 62 62 bronze badges
There's no need to use external modules, with just one line you can have the file name and
relative path. If you are using modules and need to apply a path relative to the script
directory, the relative path is enough.
It does not provide the proper full path of the script if you run it like "./myscript.pl", as
it would only show "." instead. But I still like this solution. – Keve Jun 27 '16 at 10:12
mkc , 2012-11-26 14:01:34
1
#!/usr/bin/perl -w
use strict;
my $path = $0;
$path =~ s/\.\///g;
if ($path =~ /\//){
if ($path =~ /^\//){
$path =~ /^((\/[^\/]+){1,}\/)[^\/]+$/;
$path = $1;
}
else {
$path =~ /^(([^\/]+\/){1,})[^\/]+$/;
my $path_b = $1;
my $path_a = `pwd`;
chop($path_a);
$path = $path_a."/".$path_b;
}
}
else{
$path = `pwd`;
chop($path);
$path.="/";
}
$path =~ s/\/\//\//g;
print "\n$path\n";
use strict ; use warnings ; use Cwd 'abs_path';
sub ResolveMyProductBaseDir {
# Start - Resolve the ProductBaseDir
#resolve the run dir where this scripts is placed
my $ScriptAbsolutPath = abs_path($0) ;
#debug print "\$ScriptAbsolutPath is $ScriptAbsolutPath \n" ;
$ScriptAbsolutPath =~ m/^(.*)(\\|\/)(.*)\.([a-z]*)/;
$RunDir = $1 ;
#debug print "\$1 is $1 \n" ;
#change the \'s to /'s if we are on Windows
$RunDir =~s/\\/\//gi ;
my @DirParts = split ('/' , $RunDir) ;
for (my $count=0; $count < 4; $count++) { pop @DirParts ; }
my $ProductBaseDir = join ( '/' , @DirParts ) ;
# Stop - Resolve the ProductBaseDir
#debug print "ResolveMyProductBaseDir $ProductBaseDir is $ProductBaseDir \n" ;
return $ProductBaseDir ;
} #eof sub
While a source-only answer might solve the user's question, it doesn't help them understand
why it works. You've given the user a fish, but instead you should teach them HOW to fish.
– the Tin
Man Aug 19 '13 at 21:28
Jonathan ,
0
The problem with __FILE__ is that it will print the core module ".pm" path
not necessarily the ".cgi" or ".pl" script path that is running. I guess it depends on what
your goal is.
It seems to me that Cwd just needs to be
updated for mod_perl. Here is my suggestion:
my $path;
use File::Basename;
my $file = basename($ENV{SCRIPT_NAME});
if (exists $ENV{MOD_PERL} && ($ENV{MOD_PERL_API_VERSION} < 2)) {
if ($^O =~/Win/) {
$path = `echo %cd%`;
chop $path;
$path =~ s!\\!/!g;
$path .= $ENV{SCRIPT_NAME};
}
else {
$path = `pwd`;
$path .= "/$file";
}
# add support for other operating systems
}
else {
require Cwd;
$path = Cwd::getcwd()."/$file";
}
print $path;
The problem with just using dirname(__FILE__) is that it doesn't follow
symlinks. I had to use this for my script to follow the symlink to the actual file
location.
use File::Basename;
my $script_dir = undef;
if(-l __FILE__) {
$script_dir = dirname(readlink(__FILE__));
}
else {
$script_dir = dirname(__FILE__);
}
All the library-free solutions don't actually work for more than a few ways to write a
path (think ../ or /bla/x/../bin/./x/../ etc. My solution looks like below. I have one quirk:
I don't have the faintest idea why I have to run the replacements twice. If I don't, I get a
spurious "./" or "../". Apart from that, it seems quite robust to me.
my $callpath = $0;
my $pwd = `pwd`; chomp($pwd);
# if called relative -> add pwd in front
if ($callpath !~ /^\//) { $callpath = $pwd."/".$callpath; }
# do the cleanup
$callpath =~ s!^\./!!; # starts with ./ -> drop
$callpath =~ s!/\./!/!g; # /./ -> /
$callpath =~ s!/\./!/!g; # /./ -> / (twice)
$callpath =~ s!/[^/]+/\.\./!/!g; # /xxx/../ -> /
$callpath =~ s!/[^/]+/\.\./!/!g; # /xxx/../ -> / (twice)
my $calldir = $callpath;
$calldir =~ s/(.*)\/([^\/]+)/$1/;
None of the "top" answers were right for me. The problem with using FindBin '$Bin' or Cwd
is that they return absolute path with all symbolic links resolved. In my case I needed the
exact path with symbolic links present - the same as returns Unix command "pwd" and not "pwd
-P". The following function provides the solution:
sub get_script_full_path {
use File::Basename;
use File::Spec;
use Cwd qw(chdir cwd);
my $curr_dir = cwd();
chdir(dirname($0));
my $dir = $ENV{PWD};
chdir( $curr_dir);
return File::Spec->catfile($dir, basename($0));
}
On Windows using dirname and abs_path together worked best for
me.
use File::Basename;
use Cwd qw(abs_path);
# absolute path of the directory containing the executing script
my $abs_dirname = dirname(abs_path($0));
print "\ndirname(abs_path(\$0)) -> $abs_dirname\n";
here's why:
# this gives the answer I want in relative path form, not absolute
my $rel_dirname = dirname(__FILE__);
print "dirname(__FILE__) -> $rel_dirname\n";
# this gives the slightly wrong answer, but in the form I want
my $full_filepath = abs_path($0);
print "abs_path(\$0) -> $full_filepath\n";
$ cat >testdirname
use File::Basename;
print dirname(__FILE__);
$ perl testdirname
.$ perl -v
This is perl 5, version 28, subversion 1 (v5.28.1) built for x86_64-linux-gnu-thread-multi][1]
i've thought of using substr but i can't be sure that the number above in quotes will
always be the same number of characters
You can use substr() without knowing the number of characters.
To remove the first character of $str: substr($str, 0, 1, '')
To remove the last character of $str : substr($str, -1, 1, '')
Or remove the last character of $str : substr($str, length($str) - 1, '')
Or, just use chop() to remove the last character, as you've already noted.
"what is the best way to remove the first and last character, or remove the "" from the
variable ?" [my emphasis]
If you only need to remove the leading and trailing quotes, and the example data you
provided is representative, i.e. no embedded quotes, the easiest and most efficient way to do
this would be by using transliteration
:
It's not clear to me if flieckster intends to deal only with
strings like '"foo"' (from which it is clear that 'foo' should be
extracted), or if he or she may also be dealing with strings like 'foo' '"foo' 'foo"'
'f"o"o' etc., i.e., strings not having double-quotes at both the start and end of
the string.
In the latter case, it should be noted that qr/^\"(.+)\"$/
will not match and will return an empty list, leaving $got undefined.
As with others who have commented in this thread, it's not clear to me just what
flieckster wants to
achieve.
If, and it's a big if, the aim is to remove double-quotes only when they are
paired at both the start and end of the string and never in any other circumstance, then qr{ (?| \A " (.*) " \z | (.*)) }xms # needs 5.10+
will do the trick. With this regex, '""' '""""' '"foo"' '"fo"o"' '"f"o"o"'
become '' '""' 'foo' 'fo"o' 'f"o"o'
respectively, while strings like '' '"' '"foo' 'foo"' 'f"oo' 'f"o"o'
are unchanged.
Note that this regex needs Perl version 5.10+ because it uses the (?|...) branch
reset regex extension. The regex can be made to work in pre-5.10 versions by removing the
(?|...) and adding a grep defined, ... filter to the output of the regex
match.
Re^3: What's happening in
this expression?> why $a remains uninitialized? It's not the same It's not the
same $a The lexical scope in the debugger is limited to the The lexical scope in the
debugger is limited to the The lexical scope in the debugger is limited to the
eval'ed line. Skip the
my to avoid
this effect.
The debugger is a bad place to play with scoping like this. In effect when you evaluate
single lines like this they're more like doing an
eval within the scope of the program
(more or less; I'm sure someone more familiar with the perl5db could give more
specifics).
It's kind of like (handwaving) textually shimming in say DebugDump( eval {
YOURTEXTHERE } ) into wherever you're looking at and seeing the result.
This means
that your my declaration is happening inside of a transient scope (that single
eval statement) and then it's going away. Since the my was affecting only $a when
you check for defined-ness it fails because the package $a wasn't defined (however
your modifications to $xet al changes the package versions of those and
the values do persist after the statement).
$ cat test.pl use 5.032; my $foo =
10; say qq{foo: $foo} $ perl -d test.pl Loading DB routines from perl5db.pl version 1.57
Editor support available. Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(test.pl:2): my $foo = 10; DB<1> x $foo 0 undef DB<2> n
main::(test.pl:3): say qq{foo: $foo} DB<2> x $foo 0 10 DB<3> my $foo = 20
DB<4> x $foo 0 10 DB<5> my $foo = 20; say qq{foo: $foo} foo: 20 DB<6> x
$foo 0 10[download]
Simple rule of thumb I tend to follow is just don't use
my (or
state or
our ) from the debugger command line to
try and affect anything outside of that immediate command line.
The cake is a lie. The cake is a lie. The cake is a lie.
You are right. My variables are not always treated correctly, although recently the
situation improved. I remember that in the past you just can't work with my variables
at all. I just have a utility that stripped my moving them to tail comments and then
reversed the situation. But now the usage of my "official" as it is forced by the
strict pragma. Which means that such a situation is less acceptable.
Also if you are using recursion my attribute can't be stripped at all. So this is a
clear deficiently.
That's sad, because IMHO the debugger is the crown jewel of Perl language
environment and remains in certain areas unmatched by competition(macros, flexibility
of c command, etc.) Possibility of b lineno ($var eq "value") is
indispensable for debugging complex programs. That's what I always stress in my Perl
advocacy efforts" "Unmatched by competition."
So any deficiencies here are "highly undesirable."
That's, of course, raises the question of development priorities...
This is the follow up of Perl Automateaching -- part 1: brainstorming so
read it first to have an idea of my intentions even if the pseudocode presented there is not
what I currently plan.
I have choosen the name for this project and it will be Perl::Teacher as it is
clear and explicative.
This post is a mere proof of concept about Perl teaching and, yes! it can be done! I'd like
to be billionaire to hire super skilled perl geeks to develop my idea... but let's say they are
all busy at the moment :) so the pupil ( discipulus in Latin) will squeeze his brain and
will dress teacher dresses. Contributors are welcome!
In the final form Perl::Teacher will be document oriented, ie: it will analyze perl
programs wrote by the pupil in physical files. But in the current proof of concepts various
student's attempts are hardcoded into the below program contained in scalars from
$work_01 to $work_n and with a $solution_code
Also the final form of Perl::Teacher will be a bit interactive presenting and
reviewing assignements and telling small lessons, but for the moment nothing of this is
done.
So running the below program you will see a serie of attempts to satisfy the assignemnt and
results of tests applied to provided code fragments.
Modify the $debug variable to 1 or 2 to see much more messages.
Proof of
concept
Here my efforts up now ( Ignore the warning you'll receive: Having more than one /x
regexp modifier is deprecated at
.../perl5.24-64b/perl/site/lib/Perl/Critic/Policy/ValuesAndExpressions/RequireInterpolationOfMetachars.pm
line 110. beacuse it is a problem of Perl::Critic itself: see resolved issue on github
)
use strict; use warnings; use PPI; use PPI::Dumper; use Perl::Critic; use
Test::Deep::NoTest; use Data::Dump; my $debug = 0; # 0..2 my $perl_critic_severity = 'gentle';
# 'gentle' 'stern' 'harsh' 'crue + l' 'brutal' # assignemnt print <<'EOP'; Assignement:
-Create an array named @letters with 5 elements and fill it with first + 5 letters of the
English alphabet -Remove the first element using a list operator and assign it to a sca + lar
variable -Remove the last element using a list operator and assign it to a scal + ar variable
-Join these two removed elements with a '-' (using single quotes) sign + and assign the result
to a scalar named $result NB: All variables have to be lexically scoped NB: each above steps
must be accomplished in one statement EOP # solution code my $solution_code = <<'EOC';
use strict; use warnings; my @letters = ('a'..'e'); my $first = shift @letters; my $last = pop
@letters; my $result = join '-', $first, $last; EOC # student attempts my $work_01 =
<<EOT; need to crash! EOT my $work_02 = <<EOT; # comment: no need to crash! EOT my
$work_03 = <<EOT; # comment: no need to crash! use strict; EOT my $work_04 = <<EOT;
# comment: no need to crash! use strict; use warnings; EOT my $work_05 = <<'EOT'; use
strict; use warnings; my @letters = ('a'..'e'); EOT my %tests = ( # TEST DESCRIPTION # number
=> anonymous hash (tests will be executed in a sorted + order) # name => # run => send
the code to a sub returning 0|1 plus + messages # select_child_of => given a PPI class
search each element + of such class # to see if they contain all required el + ements. #
returns 0|1 plus messages # class => the class of elements to analyze (all el + ements of
such class will be tested) # tests => anonymous array: check children of the c + urrent
element to be of the appropriate class # and to hold the desired content (string + or regex can
be used) # evaluate_to => optional but only possible if select_child + _of was used: the
DPOM fragment # extracted by select_child_of will be chec + k to hold a precise value (at
runtime: see below) # hint => # docs => 001 => { name => 'code compiles', run =>
\&test_compile, # select_child_of ... # evaluate_to ... hint => "comment the line
causing crash with a # in fro + nt of it", docs => ['perldoc perlintro',
'https://perldoc.perl.org + /perlintro.html#Basic-syntax-overview'], }, 002 => { name =>
'strictures', # run => ... select_child_of => { class => 'PPI::Statement::Include',
tests => [ #['PPI::Token::Word', 'use'], ['PPI::Token::Word', qr/^use$/],
['PPI::Token::Word', 'strict'] ], }, # evaluate_to ... hint => "search perlintro for safety
net", docs => ['https://perldoc.perl.org/perlintro.html#Safet + y-net'], }, 003 => { name
=> 'warnings', # run => ... select_child_of => { class =>
'PPI::Statement::Include', tests => [ ['PPI::Token::Word', 'use'], #['PPI::Token::Word',
qr/^use$/], ['PPI::Token::Word', 'warnings'] ], }, # evaluate_to ... hint => "search
perlintro for safety net", docs => ['https://perldoc.perl.org/perlintro.html#Safet +
y-net'], }, 004 => { name => 'array creation', select_child_of => { class =>
'PPI::Statement::Variable', tests => [ ['PPI::Token::Word', 'my'], ['PPI::Token::Symbol',
'@letters'], ['PPI::Token::Operator', '='], ], }, evaluate_to => [ ('a'..'e') ], hint =>
"search perlintro basic variable types", docs =>
['https://perldoc.perl.org/perlintro.html#Perl- + variable-types'], }, 005 => { name =>
'first element of the array', select_child_of => { class => 'PPI::Statement::Variable',
tests => [ ['PPI::Token::Word', 'my'], ['PPI::Token::Symbol', qr/\$[\S]/],
['PPI::Token::Operator', '='], ['PPI::Token::Word', 'shift'], ['PPI::Token::Symbol',
'@letters'], ], }, evaluate_to => \'a', hint => "search functions related to real
arrays", docs => ['https://perldoc.perl.org/5.32.0/perlfunc.html +
#Perl-Functions-by-Category'], }, 006 => { name => 'last element of the array',
select_child_of => { class => 'PPI::Statement::Variable', tests => [
['PPI::Token::Word', 'my'], ['PPI::Token::Symbol', qr/\$[\S]/], ['PPI::Token::Operator', '='],
['PPI::Token::Word', 'pop'], ['PPI::Token::Symbol', '@letters'], ], }, evaluate_to => \'e',
hint => "search functions related to real arrays", docs =>
['https://perldoc.perl.org/5.32.0/perlfunc.html + #Perl-Functions-by-Category'], }, 007 => {
name => 'final result', select_child_of => { class => 'PPI::Statement::Variable',
tests => [ ['PPI::Token::Word', 'my'], ['PPI::Token::Symbol', '$result'],
['PPI::Token::Operator', '='], ['PPI::Token::Word', 'join'], ['PPI::Token::Quote::Single',
"'-'"], ['PPI::Token::Operator', ','], ['PPI::Token::Symbol', qr/^\$[\S]/],
['PPI::Token::Operator', ','], ['PPI::Token::Symbol', qr/^\$[\S]/], ], }, evaluate_to =>
\'a-e', hint => "search functions related to strings", docs =>
['https://perldoc.perl.org/5.32.0/perlfunc.html + #Perl-Functions-by-Category'], }, ); #
student's attempts examination foreach my $code ( $work_01, $work_02, $work_03, $work_04,
$work_05, $ + solution_code){ $code = PPI::Document->new( \$code ); print "\n# START of
provided code:\n",$code=~s/^/| /gmr,"# END of + provided code\n# TESTS:\n";
PPI::Dumper->new($code)->print if $debug > 1; my $passed_tests; foreach my $test (sort
keys %tests){ print "DEBUG: starting test $test - $tests{ $test }{ name }\n" + if $debug; # if
run defined my $run_result; my $run_msg; if ( exists $tests{ $test }{ run } ){ ($run_result,
$run_msg) = $tests{ $test }{ run }->( $code + ); if ( $run_result ){ print "OK test [$tests{
$test }{ name }]\n"; $passed_tests++; # next test next; } else{ $run_msg =~ s/\n//; print
"FAILED test [$tests{ $test }{ name }] because: + $run_msg\n"; if ( $tests{ $test }{ hint } ){
print "HINT: $tests{ $test }{ hint }\n"; } if ( $tests{ $test }{ docs } ){ print map {"DOCS:
$_\n"} @{$tests{ $test }{ docs } + } ; } last; } } # select_child_of defined my
$candidate_pdom; my $select_child_of_msg; if ( exists $tests{ $test }{ select_child_of } ){
($candidate_pdom, $select_child_of_msg) = select_child_of( pdom => $code, wanted_class =>
$tests{ $test }{ select_child_of } + { class }, tests => $tests{ $test }{ select_child_of }{
tests + } ); } # also evaluation is required if( $candidate_pdom and exists $tests{ $test }{
evaluate_to } + ){ my ($evauleted_pdom, $eval_msg) = evaluate_to ( $candidate_pdom, $tests{
$test }{ evalua + te_to } ); if($evauleted_pdom){ print "OK test [$tests{ $test }{ name }]\n";
$passed_tests++; # jump to next test next; } else{ print "FAILED test [$tests{ $test }{ name }]
becau + se: $eval_msg\n"; if ( $tests{ $test }{ hint } ){ print "HINT: $tests{ $test }{ hint
}\n"; } if ( $tests{ $test }{ docs } ){ print map {"DOCS: $_\n"} @{$tests{ $test }{ do + cs }}
; } } } elsif( $candidate_pdom ){ print "OK test [$tests{ $test }{ name }]\n"; $passed_tests++
; # jump to next test next; } else{ print "FAILED test [$tests{ $test }{ name }] because: $sel
+ ect_child_of_msg\n"; if ( $tests{ $test }{ hint } ){ print "HINT: $tests{ $test }{ hint }\n";
} if ( $tests{ $test }{ docs } ){ print map {"DOCS: $_\n"} @{$tests{ $test }{ docs }} ; } # if
one test breaks end the testing loop last; } } # all tests passed if ( $passed_tests == scalar
keys %tests ){ print "\nALL tests passed\n"; my $critic = Perl::Critic->new( -severity =>
$perl_critic_sev + erity ); my @violations = $critic->critique($code); if ( @violations ){
print "Perl::Critic violations (with severity: $perl_criti + c_severity):\n"; print
@violations; } else{ print "No Perl::Critic violations using severity level: $p +
erl_critic_severity\n"; } } print "\n\n"; } ################################ # TESTS
################################ sub evaluate_to{ my $pdom = shift; # passed by reference my
$expected_value = shift; ############################### # VERY DIRTY TRICK - START
############################### # only last element is returned in string evaluation # so the
below code cuts the parent where the current # pdom is found. so the current statement will be
the # last one of the whole code (parent) and its value # returned by the string evaluation #
(probably I'll need to redirect STDOUT in this scope) # # NB this will fail for multiline
statements! my $pdom_parent = $pdom->parent; my @lines_od_code =
split/\n/,$pdom_parent->content; if ( $debug > 1 ){ print "ORIGINAL CODE:\n"; dd
@lines_od_code; print "FOUND current PDOM element at line: ", $pdom->line_numb + er, "\n";
print "CUTTING code at line: ", $pdom->line_number, "\n"; dd
@lines_od_code[0..$pdom->line_number-1] } $pdom = PPI::Document->new(
\join"\n",@lines_od_code[0..$pdom->lin + e_number-1] ); ############################### #
VERY DIRTY TRICK - END ############################### { local $@; my $got; # we expect a
scalar ref if ( ref $expected_value eq 'SCALAR' ){ $got = \eval $pdom ; } # we expect an array
ref elsif ( ref $expected_value eq 'ARRAY' ){ $got = [ eval $pdom ]; } # we expect a hash ref
elsif ( ref $expected_value eq 'HASH' ){ $got = { eval $pdom }; } # we expect a regexp ref
elsif ( ref $expected_value eq 'Regexp' ){ $got = eval $pdom; $got = qr/$got/; } # Not a
reference else{ $got = eval $pdom; } # check to be the same type if ( ref $expected_value ne
ref $got ){ return (0, "got and expected values are not of the same ty + pe") } else{ print
"DEBUG: OK both got and expected are of the same typ + e: ", ref $got,"\n" if $debug; } if (
eq_deeply( $got, $expected_value ) ){ if ( $debug > 1 ){ print "DEBUG: OK both got and
expected hold sa + me content: "; dd $got; } return ($pdom, "expected value found for the expre
+ ssion [$pdom]"); } else{ if ( $debug ){ print "GOT: ",ref $got,"\n"; dd $got; print
"EXPECTED: ",ref $expected_value,"\n"; dd $expected_value; #print "PARENT: ";
PPI::Dumper->new( $pdom->parent )-> + print; } return (0, "wrong value of the
expression [$pdom]") } } } sub select_child_of{ my %opt = @_; my $pdom_fragments = $opt{ pdom
}->find( $opt{ wanted_class } ); return (0, "no element found of the correct type") unless
$pdom_fr + agments; foreach my $pdom_candidate ( @$pdom_fragments ){ print "DEBUG: checking
fragment: [$pdom_candidate]\n" if $debu + g; my $expected_ok; foreach my $test ( @{$opt{ tests
}} ){ my ($class, $content) = @$test; print "DEBUG: testing for class [$class] and content
[$con + tent]\n" if $debug; if ( $pdom_candidate->find( sub { $_[1]->isa($class) and (
ref $content eq 'R + egexp' ? ( $_[1]->content = + ~ /$content/ ) : ( $_[1]->content e +
q $content ) ) } ) ){ $expected_ok++; #print "DEBUG FOUND: [",ref $_[1],"] [",$_[1]->content
+ ,"]\n"; print "DEBUG: OK..\n" if $debug; if ( $expected_ok == scalar @{$opt{ tests }} ){
print "DEBUG: found a good candidate: [$pdom_candi + date]\n" if $debug; return (
$pdom_candidate, "found expected code in: + [$pdom_candidate]" ) } } else{ print "DEBUG: FAIL
skipping to next fragment of co + de\n" if $debug; last; } } } #FAILED return (0,"element not
found") } sub test_compile{ my $code = shift; { local $@; eval $code; if ( $@ ){ # print "\$@ =
$@"; return (0, $@, "Comment the line with a # in front of it", + "perlintro" ); } else { #
$code instead of 1?????? return (1, "code compiles correctly"); } } }[download]Implementation (current)
As you can see there is a lot PPI stuff but not exclusively. Tests are execuded in
order from 001 to 00n and if a test fails the current mini program is
rejected.
Each test can contain different steps, the first one being the optional run that
simply sends the current code to a sub: this preliminary, optional test passes if the sub
returns 1 and fails otherwise. Here it is used only to check if the program compiles ( see
below for future ideas ).
The second step of a test is select_child_of and it expects a PPI class name and a
serie of subtests. Each PPI element of the specified PPI class, for example
PPI::Statement::Variable (a variable declaration) will be processed to see if they
contains PPI elemnts which satisfy all subtests. The first PPI element passing all subtests is
returned by select_child_of and becomes a candidate for further inspections.
Infact if evaluate_to is also specified, the current PPI element is, take a deep
breath, keep calm, string evaluated to see if it holds the wanted value. And hic sunt
leones or here are dragons because eval only returns the last statement
value. Search the code above for the string dirty trick to see my workaround. For me
it is a genial solution, but wait, I'm the guy who string eval'ed entire CPAN.. :) so
improvements are warmly welcome.
This form of testing is a proof of concepts: is not the final form of the testing framework
needed by Perl::Teacher
When a miniprogram passes all tests it is evaluated by Perl::Critic to give more
hints to the student. Eventual policy violations will not make the program to be marked as
wrong, but are just presented as suggestions.
A note about flexibilty: looking carefully at the assignement you will notice that
@letters and $result are constraints. Not the same for the intermediate
scalars containing the first element and the last one.
Implementation (future)module design The main Perl::Teacher module will provide only a framework to
produce courses. The $teacher will load or create a configuration will have
methods to deal with the student's input and to emit messages, but the main activity will be to
load and follow courses plugins of the class Perl::Teacher::Course
In my idea the course creator will publish
Perl::Teacher::Course::EN::BasicVariables or
Perl::Teacher::Course::IT::RegexIntroduzione all being child of the main
Perl::Teacher::Course class. These courses have to be pluggable to the
$teacher object ( Module::Pluggable probably but I have to
investigate it further)
Each course will contain a serie of lessons published a sub modules, as in
Perl::Teacher::Course::EN::BasicVariables::01_strings , ..::02_lists etc.
Yes I know: very long names.. but this will ensure a clarity of intent and of usage, in my
opinion.
Each lesson will contain an ordered serie of optional elements: zero one or more
assignement , multiple test elements possibly interleaved by one or more
discourse and direct question .
So a possible flow can be:
01 - discourse - introduction to the lesson 02 - discourse
- more words 03 - assignement 04 - test 05 - test - more test 06 - test - test test ( block
until all tests are ok ) 07 - discourse - explain and add a task 08 - assignement - the main
assignement is updated 09 - test 10 - test - more test 11 - test - test test ( block until all
tests are ok ) 12 - question 13 - question 14 - discourse - explaining answers ... nn -
discourse - TIMTOWTDI nn - discourse - see also[download]
Suggestions on module design are warmly welcome, but i want to keep it as simple as
possible, not spawning objects for everything.
tests
Tests presented in the above code are too semplicistics to cover each teaching activity. I
need beside positive tests also negative ones for example to prevent the use
of modules, or all modules but one, to prevent external program execution and so on. Theese
tests will be quite on success and will emit messages only on failure: "dont do this!".
I can use Test::Script to add tests about correct overall syntax check, behaviour
of STDOUT and STDERR given different arguments and so on.
Then Perl::Teacher will provide its own tests like ones presented above:
evaluate_to ( evaluate_at is probably a better name as it eval the code at a
certain line), is dirty but it seems to me a viable option not so risky given the super small
and controlled environment. I also plan a method named evaluate_subs which will grab
al subs to test them.
I have to mix all this features in a clean and easy to use interface. Suggetions are
welcome.
student interaction
During a lesson the student must have the possibility to review the current assignement, to
receive hints and be pointed to relevant documentation. Part of this is roughly done in the
presented code using hints and docs embedded in tests. Can be and must be
improved.
I like to add a TIMTOWTDI discourse at the end of each lesson showing more ways to
accomplish, even if not in the very same way, the assignement.
Every output, comprensive of examined code, errors and hints, emitted during
03_array_manipulation must be saved into a 03_array_manipulation.history file
so that the student can review the whole lesson including errors commited and pitfalls
and the solution alongside different approaches to the same problem. Passing the time this
becomes a good source of knoweledge.
further ideas
Testing standalone scripts is an idea haunting me since years. Modulino is an
approach. I can be mad enough to take the original PDOM of a given program, then save all subs
and use PPI method prune to delete them from the PDOM, then wrap the rest
into a new main_original_program sub, add it to a new PDOM along with all previously
saved subs. Then I could do the obtained file and test it nicely. A lot of cut 'n
paste and probably error prone, but can be a path to explore.
I'd like also my Perl::Teacher to be as much possible input/output agnostic:
implement a way to interact with the console leaving open the possibility to be used by a web
interface too: how to do this?
I'd like to ear your opinions about this project, sugesstions on module design and
implementation of its parts, comments to the above proof concepts and everything you want to
share.
L*
There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.
Taking it seriously it demonstrate an important concept: learning is a path to follow,
possibly alongside a teacher. Many of us can produce ten different ways to satisfy an
assignment using perl. But this is not the point.
As you noticed (lack of lower case specification for the array and the costraint of a
single quote for the dash) it is very important to be clear in the assignement, making it
also pedantic, and to be sure it imply the usage of already presented elements.
A teacher must introduce concepts and verify how much students have incorporated
them.
Teaching, at first, is dedicated to fill ignorant's gap with notions and concepts (then
teach how to learn and how to think, but is not my goal).
So a course (in general but also mines) starts with assumed ignorance in one field, and
step by step introduces elements and tests the overall students understanding.
To produce PPI tests making all your example to be verified is an immane task, not worth
even to plan. While teaching or learning the appropriate virtue is patience not
hubris infact to learn is fondamental to recognize somethig superior who teach
you.
So I can add this note to my Perl::Teacher project:
about assignements:
-be sure to imply only already introduced elements, possibly refering + to the lesson where
they were discussed -in the hints section put reminders to previous lessons -be pedantic in
the assignement -possibly show up what was expected by tests when datastructures are i +
nvolved (this can clarify an assignement)[download]
References are used frequently and extensively in Perl code. They're very important for a
Perl web developer to understand, as the syntax of element access changes depending on whether
you have a reference or direct access.
Q: In Perl, how do you initialize the
following?
an array
an array reference
A hash
A hash reference
Furthermore, how would you change an array to an array reference, a hash to a hash
reference, and vice versa? How do you access elements from within these variables?
A: The
use of hash and array references is a pretty basic concept for any experienced Perl developer,
but it may syntactically trip up some newer Perl developers or developers who never really
grasped the underlying basics.Initializing an Array:
my @arr = (0, 1, 2);
An array is initialized with an @ symbol prefixed to the variable name, which
denotes the variable type as an array; its elements are placed in
parentheses.
Initializing an Array Reference:
my $arr_ref = [0, 1, 2];
With an array reference, you use the $ symbol, which denotes 'scalar', and the
elements are placed in square brackets. The reference isn't specified as an array, just as a
scalar, so you have to be careful to handle the variable type appropriately.
Like an array reference, a hash reference variable is prefixed with a $ , but
the elements are placed in curly braces.
Referencing a Hash or an Array
Referencing an array or hash is pretty straightforward. In Perl, a backslash in front of a
variable will return the reference to it. You should expect something like the following:
my $arr_ref = \@arr;
my $hash_ref = \%hash;
Dereferencing
Dereferencing a referenced variable is as easy as reassigning it with the appropriate
variable identifier. For example, here's how you would dereference arrays and hashes:
my @arr = @$arr_ref;
my %hash = %$hash_ref;
Accessing Elements
The differences between accessing elements of these variable types and their reference
versions is another area where amateur developers may get tripped up.
# to access an element of an array
my $element = $arr[0];
Notice that for an array you are not using the @ prefix but rather the
$ to denote a scalar, which is the type returned when accessing any element of an
array. Accessing the elements of an array reference, a hash, and a hash reference follows a
similar syntax:
# to access an element of an array reference
my $element = ${$array_ref}[0];
# to access an element of a hash
my $element = $hash{0};
# to access an element of a hash reference
my $element = $hash_ref->{0};
This creates a breakpoint at the very first executable statement of the subroutine
valuedir.
b-command can also be used to halt a program only when a specified condition meets.
For example, below mentioned command tells the debugger to halt when it is about to execute
line 12 and the variable $vardir is equal to the null string:
DB<15> b 12 ($vardir eq "")
Any legal Perl conditional expression can be specified with the b statement.
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Stephen , 2017-10-03 16:52:57
17 7
I have never fully understood Perl's resolution of package names, but I always assumed
that the following should always work, assuming you are executing myscript.pl from within the
directory that contains it:
myscript.pl (contains the following statement: use Class1::Class2::Class3)
Class1/
Class2/
Class3.pm (contains the following package declaration: package Class1::Class2::Class3;)
However, this is not working in my code because Class3.pm cannot be located. Looking at
@INC, it does not include the current directory, only various directories of my Strawberry
Perl installation.
What is the recommended way to solve this? I suppose I could modify @INC, or I could start
using FindBin, but I'm not sure which is best. I have inherited this code and am simply
migrating it to a new location, but it doesn't look like the old code needed either such
solution (I could be wrong, still looking...) perl share edit follow edited Nov 21 '17 at 15:50
ikegami 308k 14 14 gold badges 213 213 silver badges 452 452 bronze badges asked Oct 3
'17 at 16:52 Stephen 5,308 5 5 gold badges 34 34
silver badges 66 66 bronze badges
Perl doesn't search the current directory for modules or the script's directory for
modules, at least not anymore. The current directory was removed from @INC in
5.26 for security reasons.
However, any code that relies on the current directory being in @INC was
buggy far before 5.26. Code that did so, like yours, incorrectly used the current directory
as a proxy for the script's directory. That assumption is often incorrect.
To tell Perl to look in the script's directory for modules, use the following:
use FindBin 1.51 qw( $RealBin );
use lib $RealBin;
or
use Cwd qw( abs_path );
use File::Basename qw( dirname );
use lib dirname(abs_path($0));
A tangential question, but why $RealBin and not just $Bin ? Does
having the links resolved give us any benefit here, or have you used it here just a general
good practice? – sundar - Reinstate Monica Apr
9 '18 at 12:19
melpomene , 2017-10-03 17:00:02
9
Having . (the current directory) in @INC was
removed in 5.26 for security reasons ( CVE-2016-1238 ). Some Linux
distributions have backported the change, so you might run into this problem even if you're
using e.g. 5.24. share edit follow edited Oct 3 '17 at 17:44
answered Oct 3 '17 at 17:00 melpomene 77.6k 6 6 gold badges 63 63
silver badges 117 117 bronze badges
https://tpc.googlesyndication.com/safeframe/1-0-37/html/container.html Report this ad
Stephen , 2017-10-03 16:52:57
17 7
I have never fully understood Perl's resolution of package names, but I always assumed
that the following should always work, assuming you are executing myscript.pl from within the
directory that contains it:
myscript.pl (contains the following statement: use Class1::Class2::Class3)
Class1/
Class2/
Class3.pm (contains the following package declaration: package Class1::Class2::Class3;)
However, this is not working in my code because Class3.pm cannot be located. Looking at
@INC, it does not include the current directory, only various directories of my Strawberry
Perl installation.
What is the recommended way to solve this? I suppose I could modify @INC, or I could start
using FindBin, but I'm not sure which is best. I have inherited this code and am simply
migrating it to a new location, but it doesn't look like the old code needed either such
solution (I could be wrong, still looking...) perl share edit follow edited Nov 21 '17 at 15:50
ikegami 308k 14 14 gold badges 213 213 silver badges 452 452 bronze badges asked Oct 3
'17 at 16:52 Stephen 5,308 5 5 gold badges 34 34
silver badges 66 66 bronze badges
Perl doesn't search the current directory for modules or the script's directory for
modules, at least not anymore. The current directory was removed from @INC in
5.26 for security reasons.
However, any code that relies on the current directory being in @INC was
buggy far before 5.26. Code that did so, like yours, incorrectly used the current directory
as a proxy for the script's directory. That assumption is often incorrect.
To tell Perl to look in the script's directory for modules, use the following:
use FindBin 1.51 qw( $RealBin );
use lib $RealBin;
or
use Cwd qw( abs_path );
use File::Basename qw( dirname );
use lib dirname(abs_path($0));
A tangential question, but why $RealBin and not just $Bin ? Does
having the links resolved give us any benefit here, or have you used it here just a general
good practice? – sundar - Reinstate Monica Apr
9 '18 at 12:19
melpomene , 2017-10-03 17:00:02
9
Having . (the current directory) in @INC was
removed in 5.26 for security reasons ( CVE-2016-1238 ). Some Linux
distributions have backported the change, so you might run into this problem even if you're
using e.g. 5.24. share edit follow edited Oct 3 '17 at 17:44
answered Oct 3 '17 at 17:00 melpomene 77.6k 6 6 gold badges 63 63
silver badges 117 117 bronze badges
A BLOCK by itself (labeled or not) is semantically equivalent to a loop that executes once.
Thus you can use any of the loop control statements in it to leave or restart the block. (Note
that this is NOT true in eval{} , sub{} , or contrary to
popular belief do{} blocks, which do NOT count as loops.) The
continue block is optional.
The BLOCK construct can be used to emulate case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
You'll also find that foreach loop used to create a topicalizer and a
switch:
SWITCH:
for ($var) {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
Such constructs are quite frequently used, both because older versions of Perl had no
official switch statement, and also because the new version described immediately
below remains experimental and can sometimes be confusing.
Setting PERL5LIB at runtime will not affect Perl's search path. You need to
export the variable before executing the interpreter.
Alternatively you can modify @INC at compile time (also possible to do in a
separate script/module):
This doesn't work for me, I think because the PERL5LIB environment variable is processed by
the interpreter before the script is executed, so @INC isn't modified. – Mark Mar 2 '11 at 13:25
Mark , 2011-03-02 13:11:54
4
You'd do this via 'use lib' rather than manipulating the environment:
PERL5INC is a shell environment variable, so you wouldn't set it
inside your Perl program (normally) but instead specify it before invoking Perl. The below is
a shell command where I've used PERL5LIB to instruct prove to find a Perl module residing in ~/OnePop :
$ PERL5LIB=~/OnePop prove -l t
... PERL5LIB is unset here ....
When a command is preceded by a variable assignment like this, the shell sets and exports
the variable ( PERL5LIB ) to that command, but after that the variable will be
unset again. You can also set the variable in the shell, so that all subsequent commands will
inherit it.
$ export PERL5LIB=~/OnePop
...
$ prove -l t
... PERL5LIB continues to be set here ...
If you forget the export keyword in the above example (i.e. assigns the value
using PERL5LIB=~/OnePop on a separate line) the variable will be set in the
shell, but it will not be inherited by any commands you run (meaning that
prove will not be able to see it).
Finally, if you wanted to set the environment PERL5LIB variable from inside a
Perl program you'd have to write it like this:
$ENV{PERL5LIB} = glob("~/OnePop"); # glob() expands the tilde
system(qw( prove -l t ));
Though, as other have pointed out, if you want to specify the include path from inside
Perl it is easier/better to use use lib $PATH . share improve this answer follow answered May 20
'14 at 11:05 zrajm
1,149 1 1 gold badge 11 11 silver badges 18 18 bronze badges
PERL5INC is an environment variable. Environment variables are only inherited from parents
to their children and can't (easily) be set the other way around. If you want to store extra
search paths in an external file I suggest you make it a simple list of paths and write a
simple loop to read each path from the file and manipulate @INC in the current process. If
you want this to be done early at compile time you'll have to use a BEGIN {} block.
For example
BEGIN{
open(INCFILE,"<","my.inc.file") or die($!);
foreach(<INCFILE>){
push @INC,$_;
}
close(INCFILE);
}
You could instead install the latest version of Perl 5 available (in a non-system
location, of course). After you have used a module file or done whatever is necessary to make
the new perl and cpan executables visible to your shell, you can
use cpan to install all the modules you need. I have sometimes done this for
individual applications in a similar vein to using Python Virtual Environments.
I just installed Perl 5.18, and I get a lot of warnings like this,
given is experimental at .\[...].pl line [...].
when is experimental at .\[...].pl line [...].
Smartmatch is experimental at C:/strawberry/perl/site/lib/[...] line [...].
Looking into these warnings -- which I've never heard mentioned anywhere -- I was only
able to find this in two places,
perldelta for 5.18 ,
which only really mentions insofar as to say that the feature has been downgraded to
experimental ?
The Perl Delta still does the most to give mention as to what's happening with those
features, it's halfway down buried in the pod,
Smart match, added in v5.10.0 and significantly revised in v5.10.1, has been a regular
point of complaint. Although there are a number of ways in which it is useful, it has also
proven problematic and confusing for both users and implementors of Perl. There have been a
number of proposals on how to best address the problem. It is clear that smartmatch is
almost certainly either going to change or go away in the future. Relying on its current
behavior is not recommended. Warnings will now be issued when the parser sees ~~, given, or
when.
I'm confused at how the most significant change in Perl in the past 10 years could be
pulled. I've started using given , when , and
smartmatch all over the place. Is there any more information about these
futures? How is anyone finding them "confusing?" How are these features likely to change? Is
there a plan to implement these features with a module? perl smartmatch share improve this
question follow edited Aug 23 at 19:21 HoldOffHunger
7,890 4 4 gold badges 44 44 silver badges 85 85 bronze badges asked Jun 4 '13 at 20:37
Evan
Carroll 59.4k 37 37 gold badges 193 193 silver badges 316 316 bronze badges
There are problems with the design of smart-matching. The decision of what any given
TYPE ~~ TYPE should do is most often unobvious, inconsistent and/or disputed.
The idea isn't to remove smart matching; it's to fix it.
Specifically, ~~ will be greatly simplified, as you can see in a proposal by the 5.18
pumpking. Decisions as to how two things should match will be done with helpers such as those
that already exist in Smart::Match .
... ~~ any(...)
Much more readable, much more flexible (fully extensible), and solves a number of problems
(such as "When should X be considered a number, and when should it be considered a string?").
share improve this answer
follow edited
Jul 3 '13 at 20:23 answered Jun 5 '13 at 2:25 ikegami 308k 14 14 gold badges 212 212
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Some insights might be gained by reading rjbs's proposed changes to smartmatch . He
is the pumpking (Perl release manager) after all, so his comments and his view of the future
is more relevant than most. There is also plenty of community comment on the matter, see
here for instance. The 'experimental' status is in effect because, since things are
likely to change in the future, it is responsible to inform users of that fact, even if we
don't know what those changes will be. share improve this answer follow edited Jun 5 '13 at
3:40 answered Jun 5 '13 at 3:19 Joel Berger 19.5k 4 4 gold badges 45
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Well, that's what's said in the description of the patch
that downgraded this set of features to experimental:
The behavior of given/when/~~ are likely to change in perl 5.20.0: either smart match
will be removed or stripped down. In light of this, users of these features should be
warned. A category "experimental::smartmatch" warning should be issued for these features
when they are used.
So whil you can indeed turn these warnings off, with something like this ( source ):
no if $] >= 5.018, warnings => "experimental::smartmatch";
... it's just turning your eyes off the problem. share improve this answer follow answered Jun 4
'13 at 20:43 raina77ow 86.7k 10 10 gold badges 171
171 silver badges 201 201 bronze badges
tjd ,
It's not about how to turn these warnings off, it's about what's wrong with
Smartmatch/given/when and how will they be remedying the problem. The roadmap is just "change
ahead" with no certainty of direction. – Evan Carroll Jun 4 '13 at 20:48
Monday, June 23, 2014Perl smartmatch : what now ? Sorry to wake up an old
discussion, but ... does anybody have a clear idea of what is going to happen to smartmatch
?
Our team maintains dozens of internal applications and modules containing "given/when" and
smartmatch statements. Most of this code was written between 2007 and 2012 -- remember, at that
time smartmatch was an official feature, never mentioned as being "experimental", so we happily
used it in many places. The reasons for using smartmatch were quite modest :
match a scalar against an array
match 2 scalars, without a warning when one of the scalars is undef
more readable switch statements, thanks to "given/when"
When 5.18 came out, I was quite worried about the regression of
smartmatch to "experimental" status, but I was confident that things would be settled in
5.20, so I decided not to upgrade (we still use 5.14). Now 5.20 is out .. and nothing has
changed about smartmatch, without even a clue about how this is going to evolve.
Our servers cannot easily upgrade to 5.20, because this would throw warnings all over
the place. I tried to find a way to globally turn off these warnings (like set
PERL5OPT=-M-warnings=experimental::smartmatch, or PERL5OPT=-M= experimental::smartmatch ), but this doesn't
work because the "no warnings" pragma is lexically scoped, so global settings are not taken
into account.
So my options are :
don't change anything, don't upgrade, and wait for 5.22, hoping that some reasonable form
of smartmatch will be reintroduced into the core
revise all source files, adding a line "use experimental qw/smartmatch/;" at the
beginning of each lexical scope ... but I have no guarantee that this will still work in
future versions
revise all source files, removing the given/when/smartmatch statements and replacing them
with plain old Perl, or with features from some CPAN modules like match::smart or Smart::Match ... but it would be a pity to engage
in such work if regular smartmatch comes back in a future version of Perl.
As you can see, none of these options is really satisfactory, so
I would be interested in hearing if other companies are in the same situation and how they
decided to handle it.
By the way, I love the new Perl way of introducing new features as "experimental", until
they become stable and official ... but this only works well when the experimental status
is declared from the start . The problem with smartmatch is that it had been
official for several years, before being retrograted to experimental. Agreed, the full
semantics of smartmatch as published in 10.0 had inconsistencies, but throwing away the
whole thing is a bit too harsh -- I'm sure that many users like me would be happy with a
reasonable subset of rules for matching common cases.
Thanks in advance, Laurent Dami Posted by dami at 9:07 PM13
comments:
grep will always iterate through the entire list given. any will stop as soon as it
finds a single element matching the condition, which means on average it only has to
iterate through half the elements.
Hey, I'm in the same boat, I've been using smartmatch for switch since appearance and I
like it, it works for me. The way i deal with pragmas is that i centralize them in a custom
minipackage, as in "package pragmas" and I import that, so it's easier to manage "slight
incompatibilities" like these. So when p5p marked it as experimental and issued a warning
for it, i just put warnings->unimport( 'experimental::smartmatch' ) if $] >= 5.018;
right below the "usual" warnings->import( FATAL => 'all' ); If p5p will would rip it
off entirely, I'll just fetch some variant from cpan, put it in the pragmas package and
import it from there, transparently. It's annoying, but it's not p5p's responsibility to
update my code, if I wanna keep up with all the goodies and advancements. Otherwise you
keep the code "dead", stuck on 5.14 and that's that, it works.
Hi we had the same issue with pseudhashes which was an official feature until 5.10
The problem was that old systems had huge databases with old storables containing
pseudohashes. there was no way
I had to write a module to serialize pseudohashes structures into plain hashes, and we
then saved as hashed structured when they appreared.
Or just leave them unitl all nodes in a cluster was uploaded from the perl 5.8.9.
You have to live with it, just write software that adopts.
Hello, I may be ressurect an old post, but as written here (
http://perldoc.perl.org/perl5180delta.html#The-smartmatch-family-of-features-are-now-experimental
), it is possible to stop the warning with this command:
no if $] >= 5.018, warnings => "experimental::smartmatch";
This post will be part of a new communication channel between p5p and the community. We
hope to share more with you and keep you up-to-date using this platform.
On December 20th, 2017, we released Perl 5.27.7, which included a massive change to
smartmatch . Since then it has
been reverted. What happened?
Smartmatch has a long history. It was introduced in 5.10 back in December 2007 and
significantly revised in 5.10.1. It was a good idea, but ended up causing more harm than good
to the point it was deemed unreliable.
In an unprecedented step, it was marked as "experimental" in Perl 5.18.0, released in May
2013. Here is the mention of this in perldelta :
Smartmatch, added in v5.10.0 and significantly revised in v5.10.1, has been a regular
point of complaint. Although there are some ways in which it is useful, it has also proven
problematic and confusing for both users and implementors of Perl. There have been some
proposals on how to best address the problem. It is clear that smartmatch is almost certainly
either going to change or go away in the future. Relying on its current behavior is not
recommended.
Warnings will now be issued when the parser sees ~~ , given , or
when .
Since then, various threads were raised on how to resolve it. The decided approach was to
simplify the syntax considerably. It took several rounds of discussions (with some
bike-shedding) to settle what to simplify and to reach an agreement on the new behavior.
Last year we had finally reached an agreement on the significant perspectives. The changes
were implemented by Zefram, a core developer. The work was published on a public branch for
comments.
When no objections were filed, Zefram merged the new branch. It was included in the
5.27.7
development release.
Following the release of this development version, issues started popping up with the effect
this change made. A fair portion of CPAN was breaking to the point that one of the dedicated
Perl testers decided it was unfeasible for them to continue testing. Subsequently, we decided
to revert this change.
What went wrong?
First of all, it was clear that moving smartmatch to experimental did not achieve what we
had hoped. Features are marked as experimental to allow us to freely (for some value of
"freely") adjust and tinker with them until we are comfortable making them stable. The policy
is that any experimental feature can be declared stable after two releases with no behavioral
change. With smartmatch, it was marked after numerous versions in which it existed as a stable
feature.
Secondly, the change was massive. This in and of itself is not necessarily wrong, but how we
handled it leaves room for improvement.
Thirdly, centering the communication around this change on the core mailing list was
insufficient to receive enough feedback and eyes on the problem and the proposed solution. We
should have published it off the list and sought more input and comments. We hope to use this
platform to accomplish that.
Fourthly, we could have asked our dedicated testers for help on running additional, specific
tests, to view what would break on CPAN and how damaging this change could be.
Where do
we go from here?
Despite not being the best way to learn from a mistake, there was minimal damage. The new
syntax and behavior were only available on a single development release, did not reach any
production code, and was reverted within that single release.
To address smartmatch again, we will need to reflect upon our mistakes and consider
approaching it again by communicating the change better and by receiving additional feedback to
both offer a useful feature and pleasing syntax. This will take time, and we are not rushing to
revisit smartmatch at the moment.
We apologize for the scare and we appreciate the quick responses to resolve this situation.
Thank you.
When a Perl script is executed the user can pass arguments on the command line in various
ways. For example perl program.pl file1.txt file2.txt or perl program.pl from-address
to-address file1.txt file2.txt or, the most common and most useful way:
When the scripts starts to run, Perl will automatically create an array called @ARGV and put
all the values on the command line separated by spaces in that variable. It won't include perl
and it won't include the name of our script ( program.pl in our case), that will be placed in
the $0 variable. @ARGV will only include the values located after the name of the script.
In the above case @ARGV will contain: ('-vd', '--from', 'from-address', '--to',
'to-address', 'file1.txt', 'file2.txt')
We can access @ARGV manually as described in the article about @ARGV , but there are a number of
modules that will handle most of the work for you. In this article we'll see Getopt::Long a module that also comes with the
standard installation of Perl.
Explain the command line
Just before doing that, let's see what is really our expectation from the command line
processing.
Long names with values: we would like to be able to accept parameters with long names
followed by a value. For example --to VALUE . ("Long" is relative here, it just means more
than 1 character.)
Long names without value: We would like to accept flags that by their mere existence will
turn some flag on. For example --verbose .
Short names (or single-character names) with or without values. The above two just
written -t VALUE and -v .
Combining short names: -vd should be understood as -v -d . So we want to be able to
differentiate between "long names" and "multiple short names combined". The difference here
is that "long names" start with double-dash -- while short names, even if several of them
were combined together start with a single dash - .
Non-affiliated values, values that don't have any name starting with a dash in front of
them. For example file1.txt file2.txt .
There can be lots of other requirements and Getopt::Long can handle quite a few of them, but
we'll focus on the basics.
Getopt::Long
Getopt::Long exports a
function called GetOptions , that can process the content of @ARGV based on the configuration
we give to it. It returns true or false indicating if the processing
was successful or not. During processing it removes the items from @ARGV that have been
successfully recognized. We'll take a look at possible errors later on. For now, let' see a
small example we save in cli.pl :
use strict ;
use warnings ;
use 5.010 ;
use Getopt :: Long qw ( GetOptions );
my $source_address ;
GetOptions ( 'from=s' => \$source_address ) or die "Usage: $0 --from NAME\n" ;
if ( $source_address ) {
say $source_address ;
}
After loading the module we declare a variable called $source_address where the value of the
--from command line flag will be stored. We call GetOptions with key-value pairs. The keys (in
this case one key) is the description of the flag. In this case the from=s declares that we are
expecting a command line parameter called --from with a string after it. Because in Perl
numbers can also be seen as strings, this basically means "pass me any value". This declaration
is then mapped to the variable we declared earlier. In case the syntax is unclear => is a
"fat arrow" you might be familiar from hashes and the back-slash \ in-front of the variable
indicates that we are passing a reference to the variable. You don't need to understand
references in order understand this code. Just remember that the variables on the right hand
side of the "fat comma" operators need to have a back-slash when calling GetOptions .
We can run this program in several ways: perl cli.pl --from Foo will print "Foo". The value
passed after the -from flag is assigned to the $source_address variable. On the other hand
running perl cli.pl will not print anything as we have no passed any value.
If we run it perl cli.pl Foo it won't print anything either, as GetOptions only deals with
options that start with a dash ( - ). (This is actually configurable, but let's not get there
now.)
If we run the script passing something that looks like a parameter name, but which has not
been declared when calling GetOptions . Something that starts with a dash - . For example:
perl cli.pl --to Bar
Unknown option: to
Usage: cli.pl --from NAME
The first line is a warning printed by GetOptions , the second line is the string we
generated using die .
Option requires an argument
Another case is when we run the script, pass --from , but without passing any value after
it:
perl cli.pl --from
In that case the output will look like this:
Option from requires an argument
Usage: cli.pl --from NAME
Here too, the first line was from GetOptions and the second line from our call to die . When
we called GetOptions we explicitly said =s that we are expecting a string after the --from
.
Default values
Often we would like to give a default value to one of the options. For example in the case
of the --from field we might want it to default to the word 'Maven'. We can do it by assigning
this value to the $source_address variable before calling GetOptions . For example, at the time
we declare it using my .
my $source_address = 'Maven' ;
GetOptions ( 'from=s' => \$source_address ) or die "Usage: $0 --from NAME\n" ;
if ( $source_address ) {
say $source_address ;
}
If the user does not pass the --from flag then GetOptions will not modify the value in the
$source_address variable. Running perl cli.pl will result in "Maven".
Flags without
value
In addition to parameters that require a value, we also would like to allow flags. Names,
that by their presence make a difference. These things are used when we want to allow the users
to turn on debugging, or to set the verbosity of the script.
use strict ;
use warnings ;
use 5.010 ;
use Getopt :: Long qw ( GetOptions );
my $debug ;
GetOptions ( 'debug' => \$debug ) or die "Usage: $0 --debug\n" ;
say $debug ? 'debug' : 'no debug' ;
Originally the $debug variable contained undef which is considered to be false in
Perl. If the user passes the --debug flag, the corresponding variable will be set to some
true value. (I think
it is the number one, but we should only rely on the fact that it evaluates to true.) We then
use the ternary
operator to decide what to print.
The various ways we call it and the output they produce:
The last example shows that values placed after such name are disregarded.
Multiple
flags
Obviously, in most of the scripts you will need to handle more than one flag. In those cases
we still call GetOptions once and provide it with all the parameters:
Combining the above two cases together we can have a larger example:
use strict ;
use warnings ;
use 5.010 ;
use Getopt :: Long qw ( GetOptions );
my $debug ;
my $source_address = 'Maven' ;
GetOptions (
'from=s' => \$source_address ,
'debug' => \$debug ,
) or die "Usage: $0 --debug --from NAME\n" ;
say $debug ? 'debug' : 'no debug' ;
if ( $source_address ) {
say $source_address ;
}
Running without any parameter will leave $debug as undef and the $source_address as
'Maven':
$ perl cli.pl
no debug
Maven
Passing --debug will set $debug to true, but will leave $source_address as 'Maven':
$ perl cli.pl --debug
debug
Maven
Passing --from Foo will set the $source_address but leave $debug as undef :
$ perl cli.pl --from Foo
no debug
Foo
If we provide parameters, they will both set the respective variables:
$ perl cli.pl --debug --from Foo
debug
Foo
The order of the parameters on the command line does not matter:
$ perl cli.pl --from Foo --debug
debug
Foo
Short names
Getopt::Long
automatically handles shortening of the option names up to ambiguity. We can run the above
script in the following manner:
$ perl cli.pl --fr Foo --deb
debug
Foo
We can even shorten the names to a single character:
$ perl cli.pl --f Foo --d
debug
Foo
and in that case we can even use single-dash - prefixes:
$ perl files/cli.pl -f Foo -d
debug
Foo
These however are not really single-character options, and as they are they cannot be
combined:
In order to combine them we need two do two things. First, we need to declare the options as
real single-character options. We can do this by providing alternate, single-character names in
the definition of the options:
GetOptions (
'from|f=s' => \$source_address ,
'debug|d' => \$debug ,
) or die "Usage: $0 --debug --from NAME\n" ;
The second thing is that we need to enable the gnu_getopt configuration option of
Getopt::Long by calling Getopt::Long::Configure qw(gnu_getopt);
use Getopt :: Long qw ( GetOptions );
Getopt :: Long :: Configure qw ( gnu_getopt );
After doing that we can now run
$ perl cli.pl -df Foo
debug
Foo
The full version of the script with the above changes looks like this:
use strict ;
use warnings ;
use 5.010 ;
use Getopt :: Long qw ( GetOptions );
Getopt :: Long :: Configure qw ( gnu_getopt );
use Data :: Dumper ;
my $debug ;
my $source_address = 'Maven' ;
GetOptions (
'from|f=s' => \$source_address ,
'debug|d' => \$debug ,
) or die "Usage: $0 --debug --from NAME\n" ;
say $debug ? 'debug' : 'no debug' ;
if ( $source_address ) {
say $source_address ;
}
Non-affiliated values
The GetOptions function only handles the parameters that start with a dash and their
corresponding values, when they are relevant. Once it processed the options it will remove them
from @ARGV . (Both the option name and the option value will be removed.) Any other,
non-affiliated values on the command line will stay in @ARGV . Hence if we add Data::Dumper to our script and use
that to print the content of @ARGV at the end ( print Dumper \@ARGV ) as in this script:
After processing the options, file1.txt and file2.txt were left in @ARGV . We can now do
whatever we want with them, for example we can iterate over the @ARGV array using foreach
.
Advanced
Getopt::Long has tons of
other options. You might want to check out the documentation.
We need not "assume that somebody uses this formatting". I do it frequently, and I have
often seen it in other people's code. That fact that you use it and saw it in other people code
means nothing. People often adopt and use bad programming style. Even talented programmers do.
Look at classic The Elements of Programming Style , by Brian W. Kernighan and P.
J. Plauger. They include such recommendations as ( cited from
https://en.wikipedia.org/wiki/The_Elements_of_Programming_Style ) :
Write clearly -- don't be too clever.
Say what you mean, simply and directly.
... ... ...
Write clearly -- don't sacrifice clarity for efficiency.
... ... ...
Parenthesize to avoid ambiguity.
... ... ...
Make sure special cases are truly special.
... ... ...
The real question is whether the use you advocate represents a good Perl programming
style or not.
I would understand the use of post-fix if construct in a loop to specify exit condition.
Something like:
return if ($exit_condition);
They make code more readable in comparison with the regular if statement as as such have
certain value and IMHO represent a good programming style.
In many other cases the desire to save two curly braces looks to me a very questionable
practice and a bad programming style. Your mileage may vary.
"... In your private role you are free to do whatever you wish. After all programming open source is about fun, not so much about discipline. ..."
"... The situation radically changes in commercial projects. If you are a manager of a large project you need to ensure a uniform and preferably simple style via guidelines that explicitly prohibit such "excesses" and to step on the throat of such "excessively creative" people to make them "behave". ..."
"... That's why languages that allow too much syntactic freedom are generally not welcomed in large commercial projects, even if they are able to manage large namespaces more or less OK. ..."
If I have to maintain (as only maintainer) a piece of perl code, I will *rewrite* *all*
statements as you state from action if expression; to expression and action; as that (to
me) is waaaaaaaaaay easier to read/understand/maintain. Nothing to do with "idiomatic
perl". Nothing at all!
People are extremely flexible. The same is true for programmers. Many of
talented programmers I encountered have a somewhat idiosyncratic style...
In your private role you are free to do whatever you wish. After all programming open
source is about fun, not so much about discipline.
The situation radically changes in commercial projects. If you are a manager of a large
project you need to ensure a uniform and preferably simple style via guidelines that
explicitly prohibit such "excesses" and to step on the throat of such "excessively
creative" people to make them "behave".
That's why languages that allow too much syntactic freedom are generally not welcomed in
large commercial projects, even if they are able to manage large namespaces more or less
OK.
Let's be frank: Perl lost Web applications development more or less completely. The
reasons are not clear and can be argued, but the fact is indisputable. But the problem that
I see is that Perl can lose attraction among sysadmins because of excessive push of OO
programming style by OO-fanatics and the second rate book authors, as well as due to
inability of distribute remaining scarce development resources toward modest and not fancy
(unlike closures, accessors, frameworks and other fancy staff) improvements in the
procedural programming arena (the area which this post is all about).
An interesting question is: at what point excessive syntactic flexibility stimulates
perverted programming style which is reflected in the derogative term "complexity junkies"?
When in the program simple things look complex, and complex unmanageable. "Object oriented
spaghetti" ('Lasagna code' with too many layers) is another term that addresses the same
problem. See, for example, discussion at
https://medium.com/better-programming/is-object-oriented-programming-garbage-66c4f41adcaa
Returns true if the context of the currently executing subroutine or eval is looking for a
list value. Returns false if the context is looking for a scalar. Returns the undefined value
if the context is looking for no value (void context).
Had a reeeeeaaaaly vague recollection and the Perl
Cookbook (Ch 1 section 19) does actually offer $_ = join(' ', split(' '));
as an alternative to these three substitutions:
to strip and canonicalize to single spaces; and does offer this trim
sub:
sub trim { my @out = @_ ? @_ : $_; $_ = join(' ', split(' ')) for @out; return
wantarray ? @out : "@out"; }[download]
Edit: That being said, I don't recall having seen this construct in the wild
otherwise and had the vaguest of hunches that PC mentioned anything like this so I'd
hardly call it a "common idiom" either.
At least in the US all other things being equal there is a tendency to root for the underdog
in any given conflict. Python is seen as king. Perl is seen as an "old man's language." Never
mind the two languages are contemporaries and Perl that follows best practices isn't difficult
to follow.
My first thought its we can probably leverage the psychological tendency to root for the
underdog as means to promote the language. Let's talk about that. Am I right? Am I wrong?
Second, it seems that there are very few Code Academy type sites that support and promote
Perl as a language worth learning. I keep hearing about the need for "fresh blood." Well, I'm
"fresh blood" but I used books from Amazon to get where I'm at. I'm still in the process of
learning. It seems most younger developers just want to go from one structured tutorial to
another without buying a print or even Kindle book.
So, how do we promote Perl to such sites? That's a major bottle neck, I think. Sure, Python
dominates but there is space for Rust and Go devops. I see space for Perl at the table too.
Third, there are lots of small to medium sized projects that happen to be written in Perl
that don't get a lot of visibility. Sure, they're (probably) on CPAN, but we can't all know
everything.
Someone made a point to me in another post that programming languages are like gods in some
fantasy literature: they lose power as others stop believing in them. Point taken. So, let's
increase the number of devotees by talking about these projects and their usefulness.
What are some cool projects out there that leverage the best of Perl that don't get the
visibility they deserve? Yes, you can plug your own project and/or talk about its
challenges.
ncps - A
handy colorized(optional) ps like utility that with search options. Also can be told to
display memory/CPU summary for all matched procs.
ncnetstat
- A handy colorized(optional) netstat like utility that with search options. Among other
interesting features, can be told to display CPU and memory usage for process that has
the connection.
piddler
- Grab info from the proc table for a single PID and display it and all open files and
network connections.
essearcher -
A handy command line utility for searching elasticsearch and displaying the results in a
nicely formatted manner. Can even me used as a nagios check. Comes with support for
fail2ban, HTTP access logs, Postfix, and syslog out of the box.
inidhcp
- Helps for managing DHCP subnets... adding new ones, checking for overlap, some basic
sanity checking, and generation.... I use it to basically help manage PXE boot
subnets.
Rex is an Ansible-like automation tool written in Perl
I love rex. So much more friendly than ansible and so bloody flexible. erkiferenc 3 points·
2 days ago
Rex is cool, but no one is adopting it over Ansible/Salt/Puppet/etc.
As Rex maintainer, I feel the words "no one" to be too strong in this context, so let
me clarify that claim a bit :)
I agree that the market share is relatively low. Partly because it's a volunteers-only
project, and I'm currently not aware of anybody actively flooding the advertisement
channels with Rex content in their free time (and for free). Anybody willing to change
that, please go ahead!
Another factor for relatively low visibility is that the purpose of Rex, as a
framework, is to allow one to build their own automation tool required to solve their own
needs. This means most of the Rex-based solutions are custom-made. So it's often too
specific for their own situation, or they don't have the permissions to publish it.
Personally, I have more first-hand experience with use cases where Rex is used right
form the start. Based on community reports, it's also fairly common that one of the
alternatives are replaced with Rex at some point. Overall there are known use cases
ranging from one-man shows to tens of thousands of servers.
I guess we gotta put up a success stories page or similar to the website to give those
cases more visibility :) Until then, please feel free to blog about it, tweet about it,
give a talk at the next event, star it on GitHub, favorite it on MetaCPAN, vote on
StackShare...or even hire/sponsor me to do it all! :D
level 1 DerBronco 9 points·
3 days ago
I am living in a 25k town with no metropolitan area within 200km.
My client is a leading importer and big wholesaler with a big shipping
warehouse.
Our backoffice runs in perl. The warehouse in/out is perl/mariadb.
There are dozens of retailers and dropshippers connected to our db, some pushing,
some csv/xml-generators, OpenTrans, shopify, even Lexware-odbc based stuff.
All of it individual code in perl. And not even hip modern perl, but rather
oldschool stuff, only few cpan modules etc
that makes me indespensable. There are simply no perl coders in the wider area. It
wouldnt even make sense to train somebody in perl AND my codebase thats been growing
since 2003 because that will take years.
And thats why im staying with perl.
TLDR: using underdog coding language will make my living and pay my bills till
retirement.
If I may, I'd like to challenge you asking why would you like to promote Perl
and Perl-based projects?
I think in order for this to be successful you need to formulate an answer to
that question.
I know why would it be great for me, but:
What do you gain from Perl being more popular? What do others gain from it? Who
has interest in it? Who has the energy and time to invest in this effort? Who might
have money to
financially support Perl-related promotional or development efforts?
I would advice you not to use prototypes. They have their uses, but not for most cases
and definitely not in this one.
Why might this be true (or otherwise)? I almost always supply prototypes for my Perl
functions, and I've never before seen anyone else say anything bad about using them.
perl function
prototype shareimprove this question follow edited May 23 '17 at 11:54
Community ♦ 1 1 1 silver badge asked Nov 17 '08 at 21:37 Alnitak 303k 67 67 gold badges 368 368
silver badges 458 458 bronze badges
tchrist ,
I'm curious too. The only time I don't use them is when I'm calling with a variable number of
arguments. – Paul Tomblin Nov 17 '08 at 21:56
Michael Carman , 2008-11-17 22:54:09
121
Prototypes aren't bad if used correctly. The difficulty is that Perl's prototypes don't
work the way people often expect them to. People with a background in other programming
languages tend to expect prototypes to provide a mechanism for checking that function calls
are correct: that is, that they have the right number and type of arguments. Perl's
prototypes are not well-suited for this task. It's the misuse that's bad. Perl's
prototypes have a singular and very different purpose:
Prototypes allow you to define functions that behave like built-in functions.
Parentheses are optional.
Context is imposed on the arguments.
For example, you could define a function like this:
sub mypush(\@@) { ... }
and call it as
mypush @array, 1, 2, 3;
without needing to write the \ to take a reference to the array.
In a nutshell, prototypes let you create your own syntactic sugar. For example the Moose
framework uses them to emulate a more typical OO syntax.
This is very useful but prototypes are very limited:
They have to be visible at compile-time.
They can be bypassed.
Propagating context to arguments can cause unexpected behavior.
They can make it difficult to call functions using anything other than the strictly
prescribed form.
I've accepted this answer because I feel it best answers the question - prototypes aren't
intrinsically bad, it's just how you use them. – Alnitak Nov 18 '08 at 19:53
cjm , 2008-11-17 21:55:27
69
The problem is that Perl's function prototypes don't do what people think they do. Their
purpose is to allow you to write functions that will be parsed like Perl's built-in
functions.
First of all, method calls completely ignore prototypes. If you're doing OO programming,
it doesn't matter what prototype your methods have. (So they shouldn't have any
prototype.)
Second, prototypes aren't strictly enforced. If you call a subroutine with
&function(...) , the prototype is ignored. So they don't really provide any
type safety.
Third, they're spooky action-at-a-distance. (Especially the $ prototype,
which causes the corresponding parameter to be evaluated in scalar context, instead of the
default list context.)
In particular, they make it hard to pass parameters from arrays. For example:
my @array = qw(a b c);
foo(@array);
foo(@array[0..1]);
foo($array[0], $array[1], $array[2]);
sub foo ($;$$) { print "@_\n" }
foo(@array);
foo(@array[0..1]);
foo($array[0], $array[1], $array[2]);
prints:
a b c
a b
a b c
3
b
a b c
along with 3 warnings about main::foo() called too early to check prototype
(if warnings are enabled). The problem is that an array (or array slice) evaluated in scalar
context returns the length of the array.
If you need to write a function that acts like a built-in, use a prototype. Otherwise,
don't use prototypes.
Note: Perl 6 will have completely revamped and very useful prototypes. This answer applies
only to Perl 5. share improve
this answer follow edited Nov 20 '14 at 15:11 Flimm 85.8k
28 28 gold badges 185 185 silver badges 189 189 bronze badges answered Nov 17 '08 at 21:55
cjm 58.8k 8 8 gold
badges 117 117 silver badges 164 164 bronze badges
Flimm ,
But they still provide a useful check that your caller and the sub are using the same number
of arguments, so what's wrong with that? – Paul Tomblin Nov 17 '08 at 21:57
Leon Timmermans ,
30
I agree with the above two posters. In general, using $ should be avoided.
Prototypes are only useful when using block arguments ( & ), globs (
* ), or reference prototypes ( \@ , \$ ,
\% , \* ) share improve this answer follow answered Nov 18 '08
at 17:26 Leon
Timmermans 28.9k 2 2 gold badges 57 57 silver badges 109 109 bronze badges
The Sidhekin ,
In general, perhaps, but I'd like to mention two exceptions: First, the ($)
prototype creates a named unary operator, which can be useful (certainly Perl finds them
useful; I have too, on occasion). Second, when overriding built-ins (whether through import
or using CORE::GLOBAL::), you should in general stick to whatever prototype the built-in had,
even if that includes a $ , or you might surprise the programmer (yourself,
even) with list context where the built-in would otherwise provide scalar context. –
The Sidhekin
Sep 9 '15 at 21:47
> ,
4
Some people, looking at a Perl subroutine prototype, thinks it means something that it
doesn't:
sub some_sub ($$) { ... }
To Perl, that means that the parser expects two arguments. It's Perl's way of letting you
create subroutines that behave like built-ins, all of which know what to expect from the
succeeding code. You can read about prototypes in perlsub
Without reading the documentation, people guess that the prototypes refer to run time
argument checking or something similar that they've seen in other languages. As with most
things people guess about Perl, they turn out to be wrong.
However, starting with Perl v5.20, Perl has a feature, experimental as I write this, that
gives something more like what users expect and what. Perl's subroutine
signatures does run time argument counting, variable assigning, and default setting:
use v5.20;
use feature qw(signatures);
no warnings qw(experimental::signatures);
animals( 'Buster', 'Nikki', 'Godzilla' );
sub animals ($cat, $dog, $lizard = 'Default reptile') {
say "The cat is $cat";
say "The dog is $dog";
say "The lizard is $lizard";
}
This is the feature you probably want if you're considering prototypes.
Perl lets you define your own functions to be called like Perl's built-in functions.
Consider push(@array, $item) , which must tacitly receive a reference to
@array , not just the list values held in @array , so that the array can be
modified. Prototypes let you declare subroutines to take arguments just like many of the
built-ins, that is, with certain constraints on the number and types of arguments. We call them
"prototypes", but they work more like automatic templates for the calling context than like
what C or Java programmers would think of as prototypes. With these templates, Perl will
automatically add implicit backslashes, or calls to scalar , or whatever else it takes
to get things to show up in a way that matches the template. For instance, if you declare:
sub mypush (\@@);
then mypush takes arguments exactly like push does. For this to
work, the declaration of the function to be called must be visible at compile time. The
prototype only affects the interpretation of function calls when the & character
is omitted. In other words, if you call it like a built-in function, it behaves like a built-in
function. If you call it like an old-fashioned subroutine, then it behaves like an
old-fashioned subroutine. The & suppresses prototype checks and associated
contextual effects.
Since prototypes are taken into consideration only at compile time, it naturally falls out
that they have no influence on subroutine references like \&foo or on indirect
subroutine calls like &{$subref} or $subref->() . Method calls are not
influenced by prototypes, either. That's because the actual function to be called is
indeterminate at compile time, depending as it does on inheritance, which is dynamically
determined in Perl.
Since the intent is primarily to let you define subroutines that work like built-in
functions, here are some prototypes you might use to emulate the corresponding built-ins:
Declared as
Called as
sub mylink ($$)
mylink $old, $new
sub myreverse (@)
myreverse $a,$b,$c
sub myjoin ($@)
myjoin ":",$a,$b,$c
sub mypop (\@)
mypop @array
sub mysplice (\@$$@)
mysplice @array,@array,0,@pushme
sub mykeys (\%)
mykeys %{$hashref}
sub mypipe (**)
mypipe READHANDLE, WRITEHANDLE
sub myindex ($$;$)
myindex &getstring, "substr"
myindex &getstring, "substr", $start
sub mysyswrite (*$;$$)
mysyswrite OUTF, $buf
mysyswrite OUTF, $buf, length($buf)-$off, $off
sub myopen (*;$@)
myopen HANDLE
myopen HANDLE, $name
myopen HANDLE, "-|", @cmd
sub mygrep (&@)
mygrep { /foo/ } $a,$b,$c
sub myrand ($)
myrand 42
sub mytime ()
mytime
Any backslashed prototype character (shown between parentheses in the left column above)
represents an actual argument (exemplified in the right column), which absolutely must start
with that character. Just as the first argument to keys must start with % ,
so too must the first argument to mykeys .
A semicolon separates mandatory arguments from optional arguments. (It would be redundant
before @ or % , since lists can be null.) Unbackslashed prototype characters
have special meanings. Any unbackslashed @ or % eats all the rest of the
actual arguments and forces list context. (It's equivalent to LIST in a syntax
description.) An argument represented by $ has scalar context forced on it. An
& requires a reference to a named or anonymous subroutine.
A * allows the subroutine to accept anything in that slot that would be accepted by
a built-in as a filehandle: a bare name, a constant, a scalar expression, a typeglob, or a
reference to a typeglob. The value will be available to the subroutine either as a simple
scalar or (in the latter two cases) as a reference to the typeglob. If you wish to always
convert such arguments to a typeglob reference, use Symbol::qualify_to_ref as
follows:
use Symbol 'qualify_to_ref';
sub foo (*) {
my $fh = qualify_to_ref(shift, caller);
...
}
Note how the last three examples in the table are treated specially by the parser.
mygrep is parsed as a true list operator, myrand is parsed as a true unary
operator with unary precedence the same as rand , and mytime is truly
argumentless, just like time .
That is, if you say:
mytime +2;
you'll get mytime() + 2 , not mytime(2) , which is how it would
be parsed without the prototype, or with a unary prototype.
The mygrep example also illustrates how & is treated specially when it
is the first argument. Ordinarily, an & prototype would demand an argument like
\&foo or sub{} . When it is the first argument, however, you can leave
off the sub of your anonymous subroutine, and just pass a bare block in the "indirect
object" slot (with no comma after it). So one nifty thing about the & prototype is
that you can generate new syntax with it, provided the & is in the initial
position:
sub try (&$) {
my ($try, $catch) = @_;
eval { &$try };
if ($@) {
local $_ = $@;
&$catch;
}
}
sub catch (&) { $_[0] }
try {
die "phooey";
} # not the end of the function call!
catch {
/phooey/ and print "unphooey\n";
};
This prints " unphooey ". What happens is that try is called
with two arguments, the anonymous function {die "phooey";} and the return value of the
catch function, which in this case is nothing but its own argument, the entire block
of yet another anonymous function. Within try , the first function argument is called
while protected within an eval block to trap anything that blows up. If something does
blow up, the second function is called with a local version of the global $_ variable
set to the raised exception. [3] If this all sounds
like pure gobbledygook, you'll have to read about die and eval in Chapter 29,
"Functions" , and then go check out anonymous functions and closures in Chapter 8, "References" . On
the other hand, if it intrigues you, you might check out the Error module on CPAN,
which uses this to implement elaborately structured exception handling with try ,
catch , except , otherwise , and finally clauses.
[3] Yes, there are still unresolved issues having to do with the visibility of @_
. We're ignoring that question for the moment. But if we make @_ lexically scoped
someday, as already occurs in the experimental threaded versions of Perl, those anonymous
subroutines can act like closures.
Here's a reimplementation of the grep operator (the built-in one is more efficient,
of course):
sub mygrep (&@) {
my $coderef = shift;
my @result;
foreach $_ (@_) {
push(@result, $_) if &$coderef;
}
return @result;
}
Some folks would prefer to see full alphanumeric prototypes. Alphanumerics have
been intentionally left out of prototypes for the express purpose of someday adding named,
formal parameters. (Maybe.) The current mechanism's main goal is to let module writers enforce
a certain amount of compile-time checking on module users. 6.4.1. Inlining Constant
Functions
Functions prototyped with () , meaning that they take no arguments at all, are
parsed like the time built-in. More interestingly, the compiler treats such functions
as potential candidates for inlining. If the result of that function, after Perl's optimization
and constant-folding pass, is either a constant or a lexically scoped scalar with no other
references, then that value will be used in place of calls to that function. Calls made using
&NAME are never inlined, however, just as they are not subject to any
other prototype effects. (See the use constant pragma in Chapter 31, "Pragmatic Modules" ,
for an easy way to declare such constants.)
Both version of these functions to compute will be inlined by the compiler:
sub pi () { 3.14159 } # Not exact, but close
sub PI () { 4 * atan2(1, 1) } # As good as it gets
In fact, all of the following functions are inlined because Perl can determine
everything at compile time:
sub FLAG_FOO () { 1 << 8 }
sub FLAG_BAR () { 1 << 9 }
sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
sub OPT_GLARCH () { (0x1B58 & FLAG_MASK) == 0 }
sub GLARCH_VAL () {
if (OPT_GLARCH) { return 23 }
else { return 42 }
}
sub N () { int(GLARCH_VAL) / 3 }
BEGIN { # compiler runs this block at compile time
my $prod = 1; # persistent, private variable
for (1 .. N) { $prod *= $_ }
sub NFACT () { $prod }
}
In the last example, the NFACT function is inlined because it has a void
prototype and the variable it returns is not changed by that function--and furthermore can't be
changed by anyone else, since it's in a lexical scope. So the compiler replaces uses of
NFACT with that value, which was precomputed at compile time because of the
surrounding BEGIN .
If you redefine a subroutine that was eligible for inlining, you'll get a mandatory warning.
(You can use this warning to tell whether the compiler inlined a particular subroutine.) The
warning is considered severe enough not to be optional, because previously compiled invocations
of the function will still use the old value of the function. If you need to redefine the
subroutine, ensure that it isn't inlined either by dropping the () prototype (which
changes calling semantics, so beware) or by thwarting the inlining mechanism in some other way,
such as:
sub not_inlined () {
return 23 if $$;
}
See Chapter 18, "Compiling" for more
about what happens during the compilation and execution phases of your program's life.
6.4.2. Care with Prototypes
It's probably best to put prototypes on new functions, not retrofit prototypes onto older
ones. These are context templates, not ANSI C prototypes, so you must be especially careful
about silently imposing a different context. Suppose, for example, you decide that a function
should take just one parameter, like this:
sub func ($) {
my $n = shift;
print "you gave me $n\n";
}
That makes it a unary operator (like the rand built-in) and changes how
the compiler determines the function's arguments. With the new prototype, the function consumes
just one, scalar-context argument instead of many arguments in list context. If someone has
been calling it with an array or list expression, even if that array or list contained just a
single element, where before it worked, now you've got something completely different:
func @foo; # counts @foo elements
func split /:/; # counts number of fields returned
func "a", "b", "c"; # passes "a" only, discards "b" and "c"
func("a", "b", "c"); # suddenly, a compiler error!
You've just supplied an implicit scalar in front of the argument list,
which can be more than a bit surprising. The old @foo that used to hold one thing
doesn't get passed in. Instead, 1 (the number of elements in @foo ) is now passed to
func . And the split , being called in scalar context, scribbles all over
your @_ parameter list. In the third example, because func has been
prototyped as a unary operator, only " a " is passed in; then the return value from
func is discarded as the comma operator goes on to evaluate the next two items and
return " c ." In the final example, the user now gets a syntax error at compile time
on code that used to compile and run just fine.
If you're writing new code and would like a unary operator that takes only a scalar
variable, not any old scalar expression, you could prototype it to take a scalar
reference :
sub func (\$) {
my $nref = shift;
print "you gave me $$nref\n";
}
Now the compiler won't let anything by that doesn't start with a dollar sign:
func @foo; # compiler error, saw @, want $
func split/:/; # compiler error, saw function, want $
func $s; # this one is ok -- got real $ symbol
func $a[3]; # and this one
func $h{stuff}[-1]; # or even this
func 2+5; # scalar expr still a compiler error
func ${ \(2+5) }; # ok, but is the cure worse than the disease?
If you aren't careful, you can get yourself into trouble with prototypes. But if
you are careful, you can do a lot of neat things with them. This is all very powerful, of
course, and should only be used in moderation to make the world a better place.
Subroutine signatures, in a rudimentary form, have shown up in Perl v5.20 as an experimental
feature. After a few of years of debate and a couple of competing implementations, we have
something we can use. And, because it was such a contentious subject, it got the attention a
new feature deserves. They don't have all the features we want (notably type and value
constraints), but Perl is in a good position to add those later.
Perl's roots were in simplicity and getting started as quickly as possible. We wanted to
define subroutines without much work. Instead of creating signatures in a C header file and
worrying about inputs and outputs, Larry made subroutines take in lists and return lists. Done
and done.
This simplicity means you have to do quite a bit of work yourself. You have to process the
input list, in @_ , assign your own default values, and declare the variables to
possibly store them. You ended up not saving much for the typical programmer.
To start, you need to enable the experimental feature (see Item 2. Enable new Perl features
when you need them. and
turn off the experimental warnings ):
1
usev5.20;
2
usefeature qw(signatures);
3
nowarnings qw(experimental::signatures);
To define a signature, you use the spot after the subroutine name where so far Perl has only
allowed a prototype. This is not a prototype though; it's something different. Notably, the new
subroutine signatures work with methods, whereas prototypes are compiler hints that method
calls ignore:
01
usev5.10;
02
03
subsome_sub ($$) {
04
say"I got $_[0] and $_[1]";
05
}
06
07
some_sub('Buster','Mimi');
08
09
main->some_sub();# "works" just fine
10
11
some_sub( qw(Buster) );# compile time error
But, you have a limited set of characters you can put in a prototype, and those exclude
identifier characters (those we use to make names). If you've enabled this experimental feature
and Perl see un-prototype like characters, it tries signatures instead. Note that Perl has
another feature like this: the diamond operator, <> , which might actually
be the glob operator if Perl sees glob characters in the argument. You can still
use a prototype with signatures, but you probably shouldn't use prototypes. The perlsub documentation shows you how
you can use an attribute to make a prototype.
First, be aware that using a signature does not mess with the normal argument list in
@_ . You can still play with that yourself.
The simplest prototype
The simplest signature is like the simplest prototype. Like prototypes, the signature
enforces the number of arguments. To make a constant in Perl you can use a subroutine that
takes no arguments. This is essentially what the constant pragma does:
1
usev5.20;
2
usefeature qw(signatures);
3
nowarnings qw(experimental::signatures);
4
5
subcat () {'Buster'}
6
7
say"Cat is ", cat;
If you try to pass an argument, you'll get an error but at runtime:
1
usev5.20;
2
usefeature qw(signatures);
3
nowarnings qw(experimental::signatures);
4
5
subcat () {'Buster'}
6
say"Running...";
7
say"Cat is ", cat('Mini');
The first say works, but the second fails when it calls cat
incorrectly:
1
Running...
2
Too many arguments for subroutine at cat.pl line 7.
A prototype would have raised a compile-time error because the compiler already knows how
many arguments there should be. This doesn't mean that the experimental signatures might do
that later, but the implementation now is more of a code mangler. Deparsing it (
Use B::Deparse to see what perl thinks the code is ) shows that the cat
subroutine has a die triggered by a check of @_ :
01
$ perl5.20.0 -MO=Deparse cat.pl
02
sub BEGIN {
03
require v5.20;
04
}
05
sub cat {
06
BEGIN {${^WARNING_BITS} =
"\020\001\000\000\000P\004\000\000\000\000\000\000U\005"}
Don't get too hung up on that because it might be a temporary implementation detail. This
does mean, however, that you can catch this error with eval :
1
use v5.20;
2
use feature qw(signatures);
3
no warnings qw(experimental::signatures);
4
5
sub cat () { 'Buster' }
6
say "Running...";
7
say "Cat is ", eval { cat( 'Mini') };
8
9
say "Caught error: $@" if $@;
Now we catch the error, but notice it comes from the line of the subroutine definition, not
the point where you called the subroutine like you would with a croak :
1
Running...
2
Cat is
3
Caught error: Too many arguments for subroutine at cat.pl line 5.
Mandatory, positional parameters
The meat of this feature is your ability to assign to variables in what many perceive as a
prettier way. Instead of declaring variables, usually with my , and performing
list operations on @_ , you list the variables in the signature in the order you
would assign from @_ :
1
use v5.20;
2
use feature qw(signatures);
3
no warnings qw(experimental::signatures);
4
5
cat( 'Buster' );
6
7
sub cat ($cat) {
8
say "The cat is $cat";
9
}
Again, this checks the number of parameters. With no arguments or more than one argument you
get a runtime error.
You separate multiple arguments with commas, just like a list assignment:
01
use v5.20;
02
use feature qw(signatures);
03
no warnings qw(experimental::signatures);
04
05
animals( 'Buster', 'Nikki', 'Godzilla' );
06
07
sub animals ($cat, $dog, $lizard) {
08
say "The cat is $cat";
09
say "The dog is $dog";
10
say "The lizard is $lizard";
11
}
These variables are lexical variables in the scope of the subroutine (as you'd see if you
deparsed this code). That means you can't assign to special variables, which would cause an
compile-time error. That is, except for $_ , which is experimentally lexical from
a v5.10 misadventure with given-when (
Perl v5.16 now sets proper magic on lexical $_ and Use for() instead
of given() ).
Placeholder parameters
You don't have to name all of the parameters. You can use the lone $ to not
immediately assign a value, probably because you'll process it yourself through @_
. In this example we don't care about the second argument, but the signature still needs the
right number of positions and in the right sequence:
01
use v5.20;
02
use feature qw(signatures);
03
no warnings qw(experimental::signatures);
04
05
animals( 'Buster', 'Nikki', 'Godzilla' );
06
07
sub animals ($cat, $, $lizard) { # unnamed second parameter
08
say "The cat is $cat";
09
say "The lizard is $lizard";
10
}
This is a bit tricky really. That second argument is mandatory even though you've
neglected to give it a name. It will still be in @_ even though you haven't
assigned it to a variable.
Slurpy parameters
At the end of the parameter list, you can have a slurpy parameter, which is either a
named array or hash. You can do this is a list assignment too, but a list assignment lets you
put it in the middle despite the fact that any succeeding elements get nothing:
1
my($cat,$dog,@animals,$lizard)# $lizard will never get anything
2
= qw( Buster Nikki Ginger Mimi Godzilla );
In the subroutine signature, that slurpy thing has to be at the end of the list:
1
subanimals ($cat,$dog,@animals) {# @animals
must be at the end
2
...;
3
}
The rest of the arguments past the second show up in @animals . But, here's the
interesting thing; the number of things that can show up in the array can be zero.
Sometimes you may not want to completely specify the number of arguments that your
subroutine may take, but you also don't want to create a named array, you can use a bare
@ as placeholder to mean that the argument list is unlimited:
1
subanimals ($cat,$dog, @ ) {# @ slurps the rest
2
...;
3
}
The hash can also be a slurpy parameter, and just like the slurpy array it must be at the
end of the signature:
1
subanimals ($cat,$dog,%args) {# %args
demands an even number
2
...;
3
}
For the hash, if there isn't an even number of elements left in @_ , you'll get
a runtime exception. You don't have to name the hash, and a bare % still demands
an even number of elements:
1
subanimals ($cat,$dog, % ) {# % still demands an even number
2
...;
3
}
Default values (optional parameters)
Perhaps the best feature of signatures are default values. You can use Perl to decide what
the default values, even if that is a literal. However, you can only assign default values to
optional parameters, which means that they have to appear after the mandatory arguments. In
this example, the third argument is optional and gets the default value 'MechaGodzilla'
when no argument is present:
01
usev5.20;
02
usefeature qw(signatures);
03
nowarnings qw(experimental::signatures);
04
05
say"First try-----------";
06
animals('Mimi','Nikki','Godzilla');
07
08
say"Second try-----------";
09
animals('Buster','Nikki', );
10
11
subanimals ($cat,$dog,$lizard='MechaGodzilla') {
12
say"The cat is $cat";
13
say"The dog is $dog";
14
say"The lizard is $lizard";
15
}
On the second try, you get the default value:
1
First try-----------
2
The cat is Mimi
3
The dog is Nikki
4
The lizard is Godzilla
5
Second try-----------
6
The cat is Buster
7
The dog is Nikki
8
The lizard is MechaGodzilla
This is only checking the number of arguments and assigning a value when the argument list
is too short. If you pass undef as an argument, that's the (un)value that
parameter will get:
The undef does not trigger a default value, which may surprise many of you.
Notice the third, "undef try" where $lizard gets no value:
01
First try-----------
02
The cat is Mimi
03
The dog is Nikki
04
The lizard is Godzilla
05
The bird is Poppy
06
Second try-----------
07
The cat is Buster
08
The dog is Nikki
09
The lizard is MechaGodzilla
10
The bird is Poppy
11
undef try-----------
12
The cat is Buster
13
The dog is Nikki
14
The lizard is
15
The bird is Poppy
You can also have a null default value. You might want to have one optional parameter but
not assign a value if the argument list isn't long enough. This signature allows one or two
arguments, with no defaults:
01
usev5.20;
02
usefeature qw(signatures);
03
nowarnings qw(experimental::signatures);
04
05
say"First try-----------";
06
animals('Mimi','Nikki');
07
08
say"Second try-----------";
09
animals('Buster');
10
11
subanimals ($cat, $= ) {# second argument is optional
12
say"The cat is $cat";
13
say"Second argument is $_[1]"if$#_ == 1;
14
}
You see that the second argument only exists if you specify it yourself:
1
First try-----------
2
The cat is Mimi
3
Second argument is Nikki
4
Second try-----------
5
The cat is Buster
These default values don't work with slurpy types, though. Perl will complain at
compile-time. If you want that sort of thing, though, you can make the argument a scalar and
assign an array or hash reference to it:
01
usev5.20;
02
usefeature qw(signatures);
03
nowarnings qw(experimental::signatures);
04
05
animals('Buster');
06
07
subanimals ($cat,$hash={} ) {# hashref argument is optional
08
say"The cat is $cat";
09
...;
10
}
Fancy optional values
So far your defaults have been simple values, but you can use Perl. That means you can do
almost anything. This one uses the value in another variable and increments it as it assigns
defaults:
01
usev5.20;
02
usefeature qw(signatures);
03
nowarnings qw(experimental::signatures);
04
05
animals('Mimi');
06
animals('Buster');
07
08
{
09
my$auto_id= 0;
10
subanimals ($cat,$id= ++$auto_id) {
11
say"$id: The cat is $cat";
12
}
13
}
Each cat automatically gets its own sequence value since the animals subroutine
closed over $auto_id :
1
1: The cat is Mimi
2
2: The cat is Buster
However, you can't do something tricky to bring $auto_id into the subroutine
since the parser doesn't know about the variable soon enough. Neither of these work:
You can make the default value a call to a subroutine:
01
usev5.20;
02
usefeature qw(signatures);
03
nowarnings qw(experimental::signatures);
04
05
animals('Mimi');
06
animals('Buster');
07
08
subanimals ($cat,$auto_id= get_id() ) {
09
say"$auto_id: The cat is $cat";
10
}
11
12
subget_id () {
13
state$id= 0;
14
++$id;
15
}
Method signatures
My favorite part of signatures is that they work with methods. This doesn't mean that we
have multi-dispatch in Perl (yet) (well, Perl 6 does but that's a different language). The
signatures aren't any different; they follow all the same rules:
01
usev5.20;
02
03
packageLocal::Animals {
04
usefeature qw(signatures);
05
nowarnings qw(experimental::signatures);
06
07
subnew ($class,$args= {'name'=>'Buster'} ) {
08
bless$args,$class
09
}
10
11
subname ($self) {
12
$self->{name};
13
}
14
}
15
16
my$cat= Local::Animals->new;
17
say"Cat is ",$cat->name;
A quick guide
Exactly zero arguments - empty signature
()
Exactly one named argument
($cat)
Zero or one arguments, with no default, unnamed
($=)
Zero or one arguments, with no default, named
($cat=)
Zero or one arguments, with a default
($cat='Buster')
Exactly two arguments,
($cat, $dog)
Two or more arguments, any number
($cat, $dog, @)
Two or more arguments, but an even number
($cat, $dog, %)
Two or more arguments, slurping rest into an array
($cat, $dog, @animals)
Two or more arguments, slurping rest into an hash
($cat, $dog, %animals)
Two, three, or four arguments, no defaults
($cat, $dog, $=, $=)
Two, three, or four arguments, one default
($cat, $dog, $lizard='Godzilla', $=)
Class method
( $class, ... )
Object method
( $self, ... )
Things to Remember
Subroutine signatures are experimental
Signatures are not prototypes
Signatures enforce the number of arguments, but at runtime
Default values only apply to optional parameters, and only apply to parameters past the
number of actual passed arguments
As far as I know, there's not a way to check the signature without resorting to
black magic or XS, like you could with prototype() for the
old-style prototypes.
Although there is a new Perl feature to alias different names to the same data, I
did not expect signatures to alias data. I don't see how that would work, either. I
think there would be more confusion that way.
Also the single-line constraint can become a problem with code readability (if one wants
to comment the parameters) and can be easily broken by perltidy unfortunately.
The subroutine signature feature has several more things waiting for implementation
and I think they want to figure that stuff out before they make it stable.
What is the future or roadmap of signatures? As of now 5.26 is out and the perldeltas
mention speed improvements. It sounds like signatures are here to stay, but is it still
experimental? How heavily can we rely on it when using perl 5.22 for example.
I haven't kept up with the state of signatures. They are still experimental because
there are a few more features they'd like to squeeze into them. That might change the
current syntax and they don't want to handcuff themselves to that. I think they're
pretty close to how they'll end up though. At least I hope so because I use them almost
everywhere!
is %HASH{answer}, 'forty-two', '%HASH properly filled';
A: I had the answer right, but I messed up the sigil on HASH . It should
be:
is $HASH{answer}, 'forty-two', '%HASH properly filled';
# ^ $, not %
Unfortunately, on Perl v5.20+, both statements work the same way! I didn't catch the problem
until I shipped this code and cpantesters showed me my mistake.
It was an easy fix, but it reminded me that Perl's variant
sigils can trip up programmers at any level. If I could change one thing about Perl 5, I
would change to invariant sigils.
The current situation
In Perl, the sigil tells you how many things to
expect . Scalars such as $foo are single values. Any single value in an array
@foo or hash %foo , since it is only one thing, also uses
$ , so $foo , @foo , and %foo could all
refer to different pieces of the same variable -- or to different variables. This technique of
"variant sigils" works, but confuses new Perl users and tripped up yours truly. To know what
you are accessing in an array or hash, you have to look at both the sigil and the brackets. As
a reminder:
Sigil
No brackets
[ ] (array access)
{ } (hash access)
$
$z : a scalar, i.e., a single value
$z[0] : the first element of array @z
$z{0} : the value in hash %z at key "0"
@
@z : An array, i.e., a list of value(s)
@z[0, 1] : the list ($z[0], $z[1]) of two elements from
@z (an "array slice")
@z{0, "foo"} : the list ($z{0}, $z{foo}) of two elements
from hash %z
%
%z : A hash, i.e., a list of key/value pair(s)
%z[0, 1] : the list (0, $z[0], 1, $z[1]) of keys and two
values from array @z (a "hash slice")
%z{0, "foo"} : the list ("0", $z{0}, "foo", $z{foo}) of
keys and values from hash %z
Make the sigils part of the name
To save myself from repeating my errors, I'd like the sigil to be part of a variable's name.
This is not a new idea; scalars work this way in Perl, bash, and Raku ( formerly
Perl 6 ). That would make the above table look like:
Sigil
No brackets
[ ] (array access)
{ } (hash access)
$
$z : a scalar, i.e., a single value
$z[0] : N/A
$z{0} : N/A
@
@z : An array, i.e., a list of value(s)
@z[0] : the first element of @z
@z{0} : N/A
%
%z : A hash, i.e., a list of key/value pair(s)
%z[0] : N/A
%z{0} : the value in hash %z at key 0
Simpler! Any reference to @z would always be doing something with the
array named @z .
But what about slices?
Slices such as @z[0,1] and %z{qw(hello there)} return multiple
values from an array or hash. If sigils @ and % are no longer
available for slicing, we need an alternative. The Perl family currently provides two models:
postfix dereferencing ("postderef") syntax and postfix adverbs.
Perl v5.20+ support postderef ,
which gives us one option. Postderef separates the name from the slice:
# Valid Perl v5.20+
$hashref->{a}; # Scalar, element at index "a" of the hash pointed to by $hashref
$hashref->@{a}; # List including the "a" element of the hash pointed to by $hashref
$hashref->%{a}; # List including the key "a" and the "a" element of the hash pointed to by $hashref
The type of slice comes after the reference, instead of as a sigil before the reference.
With non-references, that idea would give us slice syntax such as @array@[1,2,3]
or %hash%{a} .
Raku gives us another option: "adverbs" such as :kv . For example:
# Valid Raku
%hash{"a"} # Single value, element at index "a" of %hash
%hash{"a"}:v; # The same --- just the value
%hash{"a"}:kv; # The list including key "a" and the value of the "a" element of %hash
The adverb (e.g., :kv ) goes in postfix position, immediately after the
brackets or braces. Following this model, slices would look like @array[1,2,3]:l
or %hash{a}:kv . (For clarity, I propose :l , as in l ist, instead of
Raku's :v . Raku's :v can return a scalar or a list.)
So, the choices I see are (postderef-inspired / Raku-inspired):
What you want
No subscript
[ ] access
{ } access
Scalar
$z : a scalar, i.e., a single value
@z[0] : a single value from an array
%z{0} : the value in hash %z at key "0"
List of values
@z : an array, i.e., a list of value(s)
@z@[0, 1] / @z[0, 1]:l : the list currently written
($z[0], $z[1])
%z@{0, "foo"} / %z{0, "foo"}:l : the list currently written
($z{0}, $z{foo})
List of key/value pairs
%z : a hash, i.e., a list of key/value pair(s)
@z%[0, 1] / @z[0, 1]:kv : the list currently written
(0, $z[0], 1, $z[1])
%z%{0, "foo"} / %z{0, "foo"}:kv : the list currently
written ("0", $z{0}, "foo", $z{foo})
You can't always get what you want
I prefer the adverb syntax. It is easy to read, and it draws on all the expertise that has
gone into the design of Raku. However, my preference has to be implementable. I'm not convinced
that it is without major surgery.
The Perl parser decides how to interpret what is inside the brackets depending on the
context provided by the slice. The parser interprets the ... in
@foo[...] as a list (
ref ). In $foo[...] , the parser sees the ... as a scalar
expression (
ref ). For any slice syntax, the Perl parser needs to know the desired type of result while
parsing the subscript expression. The adverb form, unfortunately, leaves the parser guessing
until after the subscript is parsed.
You can, in fact, hack the Perl parser to save the subscript until it sees a postfix adverb.
The parser can then apply the correct context. I wrote a
proof-of-concept for @arr[expr]:v . It doesn't execute any code, but it does
parse a postfix-adverb slice without crashing! However, while writing that code, I ran across a
surprise: new syntax isn't tied to a use v5.xx directive.
It turns out the Perl parser lets code written against any Perl version use the latest
syntax. Both of the following command lines work on Perl v5.30:
$ perl -Mstrict -Mwarnings -E 'my $z; $z->@* = 10..20'
# ^ -E: use all the latest features
$ perl -Mstrict -Mwarnings -e 'my $z; $z->@* = 10..20' # (!!!)
# ^ -e: not the latest features
The second command line does not use v5.30 , so you can't use say
(introduced in v5.10). However, you can use postderef (from v5.20)!
Because the parser lets old programs use new syntax, any proposed addition to Perl's syntax
has to be meaningless in all previous Perl versions. A postfix adverb fails this test. For
example, the following is a valid Perl program:
sub kv { "kv" }
my @arr = 10..20;
print 1 ? @arr[1,2]:kv;
# ^^^^^^^^^^^^ valid Perl 5 syntax, but not a slice :(
print "\n";
My preferred slice syntax could change the meaning of existing programs, so it looks like I
can't get my first choice.
Next Steps
This is not the end of the story! In Part 2, I will dig deeper into Perl's parser and
tokenizer. I will share some surprises I discovered while investigating postderef. I will then
describe a possible path to invariant sigils and the simplicity they can provide.
Given an array listing rulers in the Kingdom of Jerusalem like this one:
@kings = ('Baldwin', 'Melisende', 'Fulk', 'Amalric', 'Guy', 'Conrad') . How can we create one
that is built from the 2nd, the 4th and then the 1st element?
One solution is:
@names = ($kings[2], $kings[4], $kings[1])
The other, the simpler solution is to use array slices:
@names = @kings[2,4,1]
In this case we use the @ prefix of the array and provide several indexes. If you are
familiar with arrays in Perl ,
you surely remember that when we talk about the whole array we put @ in front of the name, but
when we talk about a single element of an array we replace
the @ sigil by the $ sigil and put square brackets at the end.
When we want create a list of one or more of the elements of the array we use the @ sigil
again, as it represents "plural" and then we put one or more indexes in the square brackets
after the name of the array.
Scalar value @kings[2] better written as $kings[2] at array_slice.pl line 14 (#1)
(W syntax) You've used an array slice (indicated by @) to select a
single element of an array. Generally it's better to ask for a scalar
value (indicated by $). The difference is that $foo[&bar] always
behaves like a scalar, both when assigning to it and when evaluating its
argument, while @foo[&bar] behaves like a list when you assign to it,
and provides a list context to its subscript, which can do weird things
if you're expecting only one subscript.
On the other hand, if you were actually hoping to treat the array element as a list, you
need to look into how references work, because Perl will not magically convert between scalars
and lists for you. See perlref.
If you would like to create a new array using a single element of another array then you
should probably write:
my @s = $kings[2];
or if you want to make sure readers of your code won't be surprised by the assignment of a
scalar to an array, then you can even put parentheses around the value.
my @s = ($kings[2]);
Slice of an array reference
If we have out data in an ARRAY reference and not in an array, the code will be a bit more
complex:
In this case we have a variable called $kings which is a reference to an array.
In the plain version, when we use individual elements we just need to dereference the ARRAY
reference for each individual element.
my @names = ($kings->[2], $kings->[4], $kings->[1]);
If we would like to use the array slice syntax then first we need to dereference the whole
array putting the @ sigil in-front of the reference: @$kings , but then we can simply put the
square brackets behind that construct: my @slice = @$kings[2,4,1]; though I think I prefer the
version when we put curly braces around the reference, thereby making it clear that it is a
single unit of expression:
Beg, borrow, steal, buy, fabricate or otherwise obtain a rubber duck (bathtub
variety).
Place rubber duck on desk and inform it you are just going to go over some code with it,
if that's all right.
Explain to the duck what your code is supposed to do, and then go into detail and explain
your code line by line.
At some point you will tell the duck what you are doing next and then realise that that
is not in fact what you are actually doing. The duck will sit there serenely, happy in the
knowledge that it has helped you on your way.
Note : In a pinch a coworker might be able to substitute for the duck, however, it is often
preferred to confide mistakes to the duck instead of your coworker.
Original Credit : ~Andy from lists.ethernal.org
FAQs
If ducks are so smart, why don't we just let the ducks do all the work? It would be
wonderful if this were true, but the fact is that most ducks prefer to take a mentoring
role. There are a few ducks however that do choose to code, but these are the ducks that
nobody hears about because they are selected for secret government projects that are highly
classified in nature.
Where can I hire my own duck? Great question!
Amazon.com hosts a wide selection of affordable ducks that have graduated with a
technical degree from some of the world's leading universities.
Why does this site exist? As a young intern in 2008 I repeatedly pestered a mentor of
mine similar to Kevin's Rubber Duck Story and eventually
my mentor pointed me at the 2002 lists.ethernal.org post
by Andy , which paraphrased a story from the 1999 book The Pragmatic Programmer .
That night I ordered a rubber duck from Amazon and purchased this domain name as a way of
owning up to my behavior.
Match "http://stackoverflow.com/"
Group 1: "http"
Group 2: "stackoverflow.com"
Group 3: "/"
Match "https://stackoverflow.com/questions/tagged/regex"
Group 1: "https"
Group 2: "stackoverflow.com"
Group 3: "/questions/tagged/regex"
But I don't care about the protocol -- I just want the host and path of the URL. So, I
change the regex to include the non-capturing group (?:) .
(?:https?|ftp)://([^/\r\n]+)(/[^\r\n]*)?
Now, my result looks like this:
Match "http://stackoverflow.com/"
Group 1: "stackoverflow.com"
Group 2: "/"
Match "https://stackoverflow.com/questions/tagged/regex"
Group 1: "stackoverflow.com"
Group 2: "/questions/tagged/regex"
See? The first group has not been captured. The parser uses it to match the text, but
ignores it later, in the final result.
EDIT:
As requested, let me try to explain groups too.
Well, groups serve many purposes. They can help you to extract exact information from a
bigger match (which can also be named), they let you rematch a previous matched group, and
can be used for substitutions. Let's try some examples, shall we?
Imagine you have some kind of XML or HTML (be aware that
regex may not be the best tool for the job , but it is nice as an example). You want to
parse the tags, so you could do something like this (I have added spaces to make it easier to
understand):
\<(?<TAG>.+?)\> [^<]*? \</\k<TAG>\>
or
\<(.+?)\> [^<]*? \</\1\>
The first regex has a named group (TAG), while the second one uses a common group. Both
regexes do the same thing: they use the value from the first group (the name of the tag) to
match the closing tag. The difference is that the first one uses the name to match the value,
and the second one uses the group index (which starts at 1).
Let's try some substitutions now. Consider the following text:
Lorem ipsum dolor sit amet consectetuer feugiat fames malesuada pretium egestas.
Now, let's use this dumb regex over it:
\b(\S)(\S)(\S)(\S*)\b
This regex matches words with at least 3 characters, and uses groups to separate the first
three letters. The result is this:
Match "Lorem"
Group 1: "L"
Group 2: "o"
Group 3: "r"
Group 4: "em"
Match "ipsum"
Group 1: "i"
Group 2: "p"
Group 3: "s"
Group 4: "um"
...
Match "consectetuer"
Group 1: "c"
Group 2: "o"
Group 3: "n"
Group 4: "sectetuer"
...
So, if we apply the substitution string:
$1_$3$2_$4
... over it, we are trying to use the first group, add an underscore, use the third group,
then the second group, add another underscore, and then the fourth group. The resulting
string would be like the one below.
You can use named groups for substitutions too, using ${name} .
To play around with regexes, I recommend http://regex101.com/ , which offers a good amount of details on
how the regex works; it also offers a few regex engines to choose from.
You can use capturing groups to organize and parse an expression. A non-capturing group
has the first benefit, but doesn't have the overhead of the second. You can still say a
non-capturing group is optional, for example.
Say you want to match numeric text, but some numbers could be written as 1st, 2nd, 3rd,
4th,... If you want to capture the numeric part, but not the (optional) suffix you can use a
non-capturing group.
([0-9]+)(?:st|nd|rd|th)?
That will match numbers in the form 1, 2, 3... or in the form 1st, 2nd, 3rd,... but it
will only capture the numeric part.
I work quit a bit with lib ReadLine and the lib Perl Readline.
Yet, the Perl debugger refuses to save the session command line history.
Thus, each time I invoke the debugger I lose all of my previous history.
Does anyone know how to have the Perl debugger save, and hopefully, append session history
similar to the bash HISTORYFILE ?
eli ,
just for anyone else looking for that: for readline to work in perl (ctrl-p / ctrl-n / ...)
one has to apt-get install libterm-readline-gnu-perl (at least in debian)
– eli Jun 7 '18
at 14:13
ysth ,
The way I do this is by having the following line in my ~/.perldb file:
Debugger commands are then stored in ~/.perldb.hist and accessible across
sessions.
ysth ,
@bitbucket: I've had enough problems in the past getting the built-in readline support
working that I don't even bother anymore :) – ysth Jun 22 '11 at 16:59
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them
with the elements of LIST, if any. In list context, returns the elements removed from the
array. In scalar context, returns the last element removed, or undef if no elements are
removed. The array grows or shrinks as necessary. If OFFSET is negative then it starts that
far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward.
If LENGTH is negative, removes the elements from OFFSET onward except for -LENGTH elements
at the end of the array. If both OFFSET and LENGTH are omitted, removes everything. If
OFFSET is past the end of the array and a LENGTH was provided, Perl issues a warning, and
splices at the end of the array.
The following equivalences hold (assuming $#a >= $i )
Starting with Perl 5.14, an experimental feature allowed splice
to take a scalar expression. This experiment has been deemed unsuccessful, and was removed
as of Perl 5.24.
The important point to remember is the distinction between () and []. '()' gives you a
list of elements, for eg. (1, 2, 3) which you could then assign to an array variable as so
-
my @listOfElem = (1, 2, 3);
'[]' is an array reference and returns a
scalar value which you could incorporate into your list.
my $refToElem = ['a', 'b', 'c'];
In your case, if you are initializing the first array then you could simply insert the
second array elements like so,
my @listOfElem = (1, 2, ['a', 'b', 'c'], 3);
#This gives you a list of "4" elements with the third
#one being an array reference
my @listOfElem = (1, 2, $refToELem, 3);
#Same as above, here we insert a reference scalar variable
my @secondListOfElem = ('a', 'b', 'c');
my @listOfElem = (1, 2, \@secondListOfElem, 3);
#Same as above, instead of using a scalar, we insert a reference
#to an existing array which, presumably, is what you want to do.
#To access the array within the array you would write -
$listOfElem[2]->[0] #Returns 'a'
@{listOfElem[2]}[0] #Same as above.
If you have to add the array elements on the fly in the middle of the array then just use
'splice' as detailed in the other posts.
This is the sort of thing you'll understand after going through the first part of
Intermediate Perl ,
which covers references and data structures. You can also look in the Perl data structures cookbook .
In short, you store an array in another array by using a reference (which is just a
scalar):
my @big_array = ( $foo, $bar, \@other_array, $baz );
In your case, you used the anonymous array constructor and just want to splice it into an
existing array. There's nothing special about it being an array reference:
splice @big_array, $offset, $length, @new_items;
In your case, you wanted to start at element 1, remove 0 items, and add your
reference:
#!/usr/bin/perl
use strict;
use warnings;
my @array = ("element 1","element 2","element 3");
my $arrayref = ["this will", "go between", "element 1 and 2"];
splice( @array, 1, 0, $arrayref ); # Grow the array with the list (which is $arrayref)
for ( my $i = 0; $i <= $#array; $i++ ) {
print "\@array[$i] = $array[$i]\n";
}
It looks like Perl split function treats single quotes literal semantically inconsistently
with other constructs
But not always :-). For example
($line)=split(' ',$line,1)
is treated consistently (in AWK way). This is the only way I know to avoid using regex for a
very common task of trimming the leading blanks.
In general, split function should behave differently if the first argument is string and not
a regex. But right now single quoted literal is treated as regular expression. For example:
$line="head xxx tail";
say split('x+',$line);
will print
head tail
Am I missing something? BTW this would be similar to Python distinguishing between
split and re.split but in a more elegant, Perlish way. And a big help for
sysadmins.
As another special case, split emulates the default behavior of
the command line tool awk when the PATTERN is either omitted or a string
composed of a single space character (such as ' ' or "\x20" , but not
e.g. / / ). In this case, any leading whitespace in EXPR is removed
before splitting occurs, and the PATTERN is instead treated as if it were
/\s+/ ; in particular, this means that any contiguous whitespace (not just a
single space character) is used as a separator.
You also write:
Regular expressions are also treated a bit differently than regular expressions in
qr//, m// and s///.
I don't understand this statement. Can you elaborate? Give a man a fish
:<%-{-{-{-<
I can assert that conextually, splitting on all characters for split //, $string
is a lot more meaningful than splitting on nothing and returning just the original
$string . The big surprise actually happens for users (like me) who don't realize
the first parameter of split is a regular expression. But that surprise quickly turns into
joy .
> In general, split function should behave differently if the first argument is
string and not a regex.
Should ? That's pretty presumptuous. You'll notice that Perl has FAR few built in
functions (particularly string functions) than PHP, JavaScript, or Python. This is because
they've all been generalized away into regular expressions. You must also understand that the
primary design philosphy is more related to spoken linquistics than written code. The
implication here is that humans are lazy and don't want to learn more words than they need to
communicate - not true of all humans, of course. But true enough for 99% of them. This is
also reflected in the Huffmanization of most Perl syntax. This refers to Huffman
compression, which necessarily compresses more frequently used things (characters,
words, etc) into the symbols of the smallest size. I mean Perl isn't APL, but certainly gets
this idea from it.
The balkanization of built-in functions that are truly special cases of a general
case is against any philosophical underpinnings that Perl follows. I am not saying it's
perfect, but it is highly resistent to becoming a tower of babble. If that's your interest
(not accusing you of being malicious), there are more fruitful avenues to attack Perl. Most
notably, the areas of object orientation and threading. But you'll have pretty much zero
success convincing anyone who has been around Perl for a while that the approach to
split is
incorrect .
Oh, also a string (as you're calling it) is a regular expression in the
purest sense of the term . It's best described as a
concatenation of a finite set of symbols in fixed ordering. For some reason a lot of people
think this regex magic is only present in patterns that may have no beginning or no end, or
neither. In your case it just happens to have both. Doesn't make it any less of a regular
expression, though.
by you !!! on Aug 14,
2020 at 19:29 UTC Reputation: 5
The balkanization of built-in functions that are truly special cases of a general case is
against any philosophical underpinnings that Perl follows. I am not saying it's perfect,
but it is highly resistant to becoming a tower of babble. If that's your interest (not
accusing you of being malicious), there are more fruitful avenues to attack Perl
I respectfully disagree. Perl philosophy states that there should be shortcuts for
special cases if they are used often. That's the idea behind suffix conditionals (
return if (index($line,'EOL')>-1) ) and bash-style if statement ( ($debug)
&& say line; )
You also are missing the idea. My suggestion is that we can enhance the power of Perl by
treating single quoted string differently from regex in split. And do this without adding
to balkanization.
Balkanization of built-ins is generally what Python got having two different functions.
Perl can avoid this providing the same functionality with a single function. That's the
idea.
And my point is that this particular change requires minimal work in interpreter as it
already treats ' ' in a special way (AWK way).
So this is a suggestion for improving the language, not for balkanization, IMHO. And
intuitively it is logical as people understand (and expect) the difference in behavior
between single quoted literals and regex in split. So, in a way, the current situation can
be viewed as a bug, which became a feature.
To be fair, this is a lot of perl . But I can't rightfully assert that this
behavior was unintentional, in fact it appears to be very intentional (e.g., awk
emulation).
> You also are missing the idea.
My understanding is that you wish for "strings" (versus "regexes") to invoke the
awk behavior of trimming leading white space. Is that right? I'm not here to judge
your suggestion, but I can easily think of several reasons why adding another special case
to split is not
a great idea.
All I can say is you're the same guy who was looking for the trim method in Perl. If that's not a red flag
for being okay with balkanization , I don't know what is.
Finally, I must reiterate. A "string" is a regular expression . The single
quoted whitespace is most definitely a special exception since it is also a regular
expression. You're recommending not only removing one regex from the pool of potential
regexes, but an entire class of them available via quoting - i.e., fixed length strings of
a fixed ordering. I am not sure how this is really a suggestion of making all quoted
things not be regexes, because then how do you decide if it is "regex" or not?
(maybe use a regex? xD)
Comment on Why split
function treats single quotes literals as regex, instead of a special case?
Is there any way to trim both leading and trailing blanks in a text line (one of the most
common operations in text processing; often implemented as trim function which BTW was
present in Perl 6) without resorting to regular expressions (which are definitely an overkill
for this particular purpose)? This is clearly an important special case.
So far the most common solution is to use something like $line =~ s/^\s+|\s+$//g
which clearly is an abuse of regex.
See, for example, https://perlmaven.com/trim
Or install String::Util which is a not a standard module and as such creates
difficulties in enterprise env.
without resorting to regular expressions (which are definitely an overkill for this
particular purpose)?
Sure, just write your own function to do it. Having written that you will then come to the
conclusion that regular expressions are definitely not an overkill for this particular
purpose.
This is clearly an important special case. ... which clearly is an abuse of regex.
by you !!! on Aug 14,
2020 at 19:39 UTC Reputation: 6
So if you want the exact same semantic, it'll become far more complicated than this
regex.
I agree. That's a good point. Thank you !
In other words it is not easy to design a good trim function without regex, but it is
possible to design one that used regex, but treating the single quoted string as a special
case
For example
trim(' ',$line)
vs
trim(/\s/.$line)
BTW this is impossible in Python which implements regex via library, unless you add a new
lexical type to the Language (regex string instead of raw string that is used).
I will usually reach for one of Perl's string handling functions (e.g. index , rindex , substr , and so on) in preference to a
regex when that is appropriate; however, in this case, I would say that the regex makes for
much cleaner code.
You could implement a trim() function using the guts of this code (which uses
neither a regex nor any modules, standard or otherwise):
$ perl -E ' my @x = (" a b c
", "d e f ", " g h i", "j k l", " ", ""); say "*** Initial strings ***"; say "|$_|" for @x;
for my $i (0 .. $#x) { my $str = $x[$i]; while (0 == index $str, " ") { $str = substr $str,
1; } my $str_end = length($str) - 1; while ($str_end == rindex $str, " ") { $str = substr
$str, 0, $str_end; --$str_end; } $x[$i] = $str; } say "*** Final strings ***"; say "|$_|" for
@x; ' *** Initial strings *** | a b c | |d e f | | g h i| |j k l| | | || *** Final strings
*** |a b c| |d e f| |g h i| |j k l| || ||[download]
If your question was genuinely serious, please Benchmark a trim() function using
something like I've provided against another trim() function using a regex. You
could obviously do the same for ltrim() and rtrim() functions.
[As others have either asked or alluded to, please explain phrases such as "definitely
an overkill", "important special case" and "abuse of regex". Unfortunately, use of such
language makes your post come across as some sort of trollish rant -- I'm not saying that was
your intent, just how it presents itself.]
That's a valid point. My main intent with that code was really to show the complexity of
the solution when a regex or module were not used. Anyway, adding a little more complexity,
you can trim whatever blanks you want:
$ perl -E ' my @blanks = (" ", "\n",
"\r", "\t"); my @x = ( " a b c ", "d e f \r ", " \t g h i", "j k l", " ", "\n",
"\n\nXYZ\n\n", "" ); say "*** Initial strings ***"; say "|$_|" for @x; for my $i (0 .. $#x)
{ my $str = $x[$i]; while (grep { 0 == index $str, $_ } @blanks) { $str = substr $str, 1; }
my $str_end = length($str) - 1; while (grep { $str_end == rindex $str, $_ } @blanks) { $str
= substr $str, 0, $str_end; --$str_end; } $x[$i] = $str; } say "*** Final strings ***"; say
"|$_|" for @x; ' *** Initial strings *** | a b c | | e f | g h i| |j k l| | | | | | XYZ |
|| *** Final strings *** |a b c| |d e f| |g h i| |j k l| || || |XYZ| ||[download]
You're quite correct about "The OP should be clearer ..." . The word 'blank' is
often used to mean various things: a single space, multiple consecutive spaces, a whitepace
character, multiple consecutive whitepace characters, and I have also seen it used to refer
to a zero-length string. Similarly, the word 'space' can mean a single space, any gap
between visible characters, and so on. So, as with many posts, we're left with guessing the
most likely meaning from the context.
My belief, that a regex is a better option, strengthens as the complexity of the
non-regex and non-module code increases. :-)
s/^\s+|\s+$//g has been benchmarked. And I now think this is faster and "better"
than 2 statements. There is one post at Re^3: script optmization that shows some
benchmarks.
This is certainly not an "abuse" of regex. This is what regex is is for! The Perl regex
engine continually becomes better and usually faster between releases.
$rocks[0] = 'bedrock';
$rocks[1] = 'slate';
$rocks[2]= 'lava';
$rocks[3] = 'crushed rock';
$rocks[99] = 'schist';
$#rocks = 2; # forget all rocks after 'lava'
$#rocks = 99; # add 97 undef elements (the forgotten rocks are gone forever)
So I'm following the book Learning Perl, there's this code with
comments in it: If I do print $rocks[$#rocks]; it prints nothing. Why? When I comment
out $#rocks = 99; it prints 'lava' and when I comment out $#rocks = 2; it
prints 'schist'.
But when I maintain the two it prints nothing as I already said.
And what the comments "add 97 undef elements (the forgotten rocks are gone forever)" and
"forget all rocks after 'lava'" mean?
I look this up about once a month. Too bad I can't upvote it each time. – kyle Oct 29 '14 at 19:31
Ether , 2011-01-04 20:33:47
This is available in String::Util with the trim method:
Editor's note: String::Util is not a core module, but you can install it
from CPAN with [sudo] cpan
String::Util .
use String::Util 'trim';
my $str = " hello ";
$str = trim($str);
print "string is now: '$str'\n";
prints:
string is now 'hello'
However it is easy enough to do yourself:
$str =~ s/^\s+//;
$str =~ s/\s+$//;
Marki555 ,
@mklement0 nor will it ever be. But this is not relevant, since everyone should be using
modules from the CPAN. – Ether Jun 9 '15 at 21:12
> ,
UncleCarl ,
@Ether With all due respect, I really appreciate knowing that this is a non-core module. This
post is talking about using a module in lieu of a fairly simple regex one-liner. If the
module is core, I would be much more open to it. It is relevant in this case. –
UncleCarl Mar 1
'18 at 16:57
> ,
There's no built-in trim function, but you can easily implement your own using a
simple substitution:
sub trim {
(my $s = $_[0]) =~ s/^\s+|\s+$//g;
return $s;
}
Summary: This page is a printf formatting cheat sheet. I originally created this
cheat sheet for my own purposes, and then thought I would share it here.
A great thing about the printf formatting syntax is that the format specifiers
you can use are very similar -- if not identical -- between different languages, including C,
C++, Java, Perl, PHP, Ruby, Scala, and others. This means that your printf
knowledge is reusable, which is a good thing.
In this cheat sheet I'll show all the examples using Perl, but at first it might help to see
one example using both Perl and Java. Therefore, here's a simple Perl printf
example to get things started:
printf("the %s jumped over the %s, %d times", "cow", "moon", 2);
And here are three different Java printf examples, using different string
formatting methods that are available to you in the Java programming language:
System.out.format("the %s jumped over the %s, %d times", "cow", "moon", 2);
System.err.format("the %s jumped over the %s, %d times", "cow", "moon", 2);
String result = String.format("the %s jumped over the %s, %d times", "cow", "moon", 2);
As you can see in that last String.format example, that line of code doesn't
print any output, while the first line prints to standard output, and the second line prints to
standard error.
In the remainder of this document I'll use Perl examples, but again, the actual format
specifier strings can be used in many different languages.
As a summary of printf integer formatting, here's a little collection of
integer formatting examples. Several different options are shown, including a minimum width
specification, left-justified, zero-filled, and also a plus sign for positive numbers.
Description
Code
Result
At least five wide
printf("'%5d'", 10);
' 10'
At least five-wide, left-justified
printf("'%-5d'", 10);
'10 '
At least five-wide, zero-filled
printf("'%05d'", 10);
'00010'
At least five-wide, with a plus sign
printf("'%+5d'", 10);
' +10'
Five-wide, plus sign, left-justified
printf("'%-+5d'", 10);
'+10 '
Back to topformatting floating point numbers with printf
Here are several examples showing how to format floating-point numbers with
printf :
Description
Code
Result
Print one position after the decimal
printf("'%.1f'", 10.3456);
'10.3'
Two positions after the decimal
printf("'%.2f'", 10.3456);
'10.35'
Eight-wide, two positions after the decimal
printf("'%8.2f'", 10.3456);
' 10.35'
Eight-wide, four positions after the decimal
printf("'%8.4f'", 10.3456);
' 10.3456'
Eight-wide, two positions after the decimal, zero-filled
printf("'%08.2f'", 10.3456);
'00010.35'
Eight-wide, two positions after the decimal, left-justified
printf("'%-8.2f'", 10.3456);
'10.35 '
Printing a much larger number with that same format
The following character sequences have a special meaning when used as printf
format specifiers:
\a
audible alert
\b
backspace
\f
form feed
\n
newline, or linefeed
\r
carriage return
\t
tab
\v
vertical tab
\\
backslash
As you can see from that last example, because the backslash character itself is treated
specially, you have to print two backslash characters in a row to get one backslash character
to appear in your output.
Here are a few examples of how to use these special characters:
Submitted by Anonymoose (not verified) on November 5, 2009 - 10:36am
PermalinkI have to commend you, you've
I have to commend you, you've created a very easy to read manual on the basics of the printf
function. That's a feat in and of itself. Bookmarked!
Permalinkprintf formatting problems
fixed Sorry for the long delay, but hopefully I've finally fixed the formatting
problems with this article. If you see any errors please let me know, and I'll try to get them
corrected.
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
The import routine there should do exactly what you want -- just don't import any symbols
into your own namespace.
Ville M ,
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
The import routine there should do exactly what you want -- just don't import any symbols
into your own namespace.
Ville M ,
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
The import routine there should do exactly what you want -- just don't import any symbols
into your own namespace.
Ville M ,
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
The import routine there should do exactly what you want -- just don't import any symbols
into your own namespace.
Ville M ,
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
The import routine there should do exactly what you want -- just don't import any symbols
into your own namespace.
Ville M ,
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
I would like to expose all subs into my namespace without having to list them one at a time:
@EXPORT = qw( firstsub secondsub third sub etc );
Using fully qualified names would require bunch of change to existing code so I'd rather
not do that.
Is there @EXPORT_ALL?
I think documentation says it's a bad idea, but I'd like to do it anyway, or at least know
how.
To answer Jon's why: right now for quick refactoring I want to move of bunch of subs into
their own package with least hassle and code changes to the existing scripts (where those
subs are currenty used and often repeated).
Also, mostly, I was just curious. (since it seemed like that Exporter might as well have
that as standard feature, but somewhat surprisingly based on answers so far it doesn't)
brian d foy , 2009-04-08 23:58:35
Don't do any exporting at all, and don't declare a package name in your library. Just load
the file with require and everything will be in the current package. Easy peasy.
Michael Carman , 2009-04-09 00:15:10
Don't. But if you really want to... write a custom import that walks the symbol
table and export all the named subroutines.
# Export all subs in package. Not for use in production code!
sub import {
no strict 'refs';
my $caller = caller;
while (my ($name, $symbol) = each %{__PACKAGE__ . '::'}) {
next if $name eq 'BEGIN'; # don't export BEGIN blocks
next if $name eq 'import'; # don't export this sub
next unless *{$symbol}{CODE}; # export subs only
my $imported = $caller . '::' . $name;
*{ $imported } = \*{ $symbol };
}
}
Chas. Owens ,
Warning, the code following is as bad an idea as exporting everything:
package Expo;
use base "Exporter";
seek DATA, 0, 0; #move DATA back to package
#read this file looking for sub names
our @EXPORT = map { /^sub\s+([^({\s]+)/ ? $1 : () } <DATA>;
my $sub = sub {}; #make sure anon funcs aren't grabbed
sub foo($) {
print shift, "\n";
}
sub bar ($) {
print shift, "\n";
}
sub baz{
print shift,"\n";
}
sub quux {
print shift,"\n";
}
1;
__DATA__
Here is the some code that uses the module:
#!/usr/bin/perl
use strict;
use warnings;
use Expo;
print map { "[$_]\n" } @Expo::EXPORT;
foo("foo");
bar("bar");
baz("baz");
quux("quux");
And here is its output:
[foo]
[bar]
[baz]
[quux]
foo
bar
baz
quux
Jon Ericson , 2009-04-08 22:33:36
You can always call subroutines in there fully-specified form:
MyModule::firstsub();
For modules I write internally, I find this convention works fairly well. It's a bit more
typing, but tends to be better documentation.
Take a look at perldoc perlmod for more information about what you are trying
to accomplish.
More generally, you could look at Exporter 's code and see how it uses glob
aliasing. Or you can examine your module's namespace and export each subroutine. (I don't
care to search for how to do that at the moment, but Perl makes this fairly easy.) Or you
could just stick your subroutines in the main package:
package main;
sub firstsub() { ... }
(I don't think that's a good idea, but you know better than I do what you are trying to
accomplish.)
There's nothing wrong with doing this provided you know what you are doing and aren't just
trying to avoid thinking about your interface to the outside world.
ysth , 2009-04-09 01:29:04
Perhaps you would be interested in one of the Export* modules on CPAN that lets you mark subs
as exportable simply by adding an attribute to the sub definition? (Don't remember which one
it was, though.)
Although it is not usually wise to dump all sub s from module into the caller
namespace, it is sometimes useful (and more DRY!) to automatically generate
@EXPORT_OK and %EXPORT_TAGS variables.
The easiest method is to extend the Exporter. A simple example is something like this:
package Exporter::AutoOkay;
#
# Automatically add all subroutines from caller package into the
# @EXPORT_OK array. In the package use like Exporter, f.ex.:
#
# use parent 'Exporter::AutoOkay';
#
use warnings;
use strict;
no strict 'refs';
require Exporter;
sub import {
my $package = $_[0].'::';
# Get the list of exportable items
my @export_ok = (@{$package.'EXPORT_OK'});
# Automatically add all subroutines from package into the list
foreach (keys %{$package}) {
next unless defined &{$package.$_};
push @export_ok, $_;
}
# Set variable ready for Exporter
@{$package.'EXPORT_OK'} = @export_ok;
# Let Exporter do the rest
goto &Exporter::import;
}
1;
Note the use of goto that removes us from the caller stack.
A more complete example can be found here: http://pastebin.com/Z1QWzcpZ It automatically generates
tag groups from subroutine prefixes.
Sérgio , 2013-11-14 21:38:06
case 1
Library is :
package mycommon;
use strict;
use warnings;
sub onefunctionthatyoumadeonlibary() {
}
1;
you can use it, calling common:: :
#!/usr/bin/perl
use strict;
use warnings;
use mycommon;
common::onefunctionthatyoumadeonlibary()
case 2
Library is , yousimple export them :
package mycommon;
use strict;
use warnings;
use base 'Exporter';
our @EXPORT = qw(onefunctionthatyoumadeonlibary);
sub onefunctionthatyoumadeonlibary() {
}
1;
use it in same "namespace":
#!/usr/bin/perl
use strict;
use warnings;
use mycommon qw(onefunctionthatyoumadeonlibary);
onefunctionthatyoumadeonlibary()
Also we can do a mix of this two cases , we can export more common functions to use it
without calling the packages name and other functions that we only call it with package name
and that ones don't need to be exported.
> ,
You will have to do some typeglob munging. I describe something similar here:
I'm looking for advice on Perl best practices. I wrote a script which had a complicated
regular expression:
my $regex = qr/complicated/;
# ...
sub foo {
# ...
if (/$regex/)
# ...
}
where foo is a function which is called often, and $regex is not
used outside that function. What is the best way to handle situations like this? I only want
it to be interpreted once, since it's long and complicated. But it seems a bit questionable
to have it in global scope since it's only used in that sub. Is there a reasonable way to
declare it static?
A similar issue arises with another possibly-unjustified global. It reads in the current
date and time and formats it appropriately. This is also used many times, and again only in
one function. But in this case it's even more important that it not be re-initialized, since
I want all instances of the date-time to be the same from a given invocation of the script,
even if the minutes roll over during execution.
At the moment I have something like
my ($regex, $DT);
sub driver {
$regex = qr/complicated/;
$DT = dateTime();
# ...
}
# ...
driver();
which at least slightly segregates it. But perhaps there are better ways.
Again: I'm looking for the right way to do this, in terms of following best practices and
Perl idioms. Performance is nice but readability and other needs take priority if I can't
have everything.
hobbs ,
If you're using perl 5.10+, use a state variable.
use feature 'state';
# use 5.010; also works
sub womble {
state $foo = something_expensive();
return $foo ** 2;
}
will only call something_expensive once.
If you need to work with older perls, then use a lexical variable in an outer scope with
an extra pair of braces:
{
my $foo = something_expensive();
sub womble {
return $foo ** 2;
}
}
this keeps $foo from leaking to anyone except for womble .
ikegami , 2012-05-31 21:14:04
Is there any interpolation in the pattern? If not, the pattern will only be compiled once no
matter how many times the qr// is executed.
use feature qw( state );
sub foo {
state $re = qr/.../;
...
/$re/
...
}
Alan Rocker , 2014-07-02 16:25:27
Regexes can be specified with the "o" modifier, which says "compile pattern once only" - in
the 3rd. edition of the Camel, see p. 147
zoul ,
There's a state
keyword that might be a good fit for this situation:
sub foo {
state $regex = /.../;
...
}
TrueY , 2015-01-23 10:14:12
I would like to complete ikegami 's great answer. Some more words I would like
to waste on the definition of local variables in pre 5.10 perl .
Let's see a simple example code:
#!/bin/env perl
use strict;
use warnings;
{ # local
my $local = "After Crying";
sub show { print $local,"\n"; }
} # local
sub show2;
show;
show2;
exit;
{ # local
my $local = "Solaris";
sub show2 { print $local,"\n"; }
} # local
The user would expect that both sub will print the local variable, but this
is not true!
Output:
After Crying
Use of uninitialized value $local in print at ./x.pl line 20.
The reason is that show2 is parsed, but the initialization of the local
variable is not executed! (Of course if exit is removed and a show2
is added at the end, Solaris will be printed in the thirds line)
This can be fixed easily:
{ # local
my $local;
BEGIN { $local = "Solaris"; }
sub show2 { print $local,"\n"; }
} # local
This chapter introduces you to the concepts behind references to Perl modules, packages, and
classes. It also shows you how to create a few sample modules.
A Perl module is a set of Perl code that acts like a library of function calls. The term
module in Perl is synonymous with the word package . Packages are a feature of
Perl 4, whereas modules are prevalent in Perl 5.
You can keep all your reusable Perl code specific to a set of tasks in a Perl module.
Therefore, all the functionality pertaining to one type of task is contained in one file. It's
easier to build an application on these modular blocks. Hence, the word module applies a
bit more than package .
Here's a quick introduction to modules. Certain topics in this section will be covered in
detail throughout the rest of the book. Read the following paragraphs carefully to get an
overview of what lies ahead as you write and use your own modules.
What is confusing is that the terms module and package are used
interchangeably in all Perl documentation, and these two terms mean the very same thing
. So when reading Perl documents, just think "package" when you see "module" and vice
versa.
So, what's the premise for using modules? Well, modules are there to package (pardon the
pun) variables, symbols, and interconnected data items together. For example, using global
variables with very common names such as $k , $j , or $i in a
program is generally not a good idea. Also, a loop counter, $i , should be allowed to
work independently in two different portions of the code. Declaring $i as a global
variable and then incrementing it from within a subroutine will create unmanageable problems
with your application code because the subroutine may have been called from within a loop that
also uses a variable called $i . The use of modules in Perl allows variables with the
same name to be created at different, distinct places in the same program.
The symbols defined for your variables are stored in an associative array, referred to as a
symbol table . These symbol tables are unique to a package. Therefore, variables of the
same name in two different packages can have different values.
Each module has its own symbol table of all symbols that are declared within it. The symbol
table basically isolates synonymous names in one module from another. The symbol table defines
a namespace , that is, a space for independent variable names to exist in. Thus, the use
of modules, each with its own symbol table, prevents a variable declared in one section from
overwriting the values of other variables with the same name declared elsewhere in the same
program.
As a matter of fact, all variables in Perl belong to a package. The variables in a Perl
program belong to the main package. All other packages within a Perl program either
are nested within this main package or exist at the same level. There are some truly global
variables, such as the signal handler array %SIG , that are available to all other
modules in an application program and cannot be isolated via namespaces. Only those variable
identifiers starting with letters or an underscore are kept in a module's symbol table. All
other symbols, such as the names STDIN , STDOUT , STDERR ,
ARGV , ARGVOUT , ENV , Inc , and SIG are forced to
be in package _main.
Switching between packages affects only namespaces. All you are doing when you use one
package or another is declaring which symbol table to use as the default symbol table for
lookup of variable names. Only dynamic variables are affected by the use of symbol tables.
Variables declared by the use of the my keyword are still resolved with the code block
they happen to reside in and are not referenced through symbol tables. In fact, the scope of a
package declaration remains active only within the code block it is declared in. Therefore, if
you switch symbol tables by using a package within a subroutine, the original symbol table in
effect when the call was made will be restored when the subroutine returns.
Switching symbol tables affects only the default lookup of dynamic variable names. You can
still explicitly refer to variables, file handles, and so on in a specific package by
prepending a packageName :: to the variable name. You saw what a package
context was when using references in Chapter 3 . A package context
simply implies the use of the symbol table by the Perl interpreter for resolving variable names
in a program. By switching symbol tables, you are switching the package context.
Modules can be nested within other modules. The nested module can use the variables and
functions of the module it is nested within. For nested modules, you would have to use
moduleName :: nestedModuleName and so on. Using the double colon (
:: ) is synonymous with using a back quote ( ` ). However, the double colon
is the preferred, future way of addressing variables within modules.
Explicit addressing of module variables is always done with a complete reference. For
example, suppose you have a module, Investment , which is the default package in use,
and you want to address another module, Bonds , which is nested within the
Investment module. In this case, you cannot use Bond:: . Instead, you would
have to use Investment::Bond:: to address variables and functions within the
Bond module. Using Bond:: would imply the use of a package Bond that
is nested within the main module and not within the Investment module.
The symbol table for a module is actually stored in an associative array of the module's
names appended with two colons. The symbol table for a module called Bond will be
referred to as the associative array %Bond:: . The name for the symbol table for the
main module is %main:: , and can even be shortened to %:: .
Similarly, all nested packages have their symbols stored in associative arrays with double
colons separating each nesting level. For example, in the Bond module that is nested
within the Investment module, the associative array for the symbols in the
Bond module will be named %Investment::Bond:: .
A typeglob is really a global type for a symbol name. You can perform aliasing
operations by assigning to a typeglob . One or more entries in an associative array
for symbols will be used when an assignment via a typeglob is used. The actual value
in each entry of the associative array is what you are referring to when you use the *
variableName notation. Thus, there are two ways of referring to variable names in a
package:
*Investment::money = *Investment::bills;
$Investment::{'money'} = $Investment::{'bills'};
In the first method, you are referring to the variables via a typeglob reference.
The use of the symbol table, %Investment:: , is implied here, and Perl will optimize
the lookup for symbols money and bills . This is the faster and preferred way
of addressing a symbol. The second method uses a lookup for the value of a variable addressed
by 'money' and 'bills' in the associative array used for symbols,
%Investment:: explicitly. This lookup would be done dynamically and will not be
optimized by Perl. Therefore, the lookup will be forced to check the associative array every
time the statement is executed. As a result, the second method is not efficient and should be
used only for demonstration of how the symbol table is implemented internally.
Another example in this statement
*kamran = *husain;
causes variables, subroutines, and file handles that are named via the symbol
kamran to also be addressed via the symbol husain . That is, all symbol
entries in the current symbol table with the key kamran will now contain references to
those symbols addressed by the key husain . To prevent such a global assignment, you
can use explicit references. For example, the following statement will let you address the
contents of $husain via the variable $kamran :
*kamran = \$husain;
However, any arrays such @kamran and @husain will not be the same. Only
what the references specified explicitly will be changed. To summarize, when you assign one
typeglob to another, you affect all the entries in a symbol table regardless of the
type of variable being referred to. When you assign a reference from one variable type to
another, you are only affecting one entry in the symbol table.
The filename has to be called ModuleName.pm . The name of a module must end in the
string .pm by convention. The package statement is the first line of the
file. The last line of the file must contain the line with the 1; statement. This in
effect returns a true value to the application program using the module. Not using the
1; statement will not let the module be loaded correctly.
The package statement tells the Perl interpreter to start with a new namespace
domain. Basically, all your variables in a Perl script belong to a package called main
. Every variable in the main package can be referred to as $main'variable
.
Here's the syntax for such references:
$packageName'variableName
The single quote ( ' ) is synonymous with the double colon ( :: )
operator. I cover more uses of the :: operator in the next chapter. For the time
being, you must remember that the following two statements are equivalent:
The double-colon syntax is considered standard in the Perl world. Therefore, to preserve
readability, I use the double-colon syntax in the rest of this book unless it's absolutely
necessary to make exceptions to prove a point.
The default use of a variable name defers to the current package active at the time of
compilation. Thus, if you are in the package Finance.pm and specify a variable
$pv , the variable is actually equal to $Finance::$pv .
You include Perl modules in your program by using the use or the require
statement. Here's the way to use either of these statements:
use ModuleName; require ModuleName;
Note that the .pm extension is not used in the code shown above. Also note that
neither statement allows a file to be included more than once in a program. The returned value
of true ( 1; ) as the last statement is required to let Perl know that a
require d or use d module loaded correctly and lets the Perl interpreter
ignore any reloads. In general, it's better to use the use Module; statement than the
require Module; statement in a Perl program to remain compatible with future versions
of Perl.
For modules, you might want to consider continuing to use the require statement.
Here's why: The use statement does a little bit more work than the require
statement in that it alters the namespace of the module that includes another module. You want
this extra update of the namespace to be done in a program. However, when writing code for a
module, you may not want the namespace to be altered unless it's explicitly required. In this
event, you will use the require statement.
The require statement includes the full pathname of a file in the @Inc
array so that the functions and variables in the module's file are in a known location during
execution time. Therefore, the functions that are imported from a module are imported via an
explicit module reference at runtime with the require statement. The use
statement does the same thing as the require statement because it updates the
@Inc array with full pathnames of loaded modules. The code for the use
function also goes a step further and calls an import function in the module being
use d to explicitly load the list of exported functions at compile time, thus saving
the time required for an explicit resolution of a function name during execution.
Basically, the use statement is equivalent to
require ModuleName; import ModuleName [list of imported functions];
The use of the use statement does change your program's namespace because the
imported function names are inserted in the symbol table. The require statement does
not alter your program's namespace. Therefore, the following statement
use ModuleName ();
is equivalent to this statement:
require ModuleName;
Functions are imported from a module via a call to a function called import . You
can write your own import function in a module, or you can use the Exporter
module and use its import function. In almost all cases, you will use the
Exporter module to provide an import function instead of reinventing the
wheel. (You'll learn more on this in the next section.) Should you decide not to use the
Exporter module, you will have to write your own import function in each
module that you write. It's much easier to simply use the Exporter module and let Perl
do the work for you.
The best way to illustrate the semantics of how a module is used in Perl is to write a
simple module and show how to use it. Let's take the example of a local loan shark, Rudious
Maximus, who is simply tired of typing the same "request for payment" letters. Being an avid
fan of computers and Perl, Rudious takes the lazy programmer's approach and writes a Perl
module to help him generate his memos and letters.
Now, instead of typing within fields in a memo template file, all he has to do is type a few
lines to produce his nice, threatening note. Listing 4.1 shows you what he has to type.
Listing 4.1. Using theLettermodule.
1 #!/usr/bin/perl -w
2 #
3 # Uncomment the line below to include the current dir in @Inc.
4 # push (@Inc, 'pwd');
5 #
6 use Letter;
7
8 Letter::To("Mr. Gambling Man","The money for Lucky Dog, Race 2");
9 Letter::ClaimMoneyNice();
10 Letter::ThankDem();
11 Letter::Finish();
The use Letter; statement is present to force the Perl interpreter to include the
code for the module in the application program. The module should be located in the
/usr/lib/perl5/ directory, or you can place it in any directory listed in the
@Inc array. The @Inc array is the list of directories that the Perl
interpreter will look for when attempting to load the code for the named module. The commented
line (number 4) shows how to add the current working directory to include the path. The next
four lines in the file generate the subject matter for the letter.
Here's the output from using the Letter module:
To: Mr. Gambling Man
Fm: Rudious Maximus, Loan Shark
Dt: Wed Feb 7 10:35:51 CST 1996
It has come to my attention that your account is
way over due.
You gonna pay us soon?
Or would you like me to come ovah?
Thanks for your support.
Sincerely,
Rudious
The Letter module file is shown in Listing 4.2. The name of the package is declared
in the first line. Because this module's functions will be exported, I use the
Exporter module. Therefore, the statement use Exporter; is required to
inherit functionality from the Exporter module. Another required step is putting the
word Exported in the @ISA array to allow searching for Exported.pm
.
Note
The @ISA array is a special array within each package. Each item in the
array lists where else to look for a method if it cannot be found in the current
package. The order in which packages are listed in the @ISA array is the
order in which Perl searches for unresolved symbols. A class that is listed in the
@ISA array is referred to as the base class of that particular class. Perl
will cache missing methods found in base classes for future references. Modifying the
@ISA array will flush the cache and cause Perl to look up all methods again.
Let's now look at the code for Letter.pm in Listing 4.2.
Listing 4.2. TheLetter.pmmodule.
1 package Letter;
2
3 require Exporter;
4 @ISA = (Exporter);
5
6 =head1 NAME
7
8 Letter - Sample module to generate letterhead for you
9
10 =head1 SYNOPSIS
11
12 use Letter;
13
14 Letter::Date();
15 Letter::To($name,$company,$address);
16
17 Then one of the following:
18 Letter::ClaimMoneyNice() {
19 Letter::ClaimMoney();
20 Letter::ThreatBreakLeg();
21
22 Letter::ThankDem();
23 Letter::Finish();
24
25 =head1 DESCRIPTION
26
27 This module provides a short example of generating a letter for a
28 friendly neighborbood loan shark.
29
30 The code begins after the "cut" statement.
31 =cut
32
33 @EXPORT = qw( Date,
34 To,
35 ClaimMoney,
36 ClaimMoneyNice,
37 ThankDem,
38 Finish );
39
40 #
41 # Print today's date
42 #
43 sub Letter::Date {
44 $date = 'date';
45 print "\n Today is $date";
46 }
47
48 sub Letter::To {
49 local($name) = shift;
50 local($subject) = shift;
51 print "\n To: $name";
52 print "\n Fm: Rudious Maximus, Loan Shark";
53 print "\n Dt: ", `date`;
54 print "\n Re: $subject";
55 print "\n\n";
56 print "\n====================================================\n";
57 }
58 sub Letter::ClaimMoney() {
59 print "\n You owe me money. Get your act together";
60 print "\n Do you want me to send Bruno over to ";
61 print "\n collect it , or are you gonna pay up?";
62 }
63
64 sub Letter::ClaimMoneyNice() {
65 print "\n It is come to my attention that your account is ";
66 print "\n way over due.";
67 print "\n You gonna pay us soon..";
68 print "\n or would you like me to come ovah?";
69 }
70
71 sub Letter::ThreatBreakLeg() {
72 print "\n apparently letters like these dont help";
73 print "\n I will have to make an example of you";
74 print "\n \n See you in the hospital, pal!";
75 }
76
77 sub Letter::ThankDem() {
78 print "\n\n Thanks for your support";
79 }
80
81 sub Letter::Finish(){
82 printf "\n\n\n\n Sincerely";
83 printf "\n Rudious \n ";
84 }
85
86 1;
Lines containing the equal sign are used for documentation. You must document each module
for your own reference; Perl modules do not need to be documented, but it's a good idea to
write a few lines about what your code does. A few years from now, you may forget what a module
is about. Good documentation is always a must if you want to remember what you did in the
past!
I cover documentation styles used for Perl in Chapter 8 , "Documenting Perl
Scripts." For this sample module, the =head1 statement begins the documentation.
Everything up to the =cut statement is ignored by the Perl interpreter.
Next, the module lists all the functions exported by this module in the @EXPORT
array. The @EXPORT array defines all the function names that can be called by outside
code. If you do not list a function in this @EXPORT array, it won't be seen by
external code modules.
Following the @EXPORT array is the body of the code, one subroutine at a time.
After all the subroutines are defined, the final statement 1; ends the module file.
1; must be the last executable line in the file.
Let's look at some of the functions defined in this module. The first function to look at is
the simple Date function, lines 43 to 46, which prints the current UNIX date and time.
There are no parameters to this function, and it doesn't return anything meaningful back to the
caller.
Note the use of my before the $date variable in line 44. The my
keyword is used to limit the scope of the variable to within the Date function's curly
braces. Code between curly braces is referred to as a block . Variables declared within
a block are limited in scope to within the curly braces. In 49 and 50, the local variables
$name and $subject are visible to all functions.
You can also declare variables with the local qualifier. The use of local
allows a variable to be in scope for the current block as well as for other blocks of code
called from within this block. Thus, a local $x declared within one block is visible
to all subsequent blocks called from within this block and can be referenced. In the following
sample code, the ToTitled function's $name variable can be accessed but not
the data in $iphone :
The sample code for Letter.pm showed how to extract one parameter at a time. The
subroutine To() takes two parameters to set up the header for the memo.
Using functions within a module is not any different than using and defining Perl modules
within the same code file. Parameters are passed by reference unless otherwise specified.
Multiple arrays passed into a subroutine, if not explicitly dereferenced using the backslash,
are concatenated.
The @_ input array in a function is always an array of scalar values. Passing
values by reference is the preferred way in Perl to pass a large amount of data into a
subroutine. ( See Chapter 3 ,
"References.")
The Finance module, shown in Listing 4.3, is used to provide simple calculations
for loan values. Using the Finance module is straightforward. All the functions are
written with the same parameters, as shown in the formula for the functions.
Let's look at how the future value of an investment can be calculated. For example, if you
invest some dollars, $pv , in a bond that offers a fixed percentage rate, $r
, applied at known intervals for $n time periods, what is the value of the bond at the
time of its expiration? In this case, you'll be using the following formula:
$fv = $pv * (1+$r) ** $n ;
The function to get the future value is declared as FutureValue . Refer to Listing
4.3 to see how to use it.
Listing 4.3. Using theFinancemodule.
1 #!/usr/bin/perl -w
2
3 push(@Inc,'pwd');
4 use Finance;
5
6 $loan = 5000.00;
7 $apr = 3.5; # APR
8 $year = 10; # in years.
9
10 # ----------------------------------------------------------------
11 # Calculate the value at the end of the loan if interest
12 # is applied every year.
13 # ----------------------------------------------------------------
14 $time = $year;
15 $fv1 = Finance::FutureValue($loan,$apr,$time);
16 print "\n If interest is applied at end of year";
17 print "\n The future value for a loan of \$" . $loan . "\n";
18 print " at an APR of ", $apr , " for ", $time, " years";
19 printf " is %8.2f \n" , $fv1;
20
21 # ----------------------------------------------------------------
22 # Calculate the value at the end of the loan if interest
23 # is applied every month.
24 # ----------------------------------------------------------------
25 $rate = $apr / 12; # APR
26 $time = $year * 12; # in months
27 $fv2 = Finance::FutureValue($loan,$rate,$time);
28
29 print "\n If interest is applied at end of each month";
30 print "\n The future value for a loan of \$" . $loan . "\n";
31 print " at an APR of ", $apr , " for ", $time, " months";
32 printf " is %8.2f \n" , $fv2;
33
34 printf "\n The difference in value is %8.2f", $fv2 - $fv1;
35 printf "\n Therefore by applying interest at shorter time periods";
36 printf "\n we are actually getting more money in interest.\n";
Here is sample input and output of Listing 4.3.
$ testme
If interest is applied at end of year
The future value for a loan of $5000
at an APR of 3.5 for 10 years is 7052.99
If interest is applied at end of each month
The future value for a loan of $5000
at an APR of 3.5 for 120 months is 7091.72
The difference in value is 38.73
Therefore by applying interest at shorter time periods
we are actually getting more money in interest.
The revelation in the output is the result of the comparison of values between $fv1
and $fv2 . The $fv1 value is calculated with the application of interest once
every year over the life of the bond. $fv2 is the value if the interest is applied
every month at the equivalent monthly interest rate.
The Finance.pm package is shown in Listing 4.4 in its early development stages.
Listing 4.4. TheFinance.pmpackage.
1 package Finance;
2
3 require Exporter;
4 @ISA = (Exporter);
5
6 =head1 Finance.pm
7
8 Financial Calculator - Financial calculations made easy with Perl
9
10 =head 2
11 use Finance;
12
13 $pv = 10000.0;
14
15 $rate = 12.5 / 12; # APR per month.
16
17 $time = 360 ; # months for loan to mature
18
19 $fv = FutureValue();
20
21 print $fv;
22
23 =cut
24
25 @EXPORT = qw( FutureValue,
26 PresentValue,
27 FVofAnnuity,
28 AnnuityOfFV,
29 getLastAverage,
30 getMovingAverage,
31 SetInterest);
32
33 #
34 # Globals, if any
35 #
36
37 local $defaultInterest = 5.0;
38
39 sub Finance::SetInterest($) {
40 my $rate = shift(@_);
41 $defaultInterest = $rate;
42 printf "\n \$defaultInterest = $rate";
43 }
44
45 # --------------------------------------------------------------------
46 # Notes:
47 # 1. The interest rate $r is given in a value of [0-100].
48 # 2. The $n given in the terms is the rate at which the interest
49 # is applied.
50 #
51 # --------------------------------------------------------------------
52
53 # --------------------------------------------------------------------
54 # Present value of an investment given
55 # fv - a future value
56 # r - rate per period
57 # n - number of period
58 # --------------------------------------------------------------------
59 sub Finance::FutureValue($$$) {
60 my ($pv,$r,$n) = @_;
61 my $fv = $pv * ((1 + ($r/100)) ** $n);
62 return $fv;
63 }
64
65 # --------------------------------------------------------------------
66 # Present value of an investment given
67 # fv - a future value
68 # r - rate per period
69 # n - number of period
70 # --------------------------------------------------------------------
71 sub Finance::PresentValue($$$) {
72 my $pv;
73 my ($fv,$r,$n) = @_;
74 $pv = $fv / ((1 + ($r/100)) ** $n);
75 return $pv;
76
77 }
78
79 # --------------------------------------------------------------------
80 # Get the future value of an annuity given
81 # mp - Monthly Payment of Annuity
82 # r - rate per period
83 # n - number of period
84 # --------------------------------------------------------------------
85
86 sub FVofAnnuity($$$) {
87 my $fv;
88 my $oneR;
89 my ($mp,$r,$n) = @_;
90
91 $oneR = ( 1 + $r) ** $n;
92 $fv = $mp * ( ($oneR - 1)/ $r);
93 return $fv;
94 }
95
96 # --------------------------------------------------------------------
97 # Get the annuity from the following bits of information
98 # r - rate per period
99 # n - number of period
100 # fv - Future Value
101 # --------------------------------------------------------------------
102
103 sub AnnuityOfFV($$$) {
104 my $mp; # mp - Monthly Payment of Annuity
105 my $oneR;
106 my ($fv,$r,$n) = @_;
107
108 $oneR = ( 1 + $r) ** $n;
109 $mp = $fv * ( $r/ ($oneR - 1));
110 return $mp;
111 }
112
113 # --------------------------------------------------------------------
114 # Get the average of the last "n" values in an array.
115 # --------------------------------------------------------------------
116 # The last $count number of elements from the array in @values
117 # The total number of elements in @values is in $number
118 #
119 sub getLastAverage($$@) {
120 my ($count, $number, @values) = @_;
121 my $i;
122
123 my $a = 0;
124 return 0 if ($count == 0);
125 for ($i = 0; $i< $count; $i++) {
126 $a += $values[$number - $i - 1];
127 }
128 return $a / $count;
129 }
130
131 # --------------------------------------------------------------------
132 # Get a moving average of the values.
133 # --------------------------------------------------------------------
134 # The window size is the first parameter, the number of items in the
135 # passed array is next. (This can easily be calculated within the
136 # function using the scalar() function, but the subroutine shown here
137 # is also being used to illustrate how to pass pointers.) The reference to the
138 # array of values is passed next, followed by a reference to the place
139 # the return values are to be stored.
140 #
141 sub getMovingAve($$\@\@) {
142 my ($count, $number, $values, $movingAve) = @_;
143 my $i;
144 my $a = 0;
145 my $v = 0;
146
147 return 0 if ($count == 0);
148 return -1 if ($count > $number);
149 return -2 if ($count < 2);
150
151 $$movingAve[0] = 0;
152 $$movingAve[$number - 1] = 0;
153 for ($i=0; $i<$count;$i++) {
154 $v = $$values[$i];
155 $a += $v / $count;
156 $$movingAve[$i] = 0;
157 }
158 for ($i=$count; $i<$number;$i++) {
159 $v = $$values[$i];
160 $a += $v / $count;
161 $v = $$values[$i - $count - 1];
162 $a -= $v / $count;
163 $$movingAve[$i] = $a;
164 }
165 return 0;
166 }
167
168 1;
Look at the declaration of the function FutureValue with ($$$) . The three
dollar signs together signify three scalar numbers being passed into the function. This extra
scoping is present for validating the type of the parameters passed into the function. If you
were to pass a string instead of a number into the function, you would get a message very
similar to this one:
Too many arguments for Finance::FutureValue at ./f4.pl line 15, near "$time)"
Execution of ./f4.pl aborted due to compilation errors.
The use of prototypes when defining functions prevents you from sending in values other than
what the function expects. Use @ or % to pass in an array of values. If you
are passing by reference, use \@ or \% to show a scalar reference to an array
or hash, respectively. If you do not use the backslash, all other types in the argument list
prototype are ignored. Other types of disqualifiers include an ampersand for a reference to a
function, an asterisk for any type, and a semicolon to indicate that all other parameters are
optional.
Now, let's look at the lastMovingAverage function declaration, which specifies two
integers in the front followed by an array. The way the arguments are used in the function is
to assign a value to each of the two scalars, $count and $number , whereas
everything else is sent to the array. Look at the function getMovingAverage() to see
how two arrays are passed in order to get the moving average on a list of values.
The way to call the getMovingAverage function is shown in Listing 4.5.
Listing 4.5. Using the moving average function.
1 #!/usr/bin/perl -w
2
3 push(@Inc,'pwd');
4 use Finance;
5
6 @values = ( 12,22,23,24,21,23,24,23,23,21,29,27,26,28 );
7 @mv = (0);
8 $size = scalar(@values);
9 print "\n Values to work with = { @values } \n";
10 print " Number of values = $size \n";
11
12 # ----------------------------------------------------------------
13 # Calculate the average of the above function
14 # ----------------------------------------------------------------
15 $ave = Finance::getLastAverage(5,$size,@values);
16 print "\n Average of last 5 days = $ave \n";
17
18 Finance::getMovingAve(5,$size,@values,@mv);
19 print "\n Moving Average with 5 days window = \n { @mv } \n";
Here's the output from Listing 4.5:
Values to work with = { 12 22 23 24 21 23 24 23 23 21 29 27 26 28 }
Number of values = 14
Average of last 5 days = 26.2
Moving Average with 5 days window =
{ 0 0 0 0 0 19.4 21.8 22 22 21.4 23 23.8 24.2 25.2 }
The getMovingAverage() function takes two scalars and then two references to arrays
as scalars. Within the function, the two scalars to the arrays are dereferenced for use as
numeric arrays. The returned set of values is inserted in the area passed in as the second
reference. Had the input parameters not been specified with \@ for each referenced
array, the $movingAve array reference would have been empty and would have caused
errors at runtime. In other words, the following declaration is not correct:
sub getMovingAve($$@@)
The resulting spew of error messages from a bad function prototype is as follows:
Use of uninitialized value at Finance.pm line 128.
Use of uninitialized value at Finance.pm line 128.
Use of uninitialized value at Finance.pm line 128.
Use of uninitialized value at Finance.pm line 128.
Use of uninitialized value at Finance.pm line 128.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Use of uninitialized value at Finance.pm line 133.
Use of uninitialized value at Finance.pm line 135.
Values to work with = { 12 22 23 24 21 23 24 23 23 21 29 27 26 28 }
Number of values = 14
Average of last 5 days = 26.2
Moving Average with 5 days window =
{ 0 }
This is obviously not the correct output. Therefore, it's critical that you pass by
reference when sending more than one array.
Global variables for use within the package can also be declared. Look at the following
segment of code from the Finance.pm module to see what the default value of the
Interest variable would be if nothing was specified in the input. (The current module
requires the interest to be passed in, but you can change this.)
Here's a little snippet of code that can be added to the end of the program shown in Listing
4.5 to add the ability to set interest rates.
20 local $defaultInterest = 5.0;
21 sub Finance::SetInterest($) {
22 my $rate = shift(@_);
23 $rate *= -1 if ($rate < 0);
24 $defaultInterest = $rate;
25 printf "\n \$defaultInterest = $rate";
26 }
The local variable $defaultInterest is declared in line 20. The subroutine
SetInterest to modify the rate is declared in lines 21 through 26. The $rate
variable uses the values passed into the subroutine and simply assigns a positive value for it.
You can always add more error checking if necessary.
To access the defaultInterest variable's value, you could define either a
subroutine that returns the value or refer to the value directly with a call to the following
in your application program:
The variable holding the return value from the module function is declared as my
variable . The scope of this variable is within the curly braces of the function only.
When the called subroutine returns, the reference to my variable is returned. If the
calling program uses this returned reference somewhere, the link counter on the variable is not
zero; therefore, the storage area containing the returned values is not freed to the memory
pool. Thus, the function that declares
my $pv
and then later returns the value of $pv returns a reference to the value stored at
that location. If the calling routine performs a call like this one:
Finance::FVofAnnuity($monthly,$rate,$time);
there is no variable specified here into which Perl stores the returned reference;
therefore, any returned value (or a list of values) is destroyed. Instead, the call with the
returned value assigned to a local variable, such as this one:
$fv = Finance::FVofAnnuity($monthly,$rate,$time);
maintains the variable with the value. Consider the example shown in Listing 4.6, which
manipulates values returned by functions.
Listing 4.6. Sample usage of themyfunction.
1 #!/usr/bin/perl -w
2
3 push(@Inc,'pwd');
4 use Finance;
5
6 $monthly = 400;
7 $rate = 0.2; # i.e. 6 % APR
8 $time = 36; # in months
9
10 print "\n# ------------------------------------------------";
11 $fv = Finance::FVofAnnuity($monthly,$rate,$time);
12 printf "\n For a monthly %8.2f at a rate of %%%6.2f for %d periods",
13 $monthly, $rate, $time;
14 printf "\n you get a future value of %8.2f ", $fv;
15
16 $fv *= 1.1; # allow 10 % gain in the house value.
17
18 $mo = Finance::AnnuityOfFV($fv,$rate,$time);
19
20 printf "\n To get 10 percent more at the end, i.e. %8.2f",$fv;
21 printf "\n you need a monthly payment value of %8.2f",$mo,$fv;
22
23 print "\n# ------------------------------------------------ \n";
Here is sample input and output for this function:
$ testme # ------------------------------------------------
For a monthly 400.00 at a rate of % 0.20 for 36 periods
you get a future value of 1415603.75
To get 10 percent more at the end, i.e. 1557164.12
you need a monthly payment value of 440.00
# ------------------------------------------------
Modules implement classes in a Perl program that uses the object-oriented features of Perl.
Included in object-oriented features is the concept of inheritance . (You'll learn more
on the object-oriented features of Perl in Chapter 5 , "Object-Oriented
Programming in Perl .") Inheritance means
the process with which a module inherits the functions from its base classes. A module that is
nested within another module inherits its parent modules' functions. So inheritance in Perl is
accomplished with the :: construct. Here's the basic syntax:
SuperClass::NextSubClass:: ... ::ThisClass.
The file for these is stored in ./SuperClass/NextSubClass/ . Each double colon
indicates a lower-level directory in which to look for the module. Each module, in turn,
declares itself as a package with statements like the following:
For example, say that you really want to create a Money class with two subclasses,
Stocks and Finance . Here's how to structure the hierarchy, assuming you are
in the /usr/lib/perl5 directory:
Create a Money directory under the /usr/lib/perl5 directory.
Copy the existing Finance.pm file into the Money subdirectory.
Create the new Stocks.pm file in the Money subdirectory.
Edit the Finance.pm file to use the line package Money::Finance instead
of package Finance; .
Edit scripts to use Money::Finance as the subroutine prefix instead of
Finance:: .
Create a Money.pm file in the /usr/lib/perl5 directory.
The Perl script that gets the moving average for a series of numbers is presented in Listing
4.7.
Listing 4.7. Using inheriting modules.
1 #!/usr/bin/perl -w
2 $aa = 'pwd';
3 $aa .= "/Money";
4 push(@Inc,$aa);
5 use Money::Finance;
6 @values = ( 12,22,23,24,21,23,24,23,23,21,29,27,26,28 );
7 @mv = (0);
8 $size = scalar(@values);
9 print "\n Values to work with = { @values } \n";
10 print " Number of values = $size \n";
11 # ----------------------------------------------------------------
12 # Calculate the average of the above function
13 # ----------------------------------------------------------------
14 $ave = Money::Finance::getLastAverage(5,$size,@values);
15 print "\n Average of last 5 days = $ave \n";
16 Money::Finance::getMovingAve(5,$size,@values,@mv);
17 # foreach $i (@values) {
18 # print "\n Moving with 5 days window = $mv[$i] \n";
19 # }
20 print "\n Moving Average with 5 days window = \n { @mv } \n";
Lines 2 through 4 add the path to the Money subdirectory. The use
statement in line 5 now addresses the Finance.pm file in the ./Money
subdirectory. The calls to the functions within Finance.pm are now called with the
prefix Money::Finance:: instead of Finance:: . Therefore, a new subdirectory
is shown via the :: symbol when Perl is searching for modules to load.
The Money.pm file is not required. Even so, you should create a template for future
use. Actually, the file would be required to put any special requirements for initialization
that the entire hierarchy of modules uses. The code for initialization is placed in the
BEGIN() function. The sample Money.pm file is shown in Listing 4.8.
Listing 4.8. The superclass module forFinance.pm.
1 package Money;
2 require Exporter;
3
4 BEGIN {
5 printf "\n Hello! Zipping into existence for you\n";
6 }
7 1;
To see the line of output from the printf statement in line 5, you have to insert
the following commands at the beginning of your Perl script:
use Money;
use Money::Finance;
To use the functions in the Stocks.pm module, you use this line:
use Money::Stocks;
The Stocks.pm file appears in the Money subdirectory and is defined in the
same format as the Finance.pm file, with the exceptions that use Stocks is
used instead of use Finance and the set of functions to export is
different.
A number of modules are included in the Perl distribution. Check the
/usr/lib/perl5/lib directory for a complete listing after you install Perl. There are
two kinds of modules you should know about and look for in your Perl 5 release, Pragmatic and
Standard modules.
Pragmatic modules, which are also like pragmas in C compiler directives, tend to affect the
compilation of your program. They are similar in operation to the preprocessor elements of a C
program. Pragmas are locally scoped so that they can be turned off with the no
command. Thus, the command
no POSIX ;
turns off the POSIX features in the script. These features can be turned back on
with the use statement.
Standard modules bundled with the Perl package include several functioning packages of code
for you to use. Refer to appendix B, "Perl Module Archives," for a complete list of these
standard modules.
To find out all the .pm modules installed on your system, issue the following
command. (If you get an error, add the /usr/lib/perl5 directory to your path.)
Extension modules are written in C (or a mixture of Perl and C) and are dynamically loaded
into Perl if and when you need them. These types of modules for dynamic loading require support
in the kernel. Solaris lets you use these modules. For a Linux machine, check the installation
pages on how to upgrade to the ELF format binaries for your Linux kernel.
The term CPAN (Comprehensive Perl Archive Network) refers to all the hosts containing copies
of sets of data, documents, and Perl modules on the Net. To find out about the CPAN site
nearest you, search on the keyword CPAN in search engines such as Yahoo!, AltaVista, or
Magellan. A good place to start is the www.metronet.com site .
This chapter introduced you to Perl 5 modules and described what they have to offer. A more
comprehensive list is found on the Internet via the addresses shown in the Web sites
http://www.metronet.com
and http://www.perl.com
.
A Perl package is a set of Perl code that looks like a library file. A Perl module is a
package that is defined in a library file of the same name. A module is designed to be
reusable. You can do some type checking with Perl function prototypes to see whether parameters
are being passed correctly. A module has to export its functions with the @EXPORT
array and therefore requires the Exporter module. Modules are searched for in the
directories listed in the @Inc array.
Obviously, there is a lot more to writing modules for Perl than what is shown in this
chapter. The simple examples in this chapter show you how to get started with Perl modules. In
the rest of the book I cover the modules and their features, so hang in there.
I cover Perl objects, classes, and related concepts in Chapter 5 .
List::Util module provides a number of simple
and some more complex functions that can be used on lists, anything that returns a list
anything that can be seen as a list.
For example these can be used on arrays as they "return their content" in list context .
min
If given a list of numbers to it, it will return the smallest number:
# Argument "2x" isn't numeric in subroutine entry at examples/min.pl line 14.
say min ( 10 , 3 , '2x' , 21 ); # 2
If one of the arguments is a string that cannot be fully converted to a number automatically
and if you have use
warnings on as you should , then you'll see the following warnings: Argument ... isn't
numeric in subroutine entry at ...
minstr
There is a corresponding function called minstr that will accept strings and sort them
according to the ASCII order, though I guess it will work with Unicode as well if that's what
you are feeding it.
examples/minstr.pl
use 5.010 ;
use strict ;
use warnings ;
use List :: Util qw ( minstr );
say minstr ( 'f' , 'b' , 'e' ); # b
It can also accept numbers as parameters and will treat them as strings. The result might
surprise you, if you are not familiar with the automatic number to string conversion of Perl,
and that the string "11" is ahead of the string "2" because the comparison works
character-by-character and in this case the first character of "11" is ahead of the first (and
only) character of "2" in the ASCII table.
examples/minstr_numbers.pl
use 5.010 ;
use strict ;
use warnings ;
use List :: Util qw ( minstr );
say minstr ( 2 , 11 , 99 ); # 11
After all internally it uses the lt operator.
max
Similar to min just returns the biggest number.
maxstr
Similar to minstr , returns the biggest string in ASCII order.
sum
The sum function adds up the provided numbers and returns their sum. If one or more of the
values provided is a string that cannot be fully converted to a number it will generate a
warning like this: Argument ... isn't numeric in subroutine entry at ... . If the parameters of
sum are empty the function returns undef . This is unfortunate as it should be 0, but in order
to provide backwards compatibility, if the provided list is empty then undef is returned.
examples/sum.pl
use 5.010 ;
use strict ;
use warnings ;
use List :: Util qw ( sum );
say sum ( 10 , 3 , - 8 , 21 ); # 26
my @prices = ( 17.2 , 23.6 , '1.1' );
say sum ( @prices ); # 41.9
my @empty ;
# Use of uninitialized value in say at examples/sum.pl line 14.
say sum ( @empty ); # (prints nothing)
sum0
In order to fix the above issue, that sum() return undef , in version 1.26 of the module, in
2012, a new function called sum0 was introduced that behaves exactly like the sum function, but
returns 0 if no values was supplied.
examples/sum0.pl
use 5.010 ;
use strict ;
use warnings ;
use List :: Util qw ( sum0 );
say sum0 ( 10 , 3 , - 8 , 21 ); # 26
my @prices = ( 17.2 , 23.6 , '1.1' );
say sum0 ( @prices ); # 41.9
my @empty ;
say sum0 ( @empty ); # 0
product
The product function multiplies its parameters. As this function is newer it was not
constrained with backward compatibility issues so if the provided list is empty, the returned
value will be 1.
examples/product.pl
use 5.010 ;
use strict ;
use warnings ;
use List :: Util qw ( product );
my @interest = ( 1.2 , 2.6 , 4 , '1.3' );
say product ( @interest ); # 16.224
my @empty ;
say product ( @empty ); # 1
Other functions of List::Util
The module has a number of other functions that were used in various other
articles:
The any function will return true if any of the given values satisfies
the given condition. It is shown in the article Filtering values using Perl grep as
a better solution.
I will try to explain the logic behind the style decisions taken over that last 35+ years of
programming in different languages.
About programming style and layout there are as many opinions as there are people. Most
important in my opinion is to think about the reasoning behind what you, your team or
your company chooses to follow as guides.
I seriously think that way too many (young) programmers leave school, brainwashed with
GNU-style coding without realizing that the amount of indentation and the placing of braces,
brackets and parentheses were well thought about.
Several well known styles (including mine) are discussed at wikimedia . It is worth
reading through them to see the pros and cons of each.
For me personally, the GNU coding style is one of the reasons I do NOT contribute a
lot to these projects. The style does not fit my logic, and if I send patches that are rejected
simply because I wrote them in a style/layout that I think is way better because I then
understand the underlying logic, I give up.
Here I will take a tour through what I think is the only correct way of (perl) code layout,
and why. Most of this can be achieved with Perl::Tidy and a correct .perltidyrc . I'll use their configuration definitions as
a guide.
Indentation in code blocks
Opening Block Brace Right or Left
Braces Left
Because braces are just syntactic sugar to keep a block together, it should visually also
bind to the block, and not to the conditional. As the closing brace - or END
in languages like PASCAL - is visually showing me the end of the block, it should obviously
have the same indent as the block itself. An advantage is that the alignment of the closing
brace with the block emphasizes the fact that the entire block is conceptually (as well as
programmatically) a single compound statement.
In other words: I see the braces being part of the block, and as all statements inside a
block share the same indentation, in my opinion the brace - being part of the block -
should have the same indentation too.
Indent width is 4, tabs are allowed (when set to 8). I prefer having it being spaces
only, but as I cannot see the difference with good editors, I do not really
care.
Opening brace should be on the same line as the conditional
Block should be indented
Closing brace should have the same indent as the block
Of course cuddled else is not the way to go, as it makes removing either branch more
difficult and makes the indent of the closing brace go wrong. The only right way to use
if/else indent is uncuddled:
sub _directives
{
{ ENDIF => \&_endif,
IF => \&_if,
};
} # _directives
the opening brace of a sub may optionally be put on a new line. If so, it should be in column
one, for all those that use 'vi' or one of it's clones, so }, {, ]], and [[ work as expected.
if the opening brace is on the same line, which I prefer, it requires a single leading
space
sub _directives {
{ ENDIF => \&_endif,
IF => \&_if,
};
} # _directives
my @month_of_year = (qw(
Jan Feb Mar Apr May Jun
Jul Aug Sep Oct Nov Dec
));
As with the closing brace of a block, the closing parenthesis belongs to the data in the
container it closes, and thus should have the same indentation.
Define Horizontal Tightness
Of course function <space> <paren> <no-space> <first-arg>
<comma> <space>
if ((my $duration = travel ($target, $means)) > 1200) {
One of my pet-peeves. Having white-space between the function name and its opening
parenthesis is the best match to how we think. As an example, if I would ask someone to describe
his/her day, he/she might answer
I woke up
I freshened myself
I had breakfast
I got to work
I worked
I had lunch
I worked again
I went home
I had diner
I watched TV
I brushed my teeth
I went to bed
for $day in (qw( Mon Tue Wed Thu Fri )) {
wake_up ();
wash ($self);
eat ("breakfast");
:
:
Or, more extreme to show the sequence of actions
for $day in (qw( Mon Tue Wed Thu Fri )) {
wake_up ();
wash ($self);
eat ("breakfast");
:
:
Where it, IMHO, clearly shows that the actions are far more important than what it takes to
perform the action. When I read through the process, I don't care about what transport the person
uses to get to work and if eggs are part of the breakfast. These are the parameters to the actions
I will only have a look at the function's argument if I need to. In reading that I
eat , I see what action is taken. That's enough for understanding the program
flow. The arguments to the function have to be grouped together using parenthesis for the
function to know that all the arguments are for the function: the parenthesis are there to
group the arguments, not to make the function a function so the parenthesis belong to the
arguments and not to the function and therefor are to be close to the arguments ant not to
the function.
Arguments are separated by a comma and a space, just to separate the arguments more
for better readability
A rule of thumb is to NEVER use statement modifiers like
go_home () unless $work_done; # WRONG!
As it will draw the attention to going home (unconditionally) instead of to the condition,
which is more important. This is especially annoying when using exit, die, croak or return. Any of
these will visually end the current scope, so you do not have to read on. Unless there is a
statement modifier and you need to re-read the entire section.
No else after return/exit/die/croak/throw
if (expression) {
return;
}
else {
return 42;
}
As any of return, exit, die, croak, or throw will immediately exit the current scope, the
mind will read the code as to stop processing it right there, which is exactly what those keywords
are for.
In an if/else construct, the code after the construct is supposed to be executed
when either if the if/else branches where followed. If the if-branch exits the current
scope, there is no need to run the code after the construct, so the else is useless.
This is the main reason why these keywords should never have a statement modifier
(and no, you cannot come up with a valid exception to this rule).
Statement Termination Semicolon Spaces
my $i = 1;
For Loop Semicolon Spaces
for (@a = @$ap, $u = shift @a; @a; $u = $v) {
Block Comment Indentation
If comment is aligned to the left margin, leave it there
If the original comment was indented, match the indent to the surrounding code.
Never reformat comments itself. Do not wrap
Outdenting Long Quotes
if ($source_stream) {
if (@ARGV > 0) {
die "You may not specify any filenames when a source array is given\n";
}
}
if ($source_stream) {
if (@ARGV > 0) {
die "You may not specify any filenames ".
"when a source array is given\n";
}
}
for (@methods) {
push @results, {
name => $_->name,
help => $_->help,
};
}
1) Created ~/.perldb , which did not exist previously.
2) Added &parse_options("HistFile=$ENV{HOME}/.perldb.hist"); from mirod's
answer.
3) Added export PERLDB_OPTS=HistFile=$HOME/.perldb.history to ~/.bashrc from
mephinet's answer.
4) Ran source .bashrc
5) Ran perl -d my program.pl , and got this warning/error
perldb: Must not source insecure rcfile /home/ics/.perldb.
You or the superuser must be the owner, and it must not
be writable by anyone but its owner.
6) I protected ~/.perldb with owner rw chmod 700 ~/.perldb , and
the error went away.
"... Per Damien Conway’s recommendations, I always unpack all the arguments from @_in the first line of a subroutine, which ends up looking just like a subroutine signature. (I almost never use shift for this purpose.) ..."
"... Perl bashing is largely hear-say. People hear something and they say it. It doesn't require a great deal of thought. ..."
"... It may not be as common as the usual gang of languages, but there's an enormous amount of work done in Perl. ..."
Perl bashing is popular sport among a particularly vocal crowd.
Perl is extremely flexible. Perl holds up TIMTOWTDI ( There Is More Than One
Way To Do It ) as a virtue. Larry Wall's Twitter handle is @TimToady, for goodness sake!
That flexibility makes it extremely powerful. It also makes it extremely easy to write
code that nobody else can understand. (Hence, Tim Toady
Bicarbonate.)
You can pack a lot of punch in a one-liner in Perl:
It is still used, but its usage is declining. People use Python today in situations when
they would have used Perl ten years ago.
The problem is that Perl is extremely pragmatic. It is designed to be “a language to
get your job done”, and it does that well; however, that led to rejection by language
formalists. However, Perl is very well designed, only it is well designed for professionals
who grab in the dark expecting that at this place there should be a button to do the desired
functionality, and indeed, there will be the button. It is much safer to use than for example
C (the sharp knife that was delivered without a handle), but it is easy to produce quite
messy code with it if you are a newbie who doesn’t understand/feel the principles of
Perl. In the 90s and 2000s, it was the goto web language, so the web was full of terrible
programs written by those newbies, and that led to the bad reputation.
Strangely enough, PHP, which is frowned upon a lot by Perl programmers, won the favour of
the noobs, but never got the general bad reputation; in fact it is missing the design
principles I mentioned, that language is just a product of adhockery.
But today, Perl went back to its status as a niche language, and you cannot mention it in
presence of a lady, so to speak. Its support is slowly waning; I’d suggest to learn
Python, but don’t force me to learn it as well.
You should learn things that make your life easier or better. I am not an excellent Perl
user, but it is usually my go-to scripting language for important projects. The syntax is
difficult, and it's very easy to forget how to use it when you take significant time away
from it.
That being said, I love how regular expressions work in Perl. I can use sed like commands
$myvar =~ s/old/new/g for string replacement when processing or filtering strings. It's much
nicer than other languages imo.
I also like Perls foreach loops and its data structures.
I tried writing a program of moderate length in Python and it just seemed to be taking up
too much space. I stopped part way though and switched to Perl. I got the whole thing
completed in much less space (lines), and seemed to have an easier time doing it.
I am not a super fanboy, but it has just always worked for me in the past, and I can't
outright discount it because of that.
Also, look up CPAN modules. The installation of those for me on GNU is a breeze.
My last scripting project I did in Python and it went very well. I will probably shift to
Python more in the future, because I would like to build a stronger basis of knowledge with
the modules and basics of Python so that I can hop into it and create some powerful stuff
when needed. Ie I want to focus on 1–3 languages, and learn them to a higher level
instead of being "just ok" with 5–7.
Gary
Puckering , Fluent in C#, Python, and perl; rusty in C/C++ and too many others to count
Answered Apr 25, 2018 · Author has 1.1k answers and 2.5m answer views
Why is Perl so hated and not commonly used?
I think there are several reasons why Perl has a lot of detractors
Sigils . A lot of programmers seem to hate the $@% sigils! If you are coming
from a strongly typed language like C/C++, and also hate things like Hungarian notation,
you won’t like sigils.
One liners. As others have commented, writing dense and even obfuscated code
rose to the level of sport within the Perl community. The same thing happened, years
earlier, in the APL community. Programmers and managers saw that you could write
unmaintainable code, and that helped instill a fear that it was unavoidable and that
perhaps the language was flawed because it didn’t discourage the practice.
Auto-magic . The programming language PL/I, which attempted to combine the best
of COBOL and FORTRAN, went absolutely crazy with default behaviors. I remember reading an
article in the 1970’s where programming in PL/I was described as being like flying a
Boeing 747. The cockpit is filled with hundreds of buttons, knobs, switches and levers. The
autopilot does most of the work, but trying to figure out the interaction between it and
things you manually set can be bewildering. Perl, to some extent, suffers from the same
problem. In Perl 5, without enabling warnings and strict, variables spring into life simply
by naming them. A typo can instantiate and entirely new variable. Hashes get new keys
simply by an attempt to access a key. You can increment a scalar that contains a string and
it’ll try to generate a sequence using the string as a pattern (e.g. a, b, c …
z, aa, ab …). If you come from a language where you control everything, all this
auto-magic stuff can really bite you in the ass.
An odd object-oriented syntax. Until Moose (and now Moo and Mouse) came along,
writing classes in Perl meant using keywords like package and bless, as well as rolling all
your own accessor methods. If you come from C++, Java , Python or just about any
other language supporting OO your first question is going to be: where’s the
friggin’ class statement!
Dynamic typing . Some people like it. Some hate it. There are modules that let
you add typing I’d you wish, though it’ll only be enforced at run time.
No subroutine signatures . Although Perl 5 now supports subroutine signatures,
they are still considered “experimental”. This is a turn-off for most
programmers who are used to them. Per Damien Conway’s recommendations, I always
unpack all the arguments from @_in the first line of a subroutine, which ends up looking
just like a subroutine signature. (I almost never use shift for this purpose.)
Lots of magic symbols . Although you can use English names, and should do so for
more maintainable code, many Perl programmers stick to using special names like $_,
$’, $; etc. This makes Perl code look very cryptic, and increases your cognitive load
when working with the language. It’s a lot to remember. But if you use the English
names, you can largely avoid this issue.
Perl 6 is a discontinuous evolution . Although Perl 5 continues to evolve, and
some of the advances that have been put in Perl 6 have been added to Perl 5, the lack
of,upward compatibility between 5 and 6 creates uncertainly about its future.
And why should I learn it?
Despite the above, you can write maintainable code in Perl by following Damian
Comways’s Perl Best Practices. The utility perlcritic can be used to help train
yourself to write better Perl code.
Perl is multi-paradigm. In execution, it’s faster than Python. It has a superb
ecosystem in cpan , where you can find a module to help you solve almost every
imaginable problem. For command line utilities, file system administration, database
administration, data extraction-transformation-loading tasks, batch processes, connecting
disparate systems, and quick and dirty scripts, it’s often the best tool for the
job.
I frequently use Perl in connection with Excel. You can do a lot in Excel, and it provides
a great interactive UI. But complex formulas can be a pain to get right, and it can be
tedious to write code in VBA. Often, I find it much quicker to just copy cells to the
clipboard, switch to a command shell, run a Perl script over the data, sending the results to
the clipboard, switch back to Excel, and then paste the results in situ or in a new
location.
Perl is also deep. It does a good job of supporting imperative programming, OOP, and
functional programming. For more on the latter, see the book Higher-Order Perl .
Perl is powerful. Perl is fast. Perl is an effective tool to have in your toolkit. Those
are all good reasons to learn it.
Reed White , former
Engineer at Hewlett-Packard (1978-2000)
Answered Nov 7, 2017 · Author has 2.3k answers and 380.8k answer views
Yes, Perl takes verbal abuse; but in truth, it is an extremely powerful, reliable
language. In my opinion, one of its outstanding characteristics is that you don't need much
knowledge before you can write useful programs. As time goes by, you gradually learn the real
power of the language.
However, because Perl-bashing is popular, you might better put your efforts into learning
Python, which is also quite capable.
Richard Conto ,
Programmer in multiple languages. Debugger in even more
Answered Dec 18, 2017 · Author has 5.9k answers and 4.3m answer views
Perl bashing is largely hear-say. People hear something and they say it. It doesn't
require a great deal of thought.
As for Perl not commonly being used - that's BS. It may not be as common as the usual
gang of languages, but there's an enormous amount of work done in Perl.
As for you you should learn Perl, it's for the same reason you would learn any other
language - it helps you solve a particular problem better than another language available.
And yes, that can be a very subjective decision to make.
The truth is, that by any metric, more Perl is being done today than during the dot com
boom. It's just a somewhat smaller piece of a much bigger pie. In fact, I've heard from some
hiring managers that there's actually a shortage of Perl programmers, and not just for
maintaining projects, but for new greenfield deploys.
How do I change the value of a variable in the package used by a module so that subroutines
in that module can use it?
Here's my test case:
testmodule.pm:
package testmodule;
use strict;
use warnings;
require Exporter;
our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
@ISA = qw(Exporter);
@EXPORT = qw(testsub);
my $greeting = "hello testmodule";
my $var2;
sub testsub {
printf "__PACKAGE__: %s\n", __PACKAGE__;
printf "\$main::greeting: %s\n", $main::greeting;
printf "\$greeting: %s\n", $greeting;
printf "\$testmodule::greeting: %s\n", $testmodule::greeting;
printf "\$var2: %s\n", $var2;
} # End testsub
1;
testscript.pl:
#!/usr/bin/perl -w
use strict;
use warnings;
use testmodule;
our $greeting = "hello main";
my $var2 = "my var2 in testscript";
$testmodule::greeting = "hello testmodule from testscript";
$testmodule::var2 = "hello var2 from testscript";
testsub();
output:
Name "testmodule::var2" used only once: possible typo at ./testscript.pl line 11.
__PACKAGE__: testmodule
$main::greeting: hello main
$greeting: hello testmodule
$testmodule::greeting: hello testmodule from testscript
Use of uninitialized value $var2 in printf at testmodule.pm line 20.
$var2:
I expected $greeting and $testmodule::greeting to be the same
since the package of the subroutine is testmodule .
I guess this has something to do with the way use d modules are
eval d as if in a BEGIN block, but I'd like to understand it
better.
I was hoping to set the value of the variable from the main script and use it in the
module's subroutine without using the fully-qualified name of the variable.
As you found out, when you use my , you are creating a locally scoped
non-package variable. To create a package variable, you use our and not
my :
my $foo = "this is a locally scoped, non-package variable";
our $bar = "This is a package variable that's visible in the entire package";
Even better:
{
my $foo = "This variable is only available in this block";
our $bar = "This variable is available in the whole package":
}
print "$foo\n"; #Whoops! Undefined variable
print "$bar\n"; #Bar is still defined even out of the block
When you don't put use strict in your program, all variables defined are
package variables. That's why when you don't put it, it works the way you think it should and
putting it in breaks your program.
However, as you can see in the following example, using our will solve your
dilemma:
File Local/Foo.pm
#! /usr/local/bin perl
package Local::Foo;
use strict;
use warnings;
use feature qw(say);
use Exporter 'import';
our @EXPORT = qw(testme);
our $bar = "This is the package's bar value!";
sub testme {
# $foo is a locally scoped, non-package variable. It's undefined and an error
say qq(The value of \$main::foo is "$main::foo");
# $bar is defined in package main::, and will print out
say qq(The value of \$main::bar is "$main::bar");
# These both refer to $Local::Foo::bar
say qq(The value of \$Local::Foo::bar is "$Local::Foo::bar");
say qq(The value of bar is "$bar");
}
1;
File test.pl
#! /usr/local/bin perl
use strict;
use warnings;
use feature qw(say);
use Local::Foo;
my $foo = "This is foo";
our $bar = "This is bar";
testme;
say "";
$Local::Foo::bar = "This is the NEW value for the package's bar";
testme
And, the output is:
Use of uninitialized value $foo in concatenation (.) or string at Local/Foo.pm line 14.
The value of $main::foo is ""
The value of $main::bar is "This is bar"
The value of $Local::Foo::bar is "This is the package's bar value!"
The value of bar is "This is the package's bar value!"
Use of uninitialized value $foo in concatenation (.) or string at Local/Foo.pm line 14.
The value of $main::foo is ""
The value of $main::bar is "This is bar"
The value of $Local::Foo::bar is "This is the NEW value for the package's bar"
The value of bar is "This is the NEW value for the package's bar"
The error message you're getting is the result of $foo being a local
variable, and thus isn't visible inside the package. Meanwhile, $bar is a
package variable and is visible.
Sometimes, it can be a bit tricky:
if ($bar -eq "one") {
my $foo = 1;
}
else {
my $foo = 2;
}
print "Foo = $foo\n";
That doesn't work because $foo only bas a value inside the if
block. You have to do this:
Yes, it can be a bit to get your head wrapped around it initially, but the use of
use strict; and use warnings; is now de rigueur and for good
reasons. The use of use strict; and use warnings; probably has
eliminated 90% of the mistakes people make in Perl. You can't make a mistake of setting the
value of $foo in one part of the program, and attempting to use
$Foo in another. It's one of the things I really miss in Python.
> ,
After reading Variable
Scoping in Perl: the basics more carefully, I realized that a variable declared with
my isn't in the current package. For example, in a simple script with no modules
if I declare my $var = "hello"$main::var still doesn't have a
value.
The way that this applies in this case is in the module. Since my $greeting
is declared in the file, that hides the package's version of $greeting and
that's the value which the subroutine sees. If I don't declare the variable first, the
subroutine would see the package variable, but it doesn't get that far because I use
strict .
If I don't use strict and don't declare my $greeting , it works
as I would have expected. Another way to get the intended value and not break use
strict is to use our $greeting . The difference being that my declares a variable in the
current scope while our declares a variable in the current
package .
I want to repeatedly search for values in an array that does not change.
So far, I have been doing it this way: I put the values in a hash (so I have an array and a hash with essentially the same
contents) and I search the hash using exists .
I don't like having two different variables (the array and the hash) that both store the same thing; however, the hash is much
faster for searching.
I found out that there is a ~~ (smartmatch) operator in Perl 5.10. How efficient is it when searching for a scalar
in an array?
> ,
If you want to search for a single scalar in an array, you can use
List::Util 's first subroutine. It stops as soon
as it knows the answer. I don't expect this to be faster than a hash lookup if you already have the hash , but when you
consider creating the hash and having it in memory, it might be more convenient for you to just search the array you already have.
As for the smarts of the smart-match operator, if you want to see how smart it is, test it. :)
There are at least three cases you want to examine. The worst case is that every element you want to find is at the end. The
best case is that every element you want to find is at the beginning. The likely case is that the elements you want to find average
out to being in the middle.
Now, before I start this benchmark, I expect that if the smart match can short circuit (and it can; its documented in
perlsyn ), that the best case times will stay the same despite
the array size, while the other ones get increasingly worse. If it can't short circuit and has to scan the entire array every
time, there should be no difference in the times because every case involves the same amount of work.
Here's a benchmark:
#!perl
use 5.12.2;
use strict;
use warnings;
use Benchmark qw(cmpthese);
my @hits = qw(A B C);
my @base = qw(one two three four five six) x ( $ARGV[0] || 1 );
my @at_end = ( @base, @hits );
my @at_beginning = ( @hits, @base );
my @in_middle = @base;
splice @in_middle, int( @in_middle / 2 ), 0, @hits;
my @random = @base;
foreach my $item ( @hits ) {
my $index = int rand @random;
splice @random, $index, 0, $item;
}
sub count {
my( $hits, $candidates ) = @_;
my $count;
foreach ( @$hits ) { when( $candidates ) { $count++ } }
$count;
}
cmpthese(-5, {
hits_beginning => sub { my $count = count( \@hits, \@at_beginning ) },
hits_end => sub { my $count = count( \@hits, \@at_end ) },
hits_middle => sub { my $count = count( \@hits, \@in_middle ) },
hits_random => sub { my $count = count( \@hits, \@random ) },
control => sub { my $count = count( [], [] ) },
}
);
div class="answercell post-layout--right
,
Here's how the various parts did. Note that this is a logarithmic plot on both axes, so the slopes of the plunging lines aren't
as close as they look:
So, it looks like the smart match operator is a bit smart, but that doesn't really help you because you still might have to
scan the entire array. You probably don't know ahead of time where you'll find your elements. I expect a hash will perform the
same as the best case smart match, even if you have to give up some memory for it.
Okay, so the smart match being smart times two is great, but the real question is "Should I use it?". The alternative is a
hash lookup, and it's been bugging me that I haven't considered that case.
As with any benchmark, I start off thinking about what the results might be before I actually test them. I expect that if I
already have the hash, looking up a value is going to be lightning fast. That case isn't a problem. I'm more interested in the
case where I don't have the hash yet. How quickly can I make the hash and lookup a key? I expect that to perform not so well,
but is it still better than the worst case smart match?
Before you see the benchmark, though, remember that there's almost never enough information about which technique you should
use just by looking at the numbers. The context of the problem selects the best technique, not the fastest, contextless micro-benchmark.
Consider a couple of cases that would select different techniques:
You have one array you will search repeatedly
You always get a new array that you only need to search once
You get very large arrays but have limited memory
Now, keeping those in mind, I add to my previous program:
my %old_hash = map {$_,1} @in_middle;
cmpthese(-5, {
...,
new_hash => sub {
my %h = map {$_,1} @in_middle;
my $count = 0;
foreach ( @hits ) { $count++ if exists $h{$_} }
$count;
},
old_hash => sub {
my $count = 0;
foreach ( @hits ) { $count++ if exists $old_hash{$_} }
$count;
},
control_hash => sub {
my $count = 0;
foreach ( @hits ) { $count++ }
$count;
},
}
);
Here's the plot. The colors are a bit difficult to distinguish. The lowest line there is the case where you have to create
the hash any time you want to search it. That's pretty poor. The highest two (green) lines are the control for the hash (no hash
actually there) and the existing hash lookup. This is a log/log plot; those two cases are faster than even the smart match control
(which just calls a subroutine).
There are a few other things to note. The lines for the "random" case are a bit different. That's understandable because each
benchmark (so, once per array scale run) randomly places the hit elements in the candidate array. Some runs put them a bit earlier
and some a bit later, but since I only make the @random array once per run of the entire program, they move around
a bit. That means that the bumps in the line aren't significant. If I tried all positions and averaged, I expect that "random"
line to be the same as the "middle" line.
Now, looking at these results, I'd say that a smart-match is much faster in its worst case than the hash lookup is in its worst
case. That makes sense. To create a hash, I have to visit every element of the array and also make the hash, which is a lot of
copying. There's no copying with the smart match.
Here's a further case I won't examine though. When does the hash become better than the smart match? That is, when does the
overhead of creating the hash spread out enough over repeated searches that the hash is the better choice?
,
Fast for small numbers of potential matches, but not faster than the hash. Hashes are really the right tool for testing set membership.
Since hash access is O(log n) and smartmatch on an array is still O(n) linear scan (albeit short-circuiting, unlike grep), with
larger numbers of values in the allowed matches, smartmatch gets relatively worse. Benchmark code (matching against 3 values):
#!perl
use 5.12.0;
use Benchmark qw(cmpthese);
my @hits = qw(one two three);
my @candidates = qw(one two three four five six); # 50% hit rate
my %hash;
@hash{@hits} = ();
sub count_hits_hash {
my $count = 0;
for (@_) {
$count++ if exists $hash{$_};
}
$count;
}
sub count_hits_smartmatch {
my $count = 0;
for (@_) {
$count++ when @hits;
}
$count;
}
say count_hits_hash(@candidates);
say count_hits_smartmatch(@candidates);
cmpthese(-5, {
hash => sub { count_hits_hash((@candidates) x 1000) },
smartmatch => sub { count_hits_smartmatch((@candidates) x 1000) },
}
);
Is there any static code analysis module in Perl except B::Lint and Perl::Critic? How
effective is Module::Checkstyle?
> ,
There is a post on
perlmonks.org asking if PPI can be used for static analysis. PPI is the power behind
Perl::Critic, according to the reviews of this module. (I have not used it yet).
Module::Checkstyle is a tool similar to checkstyle http://checkstyle.sourceforge.net for Java. It allows
you to validate that your code confirms to a set of guidelines checking various things such as
indentation, naming, whitespace, complexity and so forth.
Module::Checkstyle is also extensible so your organization can implement custom checks that
are not provided by the standard distribution. There is a guide on how to write checks in
Module::Checkstyle::Check
Module::Checkstyle is mostly used via the provided module-checkstyle tool. You
probablly want to read module-checkstyle
.
NAME
module-checkstyle - Check that your code keeps style
SYNOPSIS
module-checkstyle [options] [file and directories ...]
This program is the command-line interface to Module::Checkstyle .
You invoke it by supplying a list of files or directories that contain Perl code that
should be checked aginst the configuration. Any problems found will be reported on standard
out.
OPTIONS
-help
Print a brief help message and exits.
-man
Prints the manual page and exists.
-config
Use an alternate config file instead of ~/.module-checkstyle/config .
-all
Don't ignore common files when traversing directories. Common files are things such as
blib/* t/* Makefile.PL etc.
Is there any static code analysis module in Perl except B::Lint and Perl::Critic? How
effective is Module::Checkstyle?
> ,
There is a post on
perlmonks.org asking if PPI can be used for static analysis. PPI is the power behind
Perl::Critic, according to the reviews of this module. (I have not used it yet).
Module::Checkstyle is a tool similar to checkstyle http://checkstyle.sourceforge.net for Java. It allows
you to validate that your code confirms to a set of guidelines checking various things such as
indentation, naming, whitespace, complexity and so forth.
Module::Checkstyle is also extensible so your organization can implement custom checks that
are not provided by the standard distribution. There is a guide on how to write checks in
Module::Checkstyle::Check
Module::Checkstyle is mostly used via the provided module-checkstyle tool. You
probablly want to read module-checkstyle
.
NAME
module-checkstyle - Check that your code keeps style
SYNOPSIS
module-checkstyle [options] [file and directories ...]
This program is the command-line interface to Module::Checkstyle .
You invoke it by supplying a list of files or directories that contain Perl code that
should be checked aginst the configuration. Any problems found will be reported on standard
out.
OPTIONS
-help
Print a brief help message and exits.
-man
Prints the manual page and exists.
-config
Use an alternate config file instead of ~/.module-checkstyle/config .
-all
Don't ignore common files when traversing directories. Common files are things such as
blib/* t/* Makefile.PL etc.
I want to repeatedly search for values in an array that does not change.
So far, I have been doing it this way: I put the values in a hash (so I have an array
and a hash with essentially the same contents) and I search the hash using
exists .
I don't like having two different variables (the array and the hash) that both store
the same thing; however, the hash is much faster for searching.
I found out that there is a ~~ (smartmatch) operator in Perl 5.10. How
efficient is it when searching for a scalar in an array?
> ,
If you want to search for a single scalar in an array, you can use List::Util 's first
subroutine. It stops as soon as it knows the answer. I don't expect this to be faster
than a hash lookup if you already have the hash , but when you consider creating
the hash and having it in memory, it might be more convenient for you to just search the
array you already have.
As for the smarts of the smart-match operator, if you want to see how smart it is,
test it. :)
There are at least three cases you want to examine. The worst case is that every
element you want to find is at the end. The best case is that every element you want to
find is at the beginning. The likely case is that the elements you want to find average
out to being in the middle.
Now, before I start this benchmark, I expect that if the smart match can short circuit
(and it can; its documented in perlsyn ), that the best case times will stay
the same despite the array size, while the other ones get increasingly worse. If it can't
short circuit and has to scan the entire array every time, there should be no difference
in the times because every case involves the same amount of work.
Here's a benchmark:
#!perl
use 5.12.2;
use strict;
use warnings;
use Benchmark qw(cmpthese);
my @hits = qw(A B C);
my @base = qw(one two three four five six) x ( $ARGV[0] || 1 );
my @at_end = ( @base, @hits );
my @at_beginning = ( @hits, @base );
my @in_middle = @base;
splice @in_middle, int( @in_middle / 2 ), 0, @hits;
my @random = @base;
foreach my $item ( @hits ) {
my $index = int rand @random;
splice @random, $index, 0, $item;
}
sub count {
my( $hits, $candidates ) = @_;
my $count;
foreach ( @$hits ) { when( $candidates ) { $count++ } }
$count;
}
cmpthese(-5, {
hits_beginning => sub { my $count = count( \@hits, \@at_beginning ) },
hits_end => sub { my $count = count( \@hits, \@at_end ) },
hits_middle => sub { my $count = count( \@hits, \@in_middle ) },
hits_random => sub { my $count = count( \@hits, \@random ) },
control => sub { my $count = count( [], [] ) },
}
);
div class="answercell post-layout--right
,
Here's how the various parts did. Note that this is a logarithmic plot on both axes, so
the slopes of the plunging lines aren't as close as they look:
So, it looks like the smart match operator is a bit smart, but that doesn't really
help you because you still might have to scan the entire array. You probably don't know
ahead of time where you'll find your elements. I expect a hash will perform the same as
the best case smart match, even if you have to give up some memory for it.
Okay, so the smart match being smart times two is great, but the real question is
"Should I use it?". The alternative is a hash lookup, and it's been bugging me that I
haven't considered that case.
As with any benchmark, I start off thinking about what the results might be before I
actually test them. I expect that if I already have the hash, looking up a value is going
to be lightning fast. That case isn't a problem. I'm more interested in the case where I
don't have the hash yet. How quickly can I make the hash and lookup a key? I expect that
to perform not so well, but is it still better than the worst case smart match?
Before you see the benchmark, though, remember that there's almost never enough
information about which technique you should use just by looking at the numbers. The
context of the problem selects the best technique, not the fastest, contextless
micro-benchmark. Consider a couple of cases that would select different techniques:
You have one array you will search repeatedly
You always get a new array that you only need to search once
You get very large arrays but have limited memory
Now, keeping those in mind, I add to my previous program:
my %old_hash = map {$_,1} @in_middle;
cmpthese(-5, {
...,
new_hash => sub {
my %h = map {$_,1} @in_middle;
my $count = 0;
foreach ( @hits ) { $count++ if exists $h{$_} }
$count;
},
old_hash => sub {
my $count = 0;
foreach ( @hits ) { $count++ if exists $old_hash{$_} }
$count;
},
control_hash => sub {
my $count = 0;
foreach ( @hits ) { $count++ }
$count;
},
}
);
Here's the plot. The colors are a bit difficult to distinguish. The lowest line there
is the case where you have to create the hash any time you want to search it. That's
pretty poor. The highest two (green) lines are the control for the hash (no hash actually
there) and the existing hash lookup. This is a log/log plot; those two cases are faster
than even the smart match control (which just calls a subroutine).
There are a few other things to note. The lines for the "random" case are a bit
different. That's understandable because each benchmark (so, once per array scale run)
randomly places the hit elements in the candidate array. Some runs put them a bit earlier
and some a bit later, but since I only make the @random array once per run
of the entire program, they move around a bit. That means that the bumps in the line
aren't significant. If I tried all positions and averaged, I expect that "random" line to
be the same as the "middle" line.
Now, looking at these results, I'd say that a smart-match is much faster in its worst
case than the hash lookup is in its worst case. That makes sense. To create a hash, I
have to visit every element of the array and also make the hash, which is a lot of
copying. There's no copying with the smart match.
Here's a further case I won't examine though. When does the hash become better than
the smart match? That is, when does the overhead of creating the hash spread out enough
over repeated searches that the hash is the better choice?
,
Fast for small numbers of potential matches, but not faster than the hash. Hashes are
really the right tool for testing set membership. Since hash access is O(log n) and
smartmatch on an array is still O(n) linear scan (albeit short-circuiting, unlike grep),
with larger numbers of values in the allowed matches, smartmatch gets relatively worse.
Benchmark code (matching against 3 values):
#!perl
use 5.12.0;
use Benchmark qw(cmpthese);
my @hits = qw(one two three);
my @candidates = qw(one two three four five six); # 50% hit rate
my %hash;
@hash{@hits} = ();
sub count_hits_hash {
my $count = 0;
for (@_) {
$count++ if exists $hash{$_};
}
$count;
}
sub count_hits_smartmatch {
my $count = 0;
for (@_) {
$count++ when @hits;
}
$count;
}
say count_hits_hash(@candidates);
say count_hits_smartmatch(@candidates);
cmpthese(-5, {
hash => sub { count_hits_hash((@candidates) x 1000) },
smartmatch => sub { count_hits_smartmatch((@candidates) x 1000) },
}
);
Racu now needs to compete on its own merits with
established languages which is extremely difficult as Ruby and Python covers the same application area
This is a positive decision for Perl5 as it slowly returns to its main niche -- the tool for
advanced Unix sysadmins. Still as the decision was made rather late in language development cycle
itt will negativly affect Racu future, if it has any. The main interest in the new language was
because of the name -- Perl6. No this is gone.
It also split the community into Perl 5 supporters and "coming to Racu" beta addicts which is
probably a good thing. But, at the same time, the loss of mindshare to Ruby and Python might
accelerate.
makecheck on Oct 7, 2015 [-]
In multiple organizations I have primarily seen Perl used in a very large, complex and
established code bases that also make significant use of things like reading/writing Perl data
structures.
Did you know that Perl is a great programming language for system administrators? Perl is
platform-independent so you can do things on different operating systems without rewriting your
scripts. Scripting in Perl is quick and easy, and its portability makes your scripts amazingly
useful. Here are a few examples, just to get your creative juices flowing! Renaming a bunch
of files
Suppose you need to rename a whole bunch of files in a directory. In this case, we've got a
directory full of .xml files, and we want to rename them all to .html
. Easy-peasy!
Then just cd to the directory where you need to make the change, and run the script. You
could put this in a cron job, if you needed to run it regularly, and it is easily enhanced to
accept parameters.
Speaking of accepting parameters, let's take a look at a script that does just
that.
Suppose you need to regularly create Linux user accounts on your system, and the format of
the username is first initial/last name, as is common in many businesses. (This is, of course,
a good idea, until you get John Smith and Jane Smith working at the same company -- or want
John to have two accounts, as he works part-time in two different departments. But humor me,
okay?) Each user account needs to be in a group based on their department, and home directories
are of the format /home/<department>/<username> . Let's take a look at a
script to do that:
# If the user calls the script with no parameters,
# give them help!
if ( not @ ARGV ) {
usage () ;
}
# Gather our options; if they specify any undefined option,
# they'll get sent some help!
my %opts ;
GetOptions ( \%opts ,
'fname=s' ,
'lname=s' ,
'dept=s' ,
'run' ,
) or usage () ;
# Let's validate our inputs. All three parameters are
# required, and must be alphabetic.
# You could be clever, and do this with a foreach loop,
# but let's keep it simple for now.
if ( not $opts { fname } or $opts { fname } !~ /^[a-zA-Z]+$/ ) {
usage ( "First name must be alphabetic" ) ;
}
if ( not $opts { lname } or $opts { lname } !~ /^[a-zA-Z]+$/ ) {
usage ( "Last name must be alphabetic" ) ;
}
if ( not $opts { dept } or $opts { dept } !~ /^[a-zA-Z]+$/ ) {
usage ( "Department must be alphabetic" ) ;
}
print "$cmd \n "
;
if ( $opts { run }) { system $cmd ;
} else { print "You need to
add the --run flag to actually execute \n " ;
}
sub usage {
my ( $msg ) = @_ ;
if ( $msg ) { print "$msg \n\n "
;
} print "Usage: $0
--fname FirstName --lname LastName --dept Department --run \n " ; exit ;
}
As with the previous script, there are opportunities for enhancement, but something like
this might be all that you need for this task.
One more, just for fun!
Change copyright text in every Perl source file in a directory
tree
Now we're going to try a mass edit. Suppose you've got a directory full of code, and each
file has a copyright statement somewhere in it. (Rich Bowen wrote a great article, Copyright
statements proliferate inside open source code a couple of years ago that discusses the
wisdom of copyright statements in open source code. It is a good read, and I recommend it
highly. But again, humor me.) You want to change that text in each and every file in the
directory tree. File::Find and File::Slurp are your
friends!
#!/usr/bin/perl
use strict ;
use warnings ;
use File :: Find qw
( find ) ;
use File :: Slurp qw (
read_file write_file ) ;
# If the user gives a directory name, use that. Otherwise,
# use the current directory.
my $dir = $ARGV [ 0 ] || '.' ;
# File::Find::find is kind of dark-arts magic.
# You give it a reference to some code,
# and a directory to hunt in, and it will
# execute that code on every file in the
# directory, and all subdirectories. In this
# case, \&change_file is the reference
# to our code, a subroutine. You could, if
# what you wanted to do was really short,
# include it in a { } block instead. But doing
# it this way is nice and readable.
find ( \&change_file , $dir ) ;
sub change_file {
my $name = $_ ;
# If the file is a directory, symlink, or other
# non-regular file, don't do anything
if ( not - f $name ) { return ;
}
# If it's not Perl, don't do anything.
# Gobble up the file, complete with carriage
# returns and everything.
# Be wary of this if you have very large files
# on a system with limited memory!
my $data = read_file ( $name ) ;
# Use a regex to make the change. If the string appears
# more than once, this will change it everywhere!
Because of Perl's portability, you could use this script on a Windows system as well as a
Linux system -- it Just Works because of the underlying Perl interpreter code. In our
create-an-account code above, that one is not portable, but is Linux-specific because it uses
Linux commands such as adduser .
In my experience, I've found it useful to have a Git repository of these things somewhere
that I can clone on each new system I'm working with. Over time, you'll think of changes to
make to the code to enhance the capabilities, or you'll add new scripts, and Git can help you
make sure that all your tools and tricks are available on all your systems.
I hope these little scripts have given you some ideas how you can use Perl to make your
system administration life a little easier. In addition to these longer scripts, take a look at
a fantastic list of Perl one-liners, and links to other
Perl magic assembled by Mischa Peterson.
(github.com)
100hondo77 notes that Larry Wall has given his approval to the
re-naming of Perl 6.
In the "Path to Raku" pull request, Larry Wall indicated his approval, leaving this comment:
I am in favor of this change, because it reflects an
ancient wisdom :
"No one sews a patch of unshrunk cloth on an old garment, for the patch will pull away
from the garment, making the tear worse. Neither do people pour new wine into old wineskins.
If they do, the skins will burst; the wine will run out and the wineskins will be ruined. No,
they pour new wine into new wineskins, and both are preserved."
"Perl 6 will become Raku, assuming the four people who haven't yet approved the pull request
give their okay," reports the Register, adding that Perl 5 will then become
simply Perl .
Dozens of comments on that pull request have now already been marked as "outdated," and
while a few contributors have made a point of abstaining from the approval process, reviewer
Alex Daniel notes that "this
pull request will be merged on October 14th if nobody in the list rejects it or requests more
changes."
Gabor Szabo is an expert in Perl who originally wrote Padre Perl Padre http://padre.perlide.org (abandonware since 2013). The last
that was available was Ubuntu 10.10.
Notable quotes:
"... This code will set element 3 (the 4th element of the array) to undef , but will NOT change the size of the array: ..."
In response to an earlier article about undef one of the readers asked me:
How do you eliminate a value in the middle of an array in Perl?
I am not sure if undef and eliminating values from an array are related, though I guess, if
we see having a value of undef as being "empty", then I can understand the connection. In
general though, setting something to be undef and deleting something is not the same.
For one-off tasks it can be very useful to be able to run a piece of Perl code without
creating a file. The code itself needs to be between quotes. Due to differences between the
Unix/Linux shell and the MS Windows Command prompt we need to use different quotes around our
code.
On Unix/Linux systsem (including Mac OSX) it is recommended to put our code in single quotes
as in the following example:
$ perl -e 'print qq{Hello World\n}'
Hello World
On MS Windows we must use double quotes around our code.
$ perl -e "print qq{Hello World\n}"
Hello World
Internally, it is probably the best to use q and qq instead of
single-quote and double-quote, respectively. That might help reduce the confusion caused by the
behavior of the shell and command prompt.
-E execute code on the command line with all
the latest features enabled
Since version 5.10 of Perl has been released, Perl includes some additional keywords (called
features) in the language. For improved backward compatibility these keywords are only enabled
if the user explicitly ask for them with use feature ... . For example by writing use feature
qw(say); , or by declaring a minimal version of Perl with use 5.010; .
On the command line we can achieve the same by using -E instead of -e . It will turn on all
the features of the version of Perl we are currently running.
For me the most important of all these features, at least in one-liners is the say
keyword introduced in perl 5.10 . It is just print with a trailing newline added. Nothing
fancy, but makes the one-liners even shorter.
The above examples would look like these:
Unix/Linux:
$ perl -E 'say q{Hello World}'
Hello World
MS Windows:
$ perl -E "say q{Hello World}"
Hello World
You can notice the change from qq to q . As we don't need to include a newline \n in our
strings we could switch from qq to q .
-n
wrap the -e/-E code in a while loop
If we provide the -n command line option it will wrap our code provided using either the -e
or the -E options in a while with a diamond operator .
So
perl -n -E 'say if /code/' file.txt
is the same as
while (<>) {
say if /code/;
}
That will go over all the lines of all the files provided on the command line (in this case
it is file.txt) and print out every line that matches the /code/ regex.
-p is like -n
with print $_
The -p option is very similar to the -n flag, but it also prints the content of $_ at the
end of each iteration.
So we could write:
perl -p -E 's/code/foobar/' file.txt
which would become
while (<>) {
s/code/foobar/
print;
}
That will print the result to the screen.
-i for in-place editing
The most common use of -p is together with the -i option that provides "in-place editing".
It means that instead of printing to the screen, all the output generated by our one-liner will
be written back to the same file it was taken from.
So this one-liner will replace the first appearance of the string "code" by "foobar" in
every line of the file "file.txt".
Prev
Next In most of the cases we either want a variable to be accessible only from inside a
small scope, inside a function or even inside a loop. These variables get created when we enter
the function (or the scope created by a a block) and destroyed when we leave the scope.
In some cases, especially when we don't want to pay attention to our code, we want variables
to be global, to be accessible from anywhere in our script and be destroyed only when the
script ends. In General having such global variables is not a good practice.
In some cases we want a variable to stay alive between function calls, but still to be
private to that function. We want it to retain its value between calls.
In the C programming language one can designate a variable to be a static variable . This means it gets
initialized only once and it sticks around retaining its old value between function calls.
In Perl, the same can be achieved using the state variable which
is available starting from version 5.10, but there is a construct that will work in every
version of Perl 5. In a way it is even more powerful.
$counter is initialized to 0
only once, the first time we call counter() . In subsequent calls, the line state $counter = 0;
does not get executed and $counter has the same value as it had when we left the function the
last time.
showing that the state $counter = say "world"; line only gets executed once. In the first
call to count() say , which was also added in version
5.10 , will return 1 upon success.
static variables in the "traditional" way
use strict ;
use warnings ;
use 5.010 ;
{
my $counter = 0 ;
sub count {
$counter ++;
return $counter ;
}
}
say count ();
say count ();
say count ();
This provides the same result as the above version using state , except that this could work
in older versions of perl as well. (Especially if I did not want to use the say keyword, that
was also introduced in 5.10.)
This version works because functions declarations are global in perl - so count() is
accessible in the main body of the script even though it was declared inside a block. On the
other hand the variable $counter is not accessible from the outside world because it was
declared inside the block. Lastly, but probably most importantly, it does not get destroyed
when we leave the count() function (or when the execution is outside the block), because the
existing count() function still references it.
Thus $count is effectively a static variable.
First assignment time
use strict ;
use warnings ;
use 5.010 ;
say "hi" ;
{
my $counter = say "world" ;
sub count {
$counter ++;
return $counter ;
}
}
say "hello" ;
say count ();
say count ();
say count ();
hi
world
hello
2
3
4
This shows that in this case too, the declaration and the initial assignment my $counter =
say "world"; happens only once, but we can also see that the assignment happens before
the first call to count() as if the my $counter = say "world"; statement was part of the
control flow of the code outside of the block.
Shared static variable
This "traditional" or "home made" static variable has an extra feature. Because it does not
belong to the the count() subroutine, but to the block surrounding it, we can declare more than
one functions in that block and we can share this static variable between two or even more
functions.
For example we could add a reset_counter() function:
use strict ;
use warnings ;
use 5.010 ;
{
my $counter = 0 ;
sub count {
$counter ++;
return $counter ;
}
sub reset_counter {
$counter = 0 ;
}
}
say count ();
say count ();
say count ();
reset_counter ();
say count ();
say count ();
1
2
3
1
2
Now both functions can access the $counter variable, but still nothing outside the enclosing
block can access it.
Static arrays and hashes
As of now, you cannot use the state declaration in list context. This means you cannot write
state @y = (1, 1); . This limitation could be overcome by some extra coding. For example in
this implementation of the Fibonacci series, we checked if the array is empty and set the
default values:
use strict ;
use warnings ;
use 5.010 ;
sub fib {
state @y ;
@y = ( 1 , 1 ) if not @y ; # workaround initialization
push @y , $y [ 0 ]+ $y [ 1 ];
return shift @y ;
}
say fib ();
say fib ();
say fib ();
say fib ();
say fib ();
Alternatively we could use the "old-style" static variable with the enclosing block.
Here is the example generating the Fibonacci series:
The problem is that use strict is complaining that there is a variable $x which is not
declared with my and that it does not know about it. So we need a way to tell strict that it is
ok. We know about the $x variable and we want to use it, but we want it to be a package
variable. We don't want to declare it using my and we don't want to always prefix it with the
package name.
With use vars ('$x') we can achieve that:
use strict ;
package VeryLongName ;
use vars ( '$x' );
$x = 23 ;
print "VeryLongName: $x\n" ;
This works, but the documentation of vars tells us that the functionality provided by
this pragma has been superseded by "our" declarations .
So how does our work?
our
use strict ;
package VeryLongName ;
our $x = 23 ;
print "VeryLongName: $x\n" ;
Caveat
The our declaration itself is lexically scoped, meaning it is limited by the file or by
enclosing curly braces. In the next example we don't have curly braces and thus the declaration
our $x = 23; will be intact even after switching namespaces. This can lead to very unpleasant
situations. My recommendation is to avoid using our (you almost always need to use my anyway)
and to put every package in its own file.
I know what my is in Perl. It defines a variable that exists only in the scope
of the block in which it is defined. What does our do? How does our
differ from my ?
Great question: How does our differ from my and what
does our do?
In Summary:
Available since Perl 5, my is a way to declare:
non-package variables, that are
private,
new ,
non-global variables,
separate from any package. So that the variable cannot be accessed in the form
of $package_name::variable .
On the other hand, our variables are:
package variables, and thus automatically
global variables,
definitely not private ,
nor are they necessarily new; and they
can be accessed outside the package (or lexical scope) with the qualified
namespace, as $package_name::variable .
Declaring a variable with our allows you to predeclare variables in
order to use them under use strict without getting typo warnings or
compile-time errors. Since Perl 5.6, it has replaced the obsolete use
vars , which was only file-scoped, and not lexically scoped as is
our .
For example, the formal, qualified name for variable $x inside package
main is $main::x . Declaring our $x allows you to use
the bare $x variable without penalty (i.e., without a resulting error), in the
scope of the declaration, when the script uses use strict or use
strict "vars" . The scope might be one, or two, or more packages, or one small
block.
The PerlMonks and PerlDoc links from cartman and Olafur are a great reference - below is my
crack at a summary:
my variables are lexically scoped within a single block defined by
{} or within the same file if not in {} s. They are not accessible
from packages/subroutines defined outside of the same lexical scope / block.
our variables are scoped within a package/file and accessible from any code
that use or require that package/file - name conflicts are resolved
between packages by prepending the appropriate namespace.
Just to round it out, local variables are "dynamically" scoped, differing
from my variables in that they are also accessible from subroutines called
within the same block.
use strict;
for (1 .. 2){
# Both variables are lexically scoped to the block.
our ($o); # Belongs to 'main' package.
my ($m); # Does not belong to a package.
# The variables differ with respect to newness.
$o ++;
$m ++;
print __PACKAGE__, " >> o=$o m=$m\n"; # $m is always 1.
# The package has changed, but we still have direct,
# unqualified access to both variables, because the
# lexical scope has not changed.
package Fubb;
print __PACKAGE__, " >> o=$o m=$m\n";
}
# The our() and my() variables differ with respect to privacy.
# We can still access the variable declared with our(), provided
# that we fully qualify its name, but the variable declared
# with my() is unavailable.
print __PACKAGE__, " >> main::o=$main::o\n"; # 2
print __PACKAGE__, " >> main::m=$main::m\n"; # Undefined.
# Attempts to access the variables directly won't compile.
# print __PACKAGE__, " >> o=$o\n";
# print __PACKAGE__, " >> m=$m\n";
# Variables declared with use vars() are like those declared
# with our(): belong to a package; not private; and not new.
# However, their scoping is package-based rather than lexical.
for (1 .. 9){
use vars qw($uv);
$uv ++;
}
# Even though we are outside the lexical scope where the
# use vars() variable was declared, we have direct access
# because the package has not changed.
print __PACKAGE__, " >> uv=$uv\n";
# And we can access it from another package.
package Bubb;
print __PACKAGE__, " >> main::uv=$main::uv\n";
Coping with Scoping
is a good overview of Perl scoping rules. It's old enough that our is not
discussed in the body of the text. It is addressed in the Notes section at the end.
The article talks about package variables and dynamic scope and how that differs from
lexical variables and lexical scope.
It's an old question, but I ever met some pitfalls about lexical declarations in Perl that
messed me up, which are also related to this question, so I just add my summary here:
1. definition or declaration?
local $var = 42;
print "var: $var\n";
The output is var: 42 . However we couldn't tell if local $var =
42; is a definition or declaration. But how about this:
use strict;
use warnings;
local $var = 42;
print "var: $var\n";
The second program will throw an error:
Global symbol "$var" requires explicit package name.
$var is not defined, which means local $var; is just a
declaration! Before using local to declare a variable, make sure that it is
defined as a global variable previously.
But why this won't fail?
use strict;
use warnings;
local $a = 42;
print "var: $a\n";
The output is: var: 42 .
That's because $a , as well as $b , is a global variable
pre-defined in Perl. Remember the sort function?
2. lexical or global?
I was a C programmer before starting using Perl, so the concept of lexical and global
variables seems straightforward to me: just corresponds to auto and external variables in C.
But there're small differences:
In C, an external variable is a variable defined outside any function block. On the other
hand, an automatic variable is a variable defined inside a function block. Like this:
int global;
int main(void) {
int local;
}
While in Perl, things are subtle:
sub main {
$var = 42;
}
&main;
print "var: $var\n";
The output is var: 42 , $var is a global variable even it's
defined in a function block! Actually in Perl, any variable is declared as global by
default.
The lesson is to always add use strict; use warnings; at the beginning of a
Perl program, which will force the programmer to declare the lexical variable explicitly, so
that we don't get messed up by some mistakes taken for granted.
Unlike my, which both allocates storage for a variable and associates a simple name with
that storage for use within the current scope, our associates a simple name with a package
variable in the current package, for use within the current scope. In other words, our has
the same scoping rules as my, but does not necessarily create a variable.
This is only somewhat related to the question, but I've just discovered a (to me) obscure bit
of perl syntax that you can use with "our" (package) variables that you can't use with "my"
(local) variables.
print "package is: " . __PACKAGE__ . "\n";
our $test = 1;
print "trying to print global var from main package: $test\n";
package Changed;
{
my $test = 10;
my $test1 = 11;
print "trying to print local vars from a closed block: $test, $test1\n";
}
&Check_global;
sub Check_global {
print "trying to print global var from a function: $test\n";
}
print "package is: " . __PACKAGE__ . "\n";
print "trying to print global var outside the func and from \"Changed\" package: $test\n";
print "trying to print local var outside the block $test1\n";
Will Output this:
package is: main
trying to print global var from main package: 1
trying to print local vars from a closed block: 10, 11
trying to print global var from a function: 1
package is: Changed
trying to print global var outside the func and from "Changed" package: 1
trying to print local var outside the block
In case using "use strict" will get this failure while attempting to run the script:
Global symbol "$test1" requires explicit package name at ./check_global.pl line 24.
Execution of ./check_global.pl aborted due to compilation errors.
#!/usr/local/bin/perl
use feature ':5.10';
#use warnings;
package a;
{
my $b = 100;
our $a = 10;
print "$a \n";
print "$b \n";
}
package b;
#my $b = 200;
#our $a = 20 ;
print "in package b value of my b $a::b \n";
print "in package b value of our a $a::a \n";
#!/usr/bin/perl -l
use strict;
# if string below commented out, prints 'lol' , if the string enabled, prints 'eeeeeeeee'
#my $lol = 'eeeeeeeeeee' ;
# no errors or warnings at any case, despite of 'strict'
our $lol = eval {$lol} || 'lol' ;
print $lol;
Let us think what an interpreter actually is: it's a piece of code that stores values in
memory and lets the instructions in a program that it interprets access those values by their
names, which are specified inside these instructions. So, the big job of an interpreter is to
shape the rules of how we should use the names in those instructions to access the values
that the interpreter stores.
On encountering "my", the interpreter creates a lexical variable: a named value that the
interpreter can access only while it executes a block, and only from within that syntactic
block. On encountering "our", the interpreter makes a lexical alias of a package variable: it
binds a name, which the interpreter is supposed from then on to process as a lexical
variable's name, until the block is finished, to the value of the package variable with the
same name.
The effect is that you can then pretend that you're using a lexical variable and bypass
the rules of 'use strict' on full qualification of package variables. Since the interpreter
automatically creates package variables when they are first used, the side effect of using
"our" may also be that the interpreter creates a package variable as well. In this case, two
things are created: a package variable, which the interpreter can access from everywhere,
provided it's properly designated as requested by 'use strict' (prepended with the name of
its package and two colons), and its lexical alias.
is it possible to import ( use ) a perl module within a different namespace?
Let's say I have a Module A (XS Module with no methods Exported
@EXPORT is empty) and I have no way of changing the module.
This Module has a Method A::open
currently I can use that Module in my main program (package main) by calling
A::open I would like to have that module inside my package main so
that I can directly call open
I tried to manually push every key of %A:: into %main:: however
that did not work as expected.
The only way that I know to achieve what I want is by using package A; inside
my main program, effectively changing the package of my program from main to
A . Im not satisfied with this. I would really like to keep my program inside
package main.
Is there any way to achieve this and still keep my program in package main?
Offtopic: Yes I know usually you would not want to import everything into your
namespace but this module is used by us extensively and we don't want to type A:: (well the
actual module name is way longer which isn't making the situation better)in front of hundreds
or thousands of calls
This is one of those "impossible" situations, where the clear solution -- to rework that
module -- is off limits.
But, you can alias that package's subs names, from its symbol table, to the same
names in main . Worse than being rude, this comes with a glitch: it catches all
names that that package itself imported in any way. However, since this package is a fixed
quantity it stands to reason that you can establish that list (and even hard-code it). It is
just this one time, right?
main
use warnings;
use strict;
use feature 'say';
use OffLimits;
GET_SUBS: {
# The list of names to be excluded
my $re_exclude = qr/^(?:BEGIN|import)$/; # ...
my @subs = grep { !/$re_exclude/ } sort keys %OffLimits::;
no strict 'refs';
for my $sub_name (@subs) {
*{ $sub_name } = \&{ 'OffLimits::' . $sub_name };
}
};
my $name = name('name() called from ' . __PACKAGE__);
my $id = id('id() called from ' . __PACKAGE__);
say "name() returned: $name";
say "id() returned: $id";
with OffLimits.pm
package OffLimits;
use warnings;
use strict;
sub name { return "In " . __PACKAGE__ . ": @_" }
sub id { return "In " . __PACKAGE__ . ": @_" }
1;
It prints
name() returned: In OffLimits: name() called from main
id() returned: In OffLimits: id() called from main
You may need that code in a BEGIN block, depending on other details.
Another option is of course to hard-code the subs to be "exported" (in @subs
). Given that the module is in practice immutable this option is reasonable and more
reliable.
This can also be wrapped in a module, so that you have the normal, selective,
importing.
WrapOffLimits.pm
package WrapOffLimits;
use warnings;
use strict;
use OffLimits;
use Exporter qw(import);
our @sub_names;
our @EXPORT_OK = @sub_names;
our %EXPORT_TAGS = (all => \@sub_names);
BEGIN {
# Or supply a hard-coded list of all module's subs in @sub_names
my $re_exclude = qr/^(?:BEGIN|import)$/; # ...
@sub_names = grep { !/$re_exclude/ } sort keys %OffLimits::;
no strict 'refs';
for my $sub_name (@sub_names) {
*{ $sub_name } = \&{ 'OffLimits::' . $sub_name };
}
};
1;
and now in the caller you can import either only some subs
use WrapOffLimits qw(name);
or all
use WrapOffLimits qw(:all);
with otherwise the same main as above for a test.
The module name is hard-coded, which should be OK as this is meant only for that
module.
The following is added mostly for completeness.
One can pass the module name to the wrapper by writing one's own import sub,
which is what gets used then. The import list can be passed as well, at the expense of an
awkward interface of the use statement.
It goes along the lines of
package WrapModule;
use warnings;
use strict;
use OffLimits;
use Exporter qw(); # will need our own import
our ($mod_name, @sub_names);
our @EXPORT_OK = @sub_names;
our %EXPORT_TAGS = (all => \@sub_names);
sub import {
my $mod_name = splice @_, 1, 1; # remove mod name from @_ for goto
my $re_exclude = qr/^(?:BEGIN|import)$/; # etc
no strict 'refs';
@sub_names = grep { !/$re_exclude/ } sort keys %{ $mod_name . '::'};
for my $sub_name (@sub_names) {
*{ $sub_name } = \&{ $mod_name . '::' . $sub_name };
}
push @EXPORT_OK, @sub_names;
goto &Exporter::import;
}
1;
what can be used as
use WrapModule qw(OffLimits name id); # or (OffLimits :all)
or, with the list broken-up so to remind the user of the unusual interface
use WrapModule 'OffLimits', qw(name id);
When used with the main above this prints the same output.
The use statement ends up using the import sub defined in the module, which
exports symbols by writing to the caller's symbol table. (If no import sub is
written then the Exporter 's import method is nicely used, which is
how this is normally done.)
This way we are able to unpack the arguments and have the module name supplied at
use invocation. With the import list supplied as well now we have to
push manually to @EXPORT_OK since this can't be in the
BEGIN phase. In the end the sub is replaced by Exporter::import via
the (good form of) goto , to complete the job.
You can forcibly "import" a function into main using glob assignment to alias the subroutine
(and you want to do it in BEGIN so it happens at compile time, before calls to that
subroutine are parsed later in the file):
use strict;
use warnings;
use Other::Module;
BEGIN { *open = \&Other::Module::open }
However, another problem you might have here is that open is a builtin function, which may
cause some problems . You can add
use subs 'open'; to indicate that you want to override the built-in function in
this case, since you aren't using an actual import function to do so.
Here is what I now came up with. Yes this is hacky and yes I also feel like I opened pandoras
box with this. However at least a small dummy program ran perfectly fine.
I renamed the module in my code again. In my original post I used the example
A::open actually this module does not contain any method/variable reserved by
the perl core. This is why I blindly import everything here.
BEGIN {
# using the caller to determine the parent. Usually this is main but maybe we want it somewhere else in some cases
my ($parent_package) = caller;
package A;
foreach (keys(%A::)) {
if (defined $$_) {
eval '*'.$parent_package.'::'.$_.' = \$A::'.$_;
}
elsif (%$_) {
eval '*'.$parent_package.'::'.$_.' = \%A::'.$_;
}
elsif (@$_) {
eval '*'.$parent_package.'::'.$_.' = \@A::'.$_;
}
else {
eval '*'.$parent_package.'::'.$_.' = \&A::'.$_;
}
}
}
I have a Perl module (Module.pm) that initializes a number of variables, some of which I'd
like to import ($VAR2, $VAR3) into additional submodules that it might load during execution.
The way I'm currently setting up Module.pm is as follows:
package Module;
use warnings;
use strict;
use vars qw($SUBMODULES $VAR1 $VAR2 $VAR3);
require Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw($VAR2 $VAR3);
sub new {
my ($package) = @_;
my $self = {};
bless ($self, $package);
return $self;
}
sub SubModules1 {
my $self = shift;
if($SUBMODULES->{'1'}) { return $SUBMODULES->{'1'}; }
# Load & cache submodule
require Module::SubModule1;
$SUBMODULES->{'1'} = Module::SubModule1->new(@_);
return $SUBMODULES->{'1'};
}
sub SubModules2 {
my $self = shift;
if($SUBMODULES->{'2'}) { return $SUBMODULES->{'2'}; }
# Load & cache submodule
require Module::SubModule2;
$SUBMODULES->{'2'} = Module::SubModule2->new(@_);
return $SUBMODULES->{'2'};
}
Each submodule is structured as follows:
package Module::SubModule1;
use warnings;
use strict;
use Carp;
use vars qw();
sub new {
my ($package) = @_;
my $self = {};
bless ($self, $package);
return $self;
}
I want to be able to import the $VAR2 and $VAR3 variables into each of the submodules
without having to reference them as $Module::VAR2 and $Module::VAR3. I noticed that the
calling script is able to access both the variables that I have exported in Module.pm in the
desired fashion but SubModule1.pm and SubModule2.pm still have to reference the variables as
being from Module.pm.
I tried updating each submodule as follows which unfortunately didn't work I was
hoping:
package Module::SubModule1;
use warnings;
use strict;
use Carp;
use vars qw($VAR2 $VAR3);
sub new {
my ($package) = @_;
my $self = {};
bless ($self, $package);
$VAR2 = $Module::VAR2;
$VAR3 = $Module::VAR3;
return $self;
}
Please let me know how I can successfully export $VAR2 and $VAR3 from Module.pm into each
Submodule. Thanks in advance for your help!
? Calling use Module from another package (say
Module::Submodule9 ) will try to run the Module::import method.
Since you don't have that method, it will call the Exporter::import method, and
that is where the magic that exports Module 's variables into the
Module::Submodule9 namespace will happen.
In your program there is only one Module namespace and only one instance of
the (global) variable $Module::VAR2 . Exporting creates aliases to this variable
in other namespaces, so the same variable can be accessed in different ways. Try this in a
separate script:
package Whatever;
use Module;
use strict;
use vars qw($VAR2);
$Module::VAR2 = 5;
print $Whatever::VAR2; # should be 5.
$VAR2 = 14; # same as $Whatever::VAR2 = 14
print $Module::VAR2; # should be 14
package M;
use strict;
use warnings;
#our is better than "use vars" for creating package variables
#it creates an alias to $M::foo named $foo in the current lexical scope
our $foo = 5;
sub inM { print "$foo\n" }
1;
In M/S.pm
package M;
#creates an alias to $M::foo that will last for the entire scope,
#in this case the entire file
our $foo;
package M::S;
use strict;
use warnings;
sub inMS { print "$foo\n" }
1;
In the script:
#!/usr/bin/perl
use strict;
use warnings;
use M;
use M::S;
M::inM();
M::S::inMS();
But I would advise against this. Global variables are not a good practice, and sharing
global variables between modules is even worse.
These are the oldest type of variables in Perl. They are still used in some cases, even
though in most cases you should just use lexical variables.
In old times, if we started to use a variable without declaring it with the my or state
keywords, we automatically got a variable in the current namespace. Thus we could write:
$x = 42 ;
print "$x\n" ; # 42
Please note, we don't use strict; in these examples. Even though you should always use strict . We'll fix this in a
bit.
The default namespace in every perl script is called "main" and you can always access
variables using their full name including the namespace:
$x = 42 ;
print "$x\n" ; # 42
print "$main::x\n" ; # 42
The package keyword is used to switch namespaces:
$x = 42 ;
print "$x\n" ; # 42
print "$main::x\n" ; # 42
package Foo ;
print "Foo: $x\n" ; # Foo:
Please note, once we switched to the "Foo" namespace, the $x name refers to the variable in
the Foo namespace. It does not have any value yet.
$x = 42 ;
print "$x\n" ; # 42
print "$main::x\n" ; # 42
package Foo ;
print "Foo: $x\n" ; # Foo:
$x = 23 ;
print "Foo: $x\n" ; # Foo 23;
Do we really have two $x-es? Can we reach the $x in the main namespace while we are in the
Foo namespace?
$x = 42 ;
print "$x\n" ; # 42
print "$main::x\n" ; # 42
package Foo ;
print "Foo: $x\n" ; # Foo:
$x = 23 ;
print "Foo: $x\n" ; # Foo 23
print "main: $main::x\n" ; # main: 42
print "Foo: $Foo::x\n" ; # Foo: 23
package main ;
print "main: $main::x\n" ; # main: 42
print "Foo: $Foo::x\n" ; # Foo: 23
print "$x\n" ; # 42
We even switched back to the main namespace (using package main; ) and if you look closely,
you can see that while we were already in the main package we could reach to the $x of the Foo
package using $Foo::x but if we accessed $x without the full package name, we reach the one in
the main namespace.
Every package (or namespace) can hold variables with the same name.
Returns the integer portion of EXPR. If EXPR is omitted, uses $_ . You should not use this function for
rounding: one because it truncates towards 0 , and two because machine
representations of floating-point numbers can sometimes produce counterintuitive
results.
For example, int(-6.725/0.025) produces -268
rather than the correct -269; that's because it's really more like
-268.99999999999994315658 instead.
Emits a warning, usually by printing it to STDERR . warn interprets its
operand LIST in the same way as die , but is slightly different in
what it defaults to when LIST is empty or makes an empty string. If it is empty and
$@ already
contains an exception value then that value is used after appending
"\t...caught" . If it is empty and $@ is also empty then the
string "Warning: Something's wrong" is used.
By default, the exception derived from the operand LIST is stringified and printed to
STDERR . This behaviour can be altered by installing a $SIG{__WARN__} handler. If there is
such a handler then no message is automatically printed; it is the handler's responsibility
to deal with the exception as it sees fit (like, for instance, converting it into a
die ).
Most handlers must therefore arrange to actually display the warnings that they are not
prepared to deal with, by calling warn again in the handler.
Note that this is quite safe and will not produce an endless loop, since
__WARN__ hooks are not called from inside one.
You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't
suppress the error text, but can instead call die again to change it).
Using a __WARN__ handler provides a powerful way to silence all warnings
(even the so-called mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN {
$SIG { '__WARN__' } = sub { warn $_ [ 0 ] if $DOWARN } }
See perlvar for
details on setting %SIG entries and for more examples. See
the Carp module for other
kinds of warnings using its carp and cluck functions.
In the Beginning, some time around 1960, every part of your program had access to all the
variables in every other part of the program. That turned out to be a problem, so language
designers invented local variables, which were visible in only a small part of the program.
That way, programmers who used a variable x could be sure that nobody was able to
tamper with the contents of x behind their back. They could also be sure that by using
x they weren't tampering with someone else's variable by mistake.
Every programming language has a philosophy, and these days most of these philosophies have
to do with the way the names of variables are managed. Details of which variables are visible
to which parts of the program, and what names mean what, and when, are of prime importance. The
details vary from somewhat baroque, in languages like Lisp, to extremely baroque, in languages
like C++. Perl unfortunately, falls somewhere towards the rococo end of this scale.
The problem with Perl isn't that it has no clearly-defined system of name management, but
rather that it two systems, both working at once. Here's the Big Secret about Perl variables
that most people learn too late: Perl has two completely separate, independent sets of
variables. One is left over from Perl 4, and the other is new. The two sets of variables are
called `package variables' and `lexical variables', and they have nothing to do with each
other.
Package variables came first, so we'll talk about them first. Then we'll see some problems
with package variables, and how lexical variables were introduced in Perl 5 to avoid these
problems. Finally, we'll see how to get Perl to automatically diagnose places where you might
not be getting the variable you meant to get, which can find mistakes before they turn into
bugs.
Here, $x is a package
variable . There are two important things to know about package variables:
Package variables are what you get if you don't say otherwise.
Package variables are always global.
Global means that package variables are
always visible everywhere in every program. After you do $x = 1 , any other part of
the program, even some other subroutine defined in some other file, can inspect and modify the
value of $x . There's no exception to this; package variables are always global.
Package variables are divided into families, called packages . Every package variable has a name with two parts. The two
parts are analogous to the variable's given name and family name. You can call the
Vice-President of the United States `Al', if you want, but that's really short for his full
name, which is `Al Gore'. Similarly, $x has a full name, which is something like
$main::x . The main part is the package qualifier , analogous to the `Gore' part of `Al
Gore'. Al Gore and Al Capone are different people even though they're both named `Al'. In the
same way, $Gore::Al and $Capone::Al are different variables, and
$main::x and $DBI::x are different variables.
You're always allowed to include the package part of the variable's name, and if you do,
Perl will know exactly which variable you mean. But for brevity, you usually like to leave the
package qualifier off. What happens if you do?
If you just say $x , perl assumes that you mean the variable $x in the
current package. What's the current package? It's normally main , but you can change
the current package by writing
package Mypackage;
in your program; from that point on, the current package is Mypackage . The only
thing the current package does is affect the interpretation of package variables that you wrote
without package names. If the current package is Mypackage , then $x really
means $Mypackage::x . If the current package is main , then $x
really means $main::x.
If you were writing a module, let's say the MyModule module, you would probably put
a line like this at the top of the module file:
package MyModule;
From there on, all the package variables you used in the module file would be in package
MyModule , and you could be pretty sure that those variables wouldn't conflict with
the variables in the rest of the program. It wouldn't matter if both you and the author of
DBI were to use a variable named $x , because one of those $x es
would be $MyModule::x and the other would be $DBI::x .
Remember that package variables are always global. Even if you're not in package DBI, even
if you've never heard of package DBI, nothing can stop you from reading from or writing
to $DBI::errstr . You don't have to do anything special. $DBI::errstr , like
all package variables, is a global variable, and it's available globally; all you have to do is
mention its full name to get it. You could even say
There are only three other things to know about package variables, and you might want to
skip them on the first reading:
The package with the empty name is the same as main . So $::x is the
same as $main::x for any x .
Some variables are always forced to be in package main. For example, if you mention
%ENV , Perl assumes that you mean %main::ENV , even if the current package
isn't main . If you want %Fred::ENV , you have to say so explicitly, even
if the current package is Fred . Other names that are special this way include
INC , all the one-punctuation-character names like $_ and $$ ,
@ARGV , and STDIN , STDOUT , and STDERR .
Package names, but not variable names, can contain :: . You can have a variable
named $DBD::Oracle::x. This means the variable x in the package
DBD::Oracle ; it has nothing at all to do with the package DBD which is
unrelated. Isaac Newton is not related to Olivia Newton-John, and Newton::Isaac is
not related to Newton::John::Olivia . Even though it appears that they both begin
with Newton , the appearance is deceptive. Newton::John::Olivia is in
package Newton::John , not package Newton.
That's all there is to know about package variables.
Package variables are global, which is dangerous, because you can never be sure that someone
else isn't tampering with them behind your back. Up through Perl 4, all variables were package
variables, which was worrisome. So Perl 5 added new variables that aren't
global.
Perl's other set of variables are called lexical variables (we'll see why later) or private
variables because they're private. They're also sometimes called my variables
because they're always declared with my . It's tempting to call them `local
variables', because their effect is confined to a small part of the program, but don't do that,
because people might think you're talking about Perl's local operator, which we'll see
later. When you want a `local variable', think my , not local .
The declaration
my $x;
creates a new variable, named x , which is totally inaccessible to most parts of
the program---anything outside the block where the variable was declared. This block is called
the scope of the variable. If the variable
wasn't declared in any block, its scope is from the place it was declared to the end of the
file.
You can also declare and initialize a my variable by writing something like
my $x = 119;
You can declare and initialize several at once:
my ($x, $y, $z, @args) = (5, 23, @_);
Let's see an example of where some private variables will be useful. Consider this
subroutine:
If lookup_salary happens to also use a variable named $employee , that's
going to be the same variable as the one used in print_report , and the works might
get gummed up. The two programmers responsible for print_report and
lookup_salary will have to coordinate to make sure they don't use the same variables.
That's a pain. In fact, in even a medium-sized project, it's an intolerable pain.
The solution: Use my variables:
sub print_report {
my @employee_list = @_;
foreach my $employee (@employee_list) {
my $salary = lookup_salary($employee);
print_partial_report($employee, $salary);
}
}
my @employee_list creates a new array variable which is totally inaccessible
outside the print_report function. for my $employee creates a new scalar
variable which is totally inaccessible outside the foreach loop, as does my
$salary . You don't have to worry that the other functions in the program are tampering
with these variables, because they can't; they don't know where to find them, because the names
have different meanings outside the scope of the my declarations. These `my variables'
are sometimes called `lexical' because their scope depends only on the program text itself, and
not on details of execution, such as what gets executed in what order. You can determine the
scope by inspecting the source code without knowing what it does. Whenever you see a variable,
look for a my declaration higher up in the same block. If you find one, you can be
sure that the variable is inaccessible outside that block. If you don't find a declaration in
the smallest block, look at the next larger block that contains it, and so on, until you do
find one. If there is no my declaration anywhere, then the variable is a package
variable.
my variables are not package variables. They're not part of a package, and they
don't have package qualifiers. The current package has no effect on the way they're
interpreted. Here's an example:
my $x = 17;
package A;
$x = 12;
package B;
$x = 20;
# $x is now 20.
# $A::x and $B::x are still undefined
The declaration my $x = 17 at the top creates a new lexical variable named x whose
scope continues to the end of the file. This new meaning of $x overrides the default
meaning, which was that $x meant the package variable $x in the current
package.
package A changes the current package, but because $x refers to the
lexical variable, not to the package variable, $x=12 doesn't have any effect on
$A::x . Similarly, after package B , $x=20 modifies the lexical
variable, and not any of the package variables.
At the end of the file, the lexical variable $x holds 20, and the package variables
$main::x , $A::x , and $B::x are still undefined. If you had wanted
them, you could still have accessed them by using their full names.
The maxim you must remember is:
Package variables are global variables.
For private variables, you must use my .
Almost everyone already knows that there's a local function that has something to
do with local variables. What is it, and how does it related to my ? The answer is
simple, but bizarre:
my creates a local variable. local doesn't.
First, here's what local $x really does: It saves the current value of the
package variable $x in a safe place, and replaces it with a new value, or with
undef if no new value was specified. It also arranges for the old value to be restored
when control leaves the current block. The variables that it affects are package variables,
which get local values. But package variables are always global, and a local
package variable is no exception. To see the difference, try this:
$lo = 'global';
$m = 'global';
A();
sub A {
local $lo = 'AAA';
my $m = 'AAA';
B();
}
sub B {
print "B ", ($lo eq 'AAA' ? 'can' : 'cannot') ,
" see the value of lo set by A.\n";
print "B ", ($m eq 'AAA' ? 'can' : 'cannot') ,
" see the value of m set by A.\n";
}
This prints
B can see the value of lo set by A.
B cannot see the value of m set by A.
What happened here? The local declaration in A saved a new temporary
value, AAA , in the package variable $lo . The old value, global ,
will be restored when A returns, but before that happens, A calls B
. B has no problem accessing the contents of $lo , because $lo is a
package variable and package variables are always available everywhere, and so it sees the
value AAA set by A .
In contrast, the my declaration created a new, lexically scoped variable named
$m , which is only visible inside of function A . Outside of A ,
$m retains its old meaning: It refers the the package variable $m ; which is
still set to global . This is the variable that B sees. It doesn't see the
AAA because the variable with that value is a lexical variable, and only exists inside
of A .
Because local does not actually create local variables, it is not very much use.
If, in the example above, B happened to modify the value of $lo , then the
value set by A would be overwritten. That is exactly what we don't want to happen. We want each
function to have its own variables that are untouchable by the others. This is what my
does.
Why have local at all? The answer is 90% history. Early versions of Perl only had
global variables. local was very easy to implement, and was added to Perl 4 as a
partial solution to the local variable problem. Later, in Perl 5, more work was done, and real
local variables were put into the language. But the name local was already taken, so
the new feature was invoked with the word my . my was chosen because it
suggests privacy, and also because it's very short; the shortness is supposed to encourage you
to use it instead of local . my is also faster than local
.
Every time control reaches a my declaration, Perl creates a new, fresh variable.
For example, this code prints x=1 fifty times:
for (1 .. 50) {
my $x;
$x++;
print "x=$x\n";
}
You get a new $x , initialized to undef , every time through the loop.
If the declaration were outside the loop, control would only pass by it once, so there would
only be one variable:
{ my $x;
for (1 .. 50) {
$x++;
print "x=$x\n";
}
}
This prints x=1 , x=2 , x=3 , ... x=50 .
You can use this to play a useful trick. Suppose you have a function that needs to remember
a value from one call to the next. For example, consider a random number generator. A typical
random number generator (like Perl's rand function) has a seed in it. The seed
is just a number. When you ask the random number generator for a random number, the function
performs some arithmetic operation that scrambles the seed, and it returns the result. It also
saves the result and uses it as the seed for the next time it is called.
Here's typical code: (I stole it from the ANSI C standard, but it behaves poorly, so don't
use it for anything important.)
There's a problem here, which is that $seed is a global variable, and that means we
have to worry that someone might inadvertently tamper with it. Or they might tamper with it on
purpose, which could affect the rest of the program. What if the function were used in a
gambling program, and someone tampered with the random number generator?
But we can't declare $seed as a my variable in the function:
sub my_rand {
my $seed;
$seed = int(($seed * 1103515245 + 12345) / 65536) % 32768;
return $seed;
}
If we did, it would be initialized to undef every time we called my_rand .
We need it to retain its value between calls to my_rand .
The declaration is outside the function, so it only happens once, at the time the program is
compiled, not every time the function is called. But it's a my variable, and it's in a
block, so it's only accessible to code inside the block. my_rand is the only other
thing in the block, so the $seed variable is only accessible to the my_rand
function.
$seed here is sometimes called a `static' variable, because it stays the same in
between calls to the function. (And because there's a similar feature in the C language that is
activated by the static keyword.)
You can't declare a variable my if its name is a punctuation character, like
$_ , @_ , or $$ . You can't declare the backreference variables
$1 , $2 , ... as my . The authors of my thought that that
would be too confusing.
Obviously, you can't say my $DBI::errstr , because that's contradictory---it
says that the package variable $DBI::errstr is now a lexical variable. But you
can say local $DBI::errstr ; it saves the current value of
$DBI::errstr and arranges for it to be restored at the end of the block.
New in Perl 5.004, you can write
foreach my $i (@list) {
instead, to confine the $i to the scope of the loop instead. Similarly,
If you're writing a function, and you want it to have private variables, you need to declare
the variables with my . What happens if you forget?
sub function {
$x = 42; # Oops, should have been my $x = 42.
}
In this case, your function modifies the global package variable $x . If you were
using that variable for something else, it could be a disaster for your program.
Recent versions of Perl have an optional protection against this that you can enable if you
want. If you put
use strict 'vars';
at the top of your program, Perl will require that package variables have an explicit
package qualifier. The $x in $x=42 has no such qualifier, so the program
won't even compile; instead, the compiler will abort and deliver this error message:
Global symbol "$x" requires explicit package name at ...
If you wanted $x to be a private my variable, you can go back and add the
my . If you really wanted to use the global package variable, you could go back and
change it to
$main::x = 42;
or whatever would be appropriate.
Just saying use strict turns on strict vars , and several other checks
besides. See perldoc strict for more details.
Now suppose you're writing the Algorithms::KnuthBendix modules, and you want the
protections of strict vars But you're afraid that you won't be able to finish the
module because your fingers are starting to fall off from typing
$Algorithms::KnuthBendix::Error all the time.
Package variables are always global. They have a name and a package qualifier. You can omit
the package qualifier, in which case Perl uses a default, which you can set with the
package declaration. For private variables, use my . Don't use local
; it's obsolete.
You should avoid using global variables because it can be hard to be sure that no two parts
of the program are using one another's variables by mistake.
To avoid using global variables by accident, add use strict 'vars' to your program.
It checks to make sure that all variables are either declared private, are explicitly qualified
with package qualifiers, or are explicitly declared with use vars .
The tech editors complained about my maxim `Never use local .' But 97% of the
time, the maxim is exactly right. local has a few uses, but only a few, and they
don't come up too often, so I left them out, because the whole point of a tutorial article is
to present 97% of the utility in 50% of the space.
I was still afraid I'd get a lot of tiresome email from people saying ``You forgot to
mention that local can be used for such-and-so, you know.'' So in the colophon at
the end of the article, I threatened to deliver Seven Useful Uses for local
in three months. I mostly said it to get people off my back about local . But it
turned out that I did write it, and it was published some time later.
Here's another potentially interesting matter that I left out for space and clarity. I
got email from Robert Watkins with a program he was writing that didn't work. The essence of
the bug looked like this:
my $x;
for $x (1..5) {
s();
}
sub s { print "$x, " }
Robert wanted this to print 1, 2, 3, 4, 5, but it did not. Instead, it printed
, , , , , . Where did the values of $x go?
The deal here is that normally, when you write something like this:
for $x (...) { }
Perl wants to confine the value of the index variable to inside the loop. If $x
is a package variable, it pretends that you wrote this instead:
{ local $x; for $x (...) { } }
But if $x is a lexical variable, it pretends you wrote this instead, instead:
{ my $x; for $x (...) { } }
This means that the loop index variable won't get propagated to subroutines, even if
they're in the scope of the original declaration.
I probably shouldn't have gone on at such length, because the perlsyn manual page
describes it pretty well:
...the variable is implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with my , it uses that
variable instead of the global one, but it's still localized to the loop. (Note that a
lexically scoped variable can cause problems if you have subroutine or format
declarations within the loop which refer to it.)
In my opinion, lexically scoping the index variable was probably a mistake. If you had
wanted that, you would have written for my $x ... in the first place. What I would
have liked it to do was to localize the lexical variable: It could save the value of the
lexical variable before the loop, and restore it again afterwards. But there may be
technical reasons why that couldn't be done, because this doesn't work either:
my $m;
{ local $m = 12;
...
}
The local fails with this error message:
Can't localize lexical variable $m...
There's been talk on P5P about making this work, but I gather it's not trivial.
Added 2000-01-05: Perl 5.6.0 introduced a new our(...) declaration. Its syntax
is the same as for my() , and it is a replacement for use vars .
Without getting into the details, our() is just like use vars ; its
only effect is to declare variables so that they are exempt from the strict 'vars'
checking. It has two possible advantages over use vars , however: Its syntax is
less weird, and its effect is lexical. That is, the exception that it creates to the
strict checking continues only to the end of the current block:
use strict 'vars';
{
our($x);
$x = 1; # Use of global variable $x here is OK
}
$x = 2; # Use of $x here is a compile-time error as usual
So whereas use vars '$x' declares that it is OK to use the global variable
$x everywhere, our($x) allows you to say that global $x should
be permitted only in certain parts of your program, and should still be flagged as an error
if you accidentally use it elsewhere.
Added 2000-01-05: Here's a little wart that takes people by surprise. Consider the
following program:
use strict 'vars';
my @lines = <>;
my @sorted = sort backwards @lines;
print @sorted;
sub backwards { $b cmp $a }
Here we have not declared $a or $b , so they are global variables. In
fact, they have to be global, because the sort operator must to be able to set
them up for the backwards function. Why doesn't strict produce a
failure?
The variables $a and $b are exempted from strict vars
checking, for exactly this reason.
A package is a collection of code which lives in its own namespace
A namespace is a named collection of unique variable names (also called a symbol table).
Namespaces prevent variable name collisions between packages
Packages enable the construction of modules which, when used, won't clobbber variables and functions outside of the modules's
own namespace
The Package Statement
package statement switches the current naming context to a specified namespace (symbol table)
If the named package does not exists, a new namespace is first created.
$i = 1; print "$i\n"; # Prints "1"
package foo;
$i = 2; print "$i\n"; # Prints "2"
package main;
print "$i\n"; # Prints "1"
The package stays in effect until either another package statement is invoked, or until the end of the end of the current
block or file.
You can explicitly refer to variables within a package using the :: package qualifier
$PACKAGE_NAME::VARIABLE_NAME
For Example:
$i = 1; print "$i\n"; # Prints "1"
package foo;
$i = 2; print "$i\n"; # Prints "2"
package main;
print "$i\n"; # Prints "1"
print "$foo::i\n"; # Prints "2"
BEGIN and END Blocks
You may define any number of code blocks named BEGIN and END which act as constructors and destructors respectively.
BEGIN { ... }
END { ... }
BEGIN { ... }
END { ... }
Every BEGIN block is executed after the perl script is loaded and compiled but before any other statement is executed
Every END block is executed just before the perl interpreter exits.
The BEGIN and END blocks are particularly useful when creating Perl modules.
What are Perl Modules?
A Perl module is a reusable package defined in a library file whose name is the same as the name of the package (with a .pm on
the end).
A Perl module file called "Foo.pm" might contain statements like this.
#!/usr/bin/perl
package Foo;
sub bar {
print "Hello $_[0]\n"
}
sub blat {
print "World $_[0]\n"
}
1;
Few noteable points about modules
The functions require and use will load a module.
Both use the list of search paths in @INC to find the module (you may modify it!)
Both call the eval function to process the code
The 1; at the bottom causes eval to evaluate to TRUE (and thus not fail)
The Require Function
A module can be loaded by calling the require function
#!/usr/bin/perl
require Foo;
Foo::bar( "a" );
Foo::blat( "b" );
Notice above that the subroutine names must be fully qualified (because they are isolated in their own package)
It would be nice to enable the functions bar and blat to be imported into our own namespace so we wouldn't have to use the Foo::
qualifier.
The Use Function
A module can be loaded by calling the use function
#!/usr/bin/perl
use Foo;
bar( "a" );
blat( "b" );
Notice that we didn't have to fully qualify the package's function names?
The use function will export a list of symbols from a module given a few added statements inside a module
require Exporter;
@ISA = qw(Exporter);
Then, provide a list of symbols (scalars, lists, hashes, subroutines, etc) by filling the list variable named @EXPORT :
For Example
package Module;
require Exporter;
@ISA = qw(Exporter);
@EXPORT = qw(bar blat);
sub bar { print "Hello $_[0]\n" }
sub blat { print "World $_[0]\n" }
sub splat { print "Not $_[0]\n" } # Not exported!
1;
Create the Perl Module Tree
When you are ready to ship your PERL module then there is standard way of creating a Perl Module Tree. This is done using h2xs
utility. This utility comes alongwith PERL. Here is the syntax to use h2xs
$h2xs -AX -n Module Name
# For example, if your module is available in Person.pm file
$h2xs -AX -n Person
This will produce following result
Writing Person/lib/Person.pm
Writing Person/Makefile.PL
Writing Person/README
Writing Person/t/Person.t
Writing Person/Changes
Writing Person/MANIFEST
Here is the descritpion of these options
-A omits the Autoloader code (best used by modules that define a large number of infrequently used subroutines)
-X omits XS elements (eXternal Subroutine, where eXternal means external to Perl, i.e. C)
-n specifies the name of the module
So above command creates the following structure inside Person directory. Actual result is shown above.
Changes
Makefile.PL
MANIFEST (contains the list of all files in the package)
README
t/ (test files)
lib/ ( Actual source code goes here
So finally you tar this directory structure into a file Person.tar and you can ship it. You would have to update README
file with the proper instructions. You can provide some test examples files in t directory.
Installing Perl Module
Installing a Perl Module is very easy. Use the following sequence to install any Perl Module.
perl Makefile.PL
make
make install
The Perl interpreter has a list of directories in which it searches for modules (global array @INC)
"... It baffles me the most because the common objection to Perl is legibility. Even if you assume that the objection is made from ignorance - i.e. not even having looked at some Perl to gauge its legibility - the nonsense you see in a complex bash script is orders of magnitude worse! ..."
"... Maybe it's not reassuring to hear that, but I took an interest in Perl precisely because it's seen as an underdog and "dead" despite having experienced users and a lot of code, kind of like TCL, Prolog, or Ada. ..."
"... There's a long history of bad code written by mediocre developers who became the only one who could maintain the codebase until they no longer worked for the organization. The next poor sap to go in found a mess of a codebase and did their best to not break it further. After a few iterations, the whole thing is ready for /dev/null and Perl gets the blame. ..."
"... All in all, Perl is still my first go-to language, but there are definitely some things I wish it did better. ..."
"... The Perl leadership Osborned itself with Perl6. 20/20 hindsight says the new project should have been given a different name at conception, that way all the "watch this space -- under construction" signage wouldn't have steered people away from perfectly usable Perl5. Again, IMO. ..."
"... I don't observe the premise at all though. Is bash really gaining ground over anything recently? ..."
"... Python again is loved, because "taught by rote" idiots. Now you can give them pretty little packages. And it's no wonder they can do little better than be glorified system admins (which id rather have a real sys admin, since he's likely to understand Perl) ..."
"... Making a new language means lots of new training. Lots of profit in this. Nobody profits from writing new books on old languages. Lots of profit in general from supporting a new language. In the end, owning the language gets you profits. ..."
"... And I still don't get why tab for blocks python is even remotely more readable than Perl. ..."
"... If anything, JavaScript is pretty dang godly at what it does, I understand why that's popular. But I don't get python one bit, except to employ millions of entry level minions who can't think on their own. ..."
"... "Every teacher I know has students using it. We do it because it's an easy language, there's only one way to do it, and with whitespace as syntax it's easy to grade. We don't teach it because it is some powerful or exceptional language. " ..."
Setting aside Perl vs. Python for the moment, how did Perl lose ground to Bash? It used to be that Bash scripts often got replaced
by Perl scripts because Perl was more powerful. Even with very modern versions of Bash, Perl is much more powerful.
The Linux Standards Base (LSB) has helped ensure that certain tools are in predictable locations. Bash has gotten a bit more powerful
since the release of 4.x, sure. Arrays, handicapped to 2-D arrays, have improved somewhat. There is a native regex engine in Bash
3.x, which admit is a big deal. There is also support for hash maps.
This is all good stuff for Bash. But, none of this is sufficient to explain why Perl isn't the thing you learn after Bash, or,
after Bash and Python; take your pick. Thoughts?
Because Perl has suffered immensely in the popularity arena and is now viewed as undesirable. It's not that Bash is seen as
an adequate replacement for Perl, that's where Python has landed.
- "thou must use Moose for everything" -> "Perl is too slow" -> rewrite in Python because the architect loves Python -> Python
is even slower -> architect shunned by the team and everything new written in Go, nobody dares to complain about speed now because
the budget people don't trust them -> Perl is slow
- "globals are bad, singletons are good" -> spaghetti -> Perl is unreadable
- "lets use every single item from the gang of four book" -> insanity -> Perl is bad
- "we must be more OOP" -> everything is a faux object with everything else as attributes -> maintenance team quits and they
all take PHP jobs, at least the PHP people know their place in the order of things and do less hype-driven-development -> Perl
is not OOP enough
- "CGI is bad" -> app needs 6.54GB of RAM for one worker -> customer refuses to pay for more RAM, fires the team, picks a PHP
team to do the next version -> PHP team laughs all the way to the bank, chanting "CGI is king"
It baffles me the most because the common objection to Perl is legibility. Even if you assume that the objection is made
from ignorance - i.e. not even having looked at some Perl to gauge its legibility - the nonsense you see in a complex bash script
is orders of magnitude worse!
Not to mention its total lack of common language features like first-class data and... Like, a compiler...
I no longer write bash scripts because it takes about 5 lines to become unmaintainable.
When I discuss projects with peers and mention that I chose to develop in Perl, the responses range from passive bemusement,
to scorn, to ridicule. The assumption is usually that I'm using a dead language that's crippled in functionality and uses syntax
that will surely make everyone's eyes bleed to read. This is the culture everywhere from the casual hackers to the C-suite.
I've proven at work that I can write nontrivial software using Perl. I'm still asked to use Python or Go (edit: or node, ugh)
for any project that'll have contributors from other teams, or to containerize apps using Docker to remove the need for Perl knowledge
for end-users (no CPAN, carton, etc.). But I'll take what I can get, and now the attitude has gone from "get with the times" or
"that's cute", to "ok but I don't expect everyone else to know it".
Perl has got a lot to offer, and I vastly enjoy using it over other languages I work with. I know that all the impassioned
figures in the Perl community love it just the same, but the community's got some major fragmentation going on. I understand that
everyone's got ideas about the future of the language, but is this really the best time to pull the community apart? I feel like
if everyone was able to let go of their ego and put their heads together to bring us to a point of stability, even a place where
we're not laughed at for professing our support for the language, it would be a major step in the right direction. I think we're
heading to the bottom fast, otherwise.
In that spirit of togetherness, I think the language, particularly the community, needs to be made more accessible to newcomers.
Not accessible to one Perl offshoot, but accessible to Perl. It needs to be decided what Perl means in today's day and age. What
can it do? Why would I want to use it over another shiny language? What are the definitive places I can go to learn more? Who
else will be there? How do I contribute and grow as a Perl developer? There need to be people talking about Perl in places that
aren't necessarily hubs for other Perl enthusiasts. It needs to be something business decision-makers can look at and feel confident
in using.
I really hope something changes. I'd be pretty sad if I had to spend the rest of my career writing whatever the trendy
language of the day is. These are just observations from someone that likes writing Perl and has been watching from the sidelines.
Maybe it's not reassuring to hear that, but I took an interest in Perl precisely because it's seen as an underdog and "dead"
despite having experienced users and a lot of code, kind of like TCL, Prolog, or Ada.
Being able to read Modern Perl for
free also helped a lot. I'm still lacking experience in Perl and I've yet to write anything of importance in it because I don't
see an area in which it's clearly better than anything else, either because of the language, a package, or a framework, and I
don't do a lot of text-munging anymore (I'm also a fan of awk so for small tasks it has the priority).
Don't call it Perl. Unfortunately. Also IME multitasking in Perl5 (or the lack thereof and/or severe issues with) has been
a detriment to it's standing in a "multithread all the things" world.
So often I see people drag themselves down that "thread my app" path. Eventually realize that they are implementing a whole
multi-processing operating system inside their app rather than taking advantage of the perfectly good one they are running on.
There are several perfectly good ways to do concurrency, multitasking, async IO and so on in perl. Many work well in the single
node case and in the multi-node case. Anyone who tells you that multitasking systems are easy because of some implementation language
choice has not made it through the whole Dunning Kruger cycle yet.
Multithreading is never easy. The processors will always manage to do things in a "wrong" order unless you are very careful
with your gatekeeping. However, other languages/frameworks have paradigms that make it seem easier such that those race conditions
show up much later in your product lifecycle.
There's a long history of bad code written by mediocre developers who became the only one who could maintain the codebase
until they no longer worked for the organization. The next poor sap to go in found a mess of a codebase and did their best to
not break it further. After a few iterations, the whole thing is ready for /dev/null and Perl gets the blame.
Bash has limitations, but that (usually) means fewer ways to mess it up. There's less domain knowledge to learn, (afaik) no
CPAN equivalent, and fewer issues with things like "I need to upgrade this but I can't because this other thing uses this older
version which is incompatible with the newer version so now we have to maintain two versions of the library and/or interpreter."
All in all, Perl is still my first go-to language, but there are definitely some things I wish it did better.
Perl has a largish executable memory-footprint*. If that gets in your way (which can happen in tight spaces such as semi/embedded),
you've got two choices: if it's shellable code, go to bash; otherwise, port to C. Or at least, that's my decision tree, and Perl5
is my go-to language. I use bash only when I must, and I hit the books every time.
The Perl leadership Osborned itself with Perl6. 20/20 hindsight says the new project should have been given a different
name at conception, that way all the "watch this space -- under construction" signage wouldn't have steered people away from perfectly
usable Perl5. Again, IMO.
*[e:] Consider, not just core here, but CPAN pull-in as well. I had one project clobbered on a smaller-memory machine when
I tried to set up a pure-Perl scp transfer -- there wasn't room enough for the full file to transfer if it was larger than about
50k, what with all the CPAN. Shelling to commandline scp worked just fine.
To be fair, wrapping a Perl script around something that's (if I read your comment right) just running SCP is adding a pointless
extra layer of complexity anyway.
It's a matter of using the best tool for each particular job, not just sticking with one. My own ~/bin directory has a big
mix of Perl and pure shell, depending on the complexity of the job to be done.
Agreed; I brought that example up to illustrate the bulk issue. In it, I was feeling my way, not sure how much finagling I
might have to do for the task (backdoor-passing legitimate sparse but possibly quite bulky email from one server to another),
which is why I initially went for the pure-Perl approach, so I'd have the mechanics exposed for any needed hackery. The experience
taught me to get by more on shelling to precompiled tooling where appropriate... and a healthy respect for CPAN pull-in, [e:]
the way that this module depends on that module so it gets pulled in along with its dependencies in turn, and the pileup
grows in memory. There was a time or two here and there where I only needed a teeny bit of what a module does, so I went in and
studied the code, then implemented it internally as a function without the object's generalities and bulk. The caution learned
on ancient x86 boxes now seems appropriate on ARM boards like rPi; what goes around comes around.
wouldn't have steered people away from perfectly usable Perl5
Perl5 development was completely stalled at the time. Perl6 brought not only new blood into it's own effort, it reinvigorated
Perl5 in the process.
It's completely backwards to suggest Perl 5 was fine until perl6 came along. It was almost dormant and became a lively language
after Perl 6 was announced.
Perl is better than pretty much everything g out there at what it does.
But keep in mind,
They say C sharp is loved by everyone, when in reality it's Microsoft pushing their narrative and the army of "learn by rote"
engineers In developing countries
Python again is loved, because "taught by rote" idiots. Now you can give them pretty little packages. And it's no wonder
they can do little better than be glorified system admins (which id rather have a real sys admin, since he's likely to understand
Perl)
Making a new language means lots of new training. Lots of profit in this. Nobody profits from writing new books on old
languages. Lots of profit in general from supporting a new language. In the end, owning the language gets you profits.
And I still don't get why tab for blocks python is even remotely more readable than Perl.
If anything, JavaScript is pretty dang godly at what it does, I understand why that's popular. But I don't get python one
bit, except to employ millions of entry level minions who can't think on their own.
I know a comp sci professor. I asked why he thought Python was so popular.
"Every teacher I know has students using it. We do it because it's an easy language, there's only one way to do it, and
with whitespace as syntax it's easy to grade. We don't teach it because it is some powerful or exceptional language. "
Then he said if he really needs to get something done, it's Perl or C.
Perl has a steeper and longer learning with it. curve than Python, and there is more than one way to do anything. And there
quite a few that continue coding
Min and max
functions are available in perl, but you need to load them first. To do this, add
use List::Util qw[min max];
to the top of the script. These functions take a list of numbers and return the min/max of
that list. The list can have 2 numbers or 100 – it doesn't matter:
Sunday, August 2, 2009Cute Perl Gem to Get the Minimum/Maximum Value Saw this
little nugget on #[email protected] the other night. It determines the minimum of two
values:
[$b, $a]->[$a <= $b]
It takes advantage of the fact that Perl doesn't have a Boolean return type for true or false,
so the comparison operators return 1 or 0 for true and false, respectively, which are then used
by this code to index the array ref.
To get the maximum of the two values, just flip the operator to >= Posted by Luke at
9:41 PM
Perl enables you to write powerful programs right from the start, whether you're a
programming novice or expert. Perl offers the standard programming tools -- comparison
operators, pattern-matching quantifiers, list functions -- and has shortcuts for inputting
character ranges. Perl also offers file tests so you can find what you want fast.
The
Most Useful File Tests in Perl
Programming with Perl is fairly straightforward, which runs to the letters you use for file
tests. For example, r tests whether a file can be r ead, and T looks for a
t ext file. Here are most useful file tests in Perl:
Test
Description
-e
File exists.
-r
File can be read.
-w
File can be written to.
-z
File is exactly zero bytes long.
-d
Named item is a directory, not a file.
-T
File is a text file. (The first chunk of a file is examined,
and it's a text file if fewer than 30 percent or so of the
characters are nonprintable.)
-B
File is a binary file. (This is the exact opposite of the -T
test -- it's a binary file if more than 30 percent or so
of the characters are nonprintable.)
-s
Size of the file in bytes.
-C
Creation age of file.
-A
Access age of file.
-M
Modification age of file.
Special Characters in Perl
Like any programming language, Perl uses special commands for special characters, such as
backspaces or vertical tabs. So, if you need to program in a bell or a beep or just a carriage
return, check the following table for the character that will produce it:
Character
Meaning
n
Newline
r
Carriage return
t
Tab character
f
Formfeed character
b
Backspace character
v
Vertical tab
a
Bell or beep
e
Escape character
Perl True-False Comparison Operators
When you're programming with Perl -- or any other language -- you use comparison operators
all the time. The following table shows the common comparisons for Perl in both math and string
form:
Comparison
Math
String
Equal to
==
eq
Not equal to
!=
ne
Less than
<
lt
Greater than
>
gt
Less than or equal to
<=
le
Greater than or equal to
>=
ge
Common List Functions in Perl
Perl was originally designed to help process reports more easily. Reports often contain
lists, and you may want to use Perl to perform certain functions within a list. The following
table shows you common list functions, their splice equivalents, and explains what the function
does:
Function
splice Equivalent
What It Does
push (@r, @s)
splice(@r, $#r+1,0, @s)
Adds to the right of the list
pop (@r)
splice(@r, $#r, 1)
Removes from the right of the list
shift (@r)
splice(@r, 0, 1)
Removes from the left of the list
unshift (@r, @s)
splice(@r, 0, 0,@s)
Adds to the left of the list
Shortcuts for Character Ranges in Perl
You're programming along in Perl and want to use a code shortcut to represent anything from
a number to a non-number to any letter or number. You're in luck, because the following table
gives you the code, shows you what it's a shortcut for, and describes it.
Code
Replaces
Description
d
[0..9]
Any digit
w
[a-zA-Z_0-9]
Any alphanumeric character
s
[ tnrf]
A whitespace character
D
^[0..9]
Any non-digit
W
^[a-zA-Z_0-9]
Any non-alphanumeric character
S
^[ tnrf]
A non-whitespace character
Perl Pattern-Matching Quantifiers
Perl enables you to use common symbols to instruct the program you're writing to match data
once, never, or up to a certain number of times. The following table shows you which symbol to
use to get the match you want:
Symbol
Meaning
+
Match 1 or more times
*
Match 0 or more times
?
Match 0 or 1 time
{n}
Match exactly n times
{n,}
Match at least n times
{n,m}
Match at least n, but not more than m, times (these values must
be less than 65,536)
I'm trying to parse a single string and get multiple chunks of data out from the same string
with the same regex conditions. I'm parsing a single HTML doc that is static (For an
undisclosed reason, I can't use an HTML parser to do the job.) I have an expression that
looks like:
$string =~ /\<img\ssrc\="(.*)"/;
and I want to get the value of $1. However, in the one string, there are many img tags
like this, so I need something like an array returned (@1?) is this possible?
Entering a typo or two during the course of writing a Perl program is not uncommon. But when
you attempt to run a program containing a text-entry slip-up, Perl usually becomes confused and
tells you so by reporting an error. The natural reaction for most people, even those with years
of programming experience, is to get worried or angry or both when an error message pops
up.
Don't panic. Take a deep breath. Take another slow, deep breath. Seriously, you can't get to
the root of the problem if you're all tense and bothered. No matter how many years you program,
you always end up finding some errors in the code you're written.
So, now that you are (hopefully!) a bit calmer, you can start to appreciate the fact that
Perl has more helpful error messages than almost any other programming language. The messages
aren't always right on the money, but they can get you pretty close to the spot where the
problem lies with minimal searching on your part.
Perl has myriad error messages, but a few definitely crop up more than others owing to some
common typos that everyone seems to make. The following errors result from minor text-entry
goofs that you can easily avoid.
Forgetting a semicolon
Probably the most common error message you see when programming in Perl looks something like
this:
# syntax error, near "open"
File 'counter1.pl'; Line 10
# Execution aborted due to compilation errors.
You can look and look at Line 10, the one with the open statement, and you won't see
anything wrong with it. The trick here is to examine the statement that comes before the
open statement and see whether it ends with a semicolon. (Perl knows that a statement ends only
when it encounters a semicolon.) In this case, the error is caused by a missing semicolon at
the end of Line 7 of the program:
$TheFile = "sample.txt"
Forgetting a quotation mark
The following sort of error message can be extremely frustrating if you don't know of a
quick fix:
# Bare word found where operator expected, near
# "open(INFILE, $TheFile) or die "The"
# (Might be a runaway multi-line " string starting on
# line 7)
File 'counter1.pl'; Line 10
This error is similar to forgetting a semicolon; instead, it's a quotation mark that's
accidentally omitted:
$TheFile = "sample.txt;
In this case, Perl did a good job of guessing what is wrong, suggesting that a runaway
multi-line " string on Line 7 is the problem, which is precisely right.
Entering one
parenthesis too many or too few
When you have loads of opening and closing parentheses in a program, it's easy to slip an
extra one in by accident. If that's the case, you may see a message from Perl that reads
something like this:
# syntax error, near ") eq"
File 'counter1.pl'; Line 38
# syntax error, near "}"
File 'counter1.pl'; Line 42
Here, Perl can't determine where the error is exactly, but it actually got it right on the
first guess: Line 38 contains an extra right parenthesis:
if(substr($TheLine, $CharPos, 1)) eq " ")
Having one parenthesis too few in a Perl program can cause harder-to-find problems:
# Can't use constant item as left arg of implicit -- >,
# near "1 }"
File 'counter1.pl'; Line 39
# Scalar found where operator expected, near "$CharPos"
File 'counter1.pl'; Line 40
# (Missing semicolon on previous line?)
# syntax error, near "$CharPos "
File 'counter1.pl'; Line 40
Yarp! All this was produced because the last parenthesis on Line 38 is missing:
if(substr($TheLine, $CharPos, 1) eq " "
Here is another good lesson in hunting down typing errors: Start where Perl says it found an
error. If you don't find the error there, go up a line or two and see if the problem started
earlier.
A final word of advice: Trust Perl to find the simple typos for you (where it can), and
remember that it's giving you all the help it can, which is more than you can say for many
programming languages.
to enable an experimental switch feature. This is loosely based on an old version of a Perl
6 proposal, but it no longer resembles the Perl 6 construct. You also get the switch feature
whenever you declare that your code prefers to run under a version of Perl that is 5.10 or
later. For example:
Under the "switch" feature, Perl gains the experimental keywords given , when , default ,
continue , and break . Starting from
Perl 5.16, one can prefix the switch keywords with CORE:: to access the feature
without a use
feature statement. The keywords given and when are analogous to
switch and case in other languages -- though continue is not -- so the
code in the previous section could be rewritten as
The arguments to given and when are in scalar context, and
given
assigns the $_ variable its topic value.
Exactly what the EXPR argument to when does is hard to describe
precisely, but in general, it tries to guess what you want done. Sometimes it is interpreted as
$_ ~~ EXPR , and sometimes it is not. It also behaves differently when
lexically enclosed by a given block than it does when
dynamically enclosed by a foreach loop. The rules are far
too difficult to understand to be described here. See Experimental
Details on given and when later on.
Due to an unfortunate bug in how given was implemented between Perl
5.10 and 5.16, under those implementations the version of $_ governed by
given
is merely a lexically scoped copy of the original, not a dynamically scoped alias to the
original, as it would be if it were a foreach or under both the original
and the current Perl 6 language specification. This bug was fixed in Perl 5.18 (and lexicalized
$_ itself was removed in Perl 5.24).
If your code still needs to run on older versions, stick to foreach for your topicalizer and
you will be less unhappy.
die " Reports of my death are greatly exaggerated . \n "
Perl is alive and well, but it has steadily been losing promise over the past 20
years.
It's still heavily used for the tasks it was used for when I learnt it, in 1994–1995,
but at that time, it looked set for an even brighter future: it was developing into one of the
top-5 languages, a universal scripting language, a language you expect to find wherever
scripting or dynamically typed languages are appropriate.
You can still find evidence of that today: some software has an extension API in Perl, some
web applications are written in Perl, some larger system administration software is written in
Perl, etcetera. But these systems are typically 20 years old. If you do this today, be prepared
to justify yourself.
This is not because Perl has become any less suitable for doing these things. On the
contrary, it has continued to improve. Yet, people have turned away from Perl, towards newer
scripting languages such as Python, PHP, Ruby, and Lua, for tasks that in 1995 they would
probably have used Perl for.
Why?
I believe the reason is simple: Perl is very free, syntactically and semantically. This
makes it very good at what it was designed to do (scripting) but less suited for larger-scale
programming.
Perl's syntactic freedom mostly originates from its mimicking idioms from other languages.
It was designed to be a suitable replacement for other scripting languages, most notably the
Bourne shell (
/bin/ sh ) and awk , so it adopts some of their idioms. This is
perfect if you like these idioms for their compactness.
For instance, in the Bourne shell, we can write
if mkdir $directory
then
echo successfully created directory : $directory
elif test - d $directory
then
echo pre - existing directory : $directory
else
echo cannot create directory : $directory
fi
In the Bourne shell, every statement is a Unix command invocation; in this case,
test and mkdir . (Some commands, such as test , were
built into the shell later.) Every command will succeed or fail, so we can use it in the
condition of an if statement.
Now what if we only want to print a warning when something went wrong? We can write
this:
if mkdir $directory
then
: # nothing
elif test - d $directory
then
: # nothing
else
echo cannot create directory : $directory
fi
or we can combine the two conditions:
if mkdir $directory || test - d $directory
then
: # nothing
else
echo cannot create directory : $directory
fi
or we can combine them even further:
mkdir $directory ||
test - d $directory ||
echo cannot create directory : $directory
These all do the same exact thing; clearly, the last version is the most compact. In a shell
script with a lot of tests like this, writing things this way can save a considerable amount of
space. Especially in throwaway scripts of a few lines, it's a lot easier to use more compact
syntax.
Most programmers are familiar with seeing some special syntax for conditions in
if statements. For this reason, Unix has the [ command, which scans
its arguments for a matching ], and then invokes test with the arguments up to
that point. So we can always replace
test - d $directory
with
[ - d $directory ]
in the pieces of code above. It means the same thing.
Now, Perl comes onto the scene. It is designed to be easy to replace Bourne shell scripts
with. This is a very frequent use case for Perl, even today: I regularly find myself rewriting
my Bourne shell scripts into Perl by going through them line by line.
So what do the Perl replacements of the above look like?
Here we go:
if ( mkdir $directory )
{
# nothing
} elsif (- d $directory )
{
# nothing
} else {
say "cannot create directory: $directory"
}
or we can combine the two conditions:
if ( mkdir $directory || - d $directory )
{
# nothing
} else {
say "cannot create directory: $directory"
}
or we can combine them even further:
mkdir $directory or
- d $directory or
say "cannot create directory: $directory"
As you can see, these are literal transliterations of the corresponding Bourne shell
fragments.
In a language such as Java, you can use the first two forms, but not the third one. In such
languages, there is a syntactic separation between expressions , which yield a value,
and must be used in a context that demands such a value, and statements , which do not
yield a value, and must be used in contexts that do not demand one. The third form is
syntactically an expression, used in a context that demands a statement, which is invalid in
such a language.
No such distinction is made in Perl, a trait it inherited from the Bourne shell, which in
turn took it from Algol 68.
So here we have an example of syntactic freedom in Perl that many other languages lack, and
in this case, Perl took it from the Bourne shell.
Allowing more compactness isn't the only reason for this freedom. The direct reason the
Bourne shell doesn't make the distinction is that it relies on Unix commands, which do not make
the distinction, either. Every Unix command can return a value (a return code) to indicate
whether it failed and how. Therefore, it acts both as a statement and as a condition. There is
a deeper reason behind this: concurrency.
For instance, when we want to create a directory, we can't separate doing it from testing
whether it can/could be done. We could try and write something like
if ( some test to see if we can mkdir $directory )
then
mkdir directory
fi
if ( some test to see if we managed to mkdir directory )
then
[...]
fi
but that logic isn't correct. Unix is a multiprogramming environment, so anything could
happen between our first test and our mkdir command, and before our mkdir command
and the second test. Someone else might create that directory or remove it, or do something
else that causes problems. Therefore, the only correct way to write code that tries to create a
directory and determines whether it succeeds is to actually issue the mkdir command and
check the value it returned. Which is what the constructs above do.
A shortcut like
mkdir $directory or
- d $directory or
say "cannot create directory: $directory"
is just a consequence. Of course, you can still object to using it for stylistic reasons,
but at least the construct makes sense once you know its origins.
Programmers who are unfamiliar with the paradigm of mixing statements and expressions, who
have never seen any but the simplest of Bourne shell scripts, who have only been given
programming tasks in which their program calls all the shots and nothing else can interfere,
have never encountered a reason to treat statements and expressions as the same thing. They
will be taken aback by a construct like this. I can't read this , they will mutter,
it's incomprehensible gibberish . And if Perl is the first language they've seen that
allows it, they will blame Perl. Only because they were never subjected to a large amount of
Bourne shell scripting. Once you can read that, you can read anything ; Perl will look
pretty tame in comparison.
Similar reasons can be given for most of the other syntactical freedom in Perl. I must say,
Perl sometimes seems to make a point of being quirky, and I find some of the resulting oddities
hard to justify, but they do make sense in context. The overall motivation is compactness. In
scripting, where you type a lot and throw away a lot, the ability to write compact code is a
great virtue.
Due to these syntactic quirks, Perl got a reputation for being a write-only language -
meaning that when programmer A is faced with programmer B 's code, B may
have used all kinds of idioms that A is unfamiliar with, causing delays for A .
There is some truth to this, but the problem is exaggerated: syntax is the first thing you
notice about a program, which is why it sticks out, but it's pretty superficial: new syntax
really isn't so hard to learn.
So I'm not really convinced Perl's syntactic freedom is such a bad thing, except that people
tend to blow it out of proportion.
However, Perl is also very free semantically : it is a truly dynamic language,
allowing programmers to do all kinds of things that stricter languages forbid. For instance, I
can monkey-patch
functions and methods in arbitrary code that I'm using. This can make it very hard for
programmers to understand how a piece of code is working, or whether it is working as
intended.
This becomes more important when a software system grows larger or when others than the
original author start to rely on it. The code doesn't just need to work, but it must be
understandable to others. Consequently, in large, stable code bases, compactness and freedom of
expression are less important than consistency, a smooth learning curve for beginners, and
protection against routine errors. Therefore, many software development teams prefer languages
such as Java, with its very limited syntactic freedom and strict compile-time type checking.
Perl is at the opposite end of the spectrum, with its extreme syntactic and semantic
freedom.
This wouldn't be a problem if there were ways to straitjacket Perl if you wanted to; if
there was a way to say: for this project, be as rigid as Java syntactically or semantically; I
want as few surprises as possible in code that I didn't write. Sure enough, Perl has support
for compile-time checking ( use strict ; use warnings , and the
perlcritic utility) and consistent code formatting (the perltidy
utility), but they were added as afterthoughts and cannot come anywhere near the level of
strictness a Java programmer would expect.
To support that, the language needed to be redesigned from scratch, and the result would be
incompatible with the original. This effort has been made, producing Perl 6, but in the
meantime, many other languages sprung up and became popular for the cases Perl programmers
wanted to use Perl for, and if you're going to switch to an incompatible language anyway, why
not use one of those instead?
The fate of Perl 6 is unclear but Perl 5.10 is here to stay. Some thing were screwed after Perl 5.10, but they might be
eventually corrected. OO-enthusiasts did a every bad service to Perl trying to enforce unsuitable for programming, say, utilities
paradigm on everybody. That led to huge inefficiencies and bloated difficult to maintain code. That also somewhat devalued
Perl standard library as the conversion to OO spoiled the broth.
Notable quotes:
"... I'm keeping up with Perl, but not really, I still see a feature, like in Perl 5.16, and I go, Oh, that's in relatively modern Perl, no wonder I don't know about it. I think of Perl as whatever was back in 5.10 and 5.12, that's the latest that I was writing my books for, my trainings for. ..."
"... So the stuff that's coming out in 5.18 and 5.20 and 5.22 now, is sort of beyond me, I just can't keep up with Perl-delta, and that's a scary thing for the number one prolific author about Perl, to not be able to keep up with what's happening in the Perl community, this is clearly an indication that Perl is alive and well, and I've kind of missed the boat, now. ..."
"... And every time I go to YAPC or some other place where they're talking about Perl 6, I get excited about it, for all of a month, and then I come back and then I go, How am I going to use this practically? None of my current clients are demanding that. ..."
Yeah, I think a few years ago, it was all about cloud stuff. So it
was all about running your application in cloud. Starting probably a couple years ago, with the
Docker revolution, it's all about
containers now.
But we're also seeing a revolution in smart, JavaScript-based ultimately, front-ends, that
are doing things like single-page applications and stuff,
and I'm really pretty excited about that. Not that I ever really wanted to spend a lot of time
playing with JavaScript, but unfortunately I guess that that's a requirement, so I'm continuing
to hone my JavaScript skills.
I'm also honing my Dart skills,
because that language out of Google, is really gaining some traction, in terms of being able to
do server-side stuff, essentially replacing Node.JS with a reasonable language. And also client-side stuff for
all the modern browsers, and it translating down into JavaScript, so as long as there's a
reasonable ECMA 5 or
something available in the browser, Dart works really nicely. But Dart looks closer, as a
language, to something like Java, with optional typing, so if you add types to variables, you
can actually get hints from your development environment and that's pretty slick. So I'm
learning Dart in the background, I actually have a couple applications for it already, that as
I learn more, I'll be able to deploy. I'm also learning things like Angular , so I can have reactive front-ends, and again, it's like
there's not enough hours in the day for me to learn everything I want to learn.
I'm keeping up with Perl, but not really, I still see a feature, like in Perl 5.16, and
I go, Oh, that's in relatively modern Perl, no wonder I don't know about it. I think of
Perl as whatever was back in 5.10 and 5.12, that's the latest that I was writing my books for,
my trainings for.
So the stuff that's coming out in 5.18 and 5.20 and 5.22 now, is sort of beyond me, I
just can't keep up with Perl-delta, and that's a scary thing for the number one prolific author
about Perl, to not be able to keep up with what's happening in the Perl community, this is
clearly an indication that Perl is alive and well, and I've kind of missed the boat,
now.
17:53 Gabor Szabo Yeah, so as a closing question, I would like to go back a little bit to
the languages and the things you do with open source, and ask you, where are you heading? Are
you going to go back to Perl and learn what the new things in Perl are, or are you more
interested in other languages, and which ones?
18:16 Randal Schwartz
Well, I download and compile Perl 6 every day. And every time I go to YAPC or some other place where they're talking about Perl 6, I get
excited about it, for all of a month, and then I come back and then I go, How am I going to
use this practically? None of my current clients are demanding that.
Clearly if I were to write training materials for that, I'd have to present it at least to 200
people, whether that's 10 classes of 20, or a giant 200 person week-end event, that's sort of
the minimum for amortizing the inception cost for any class that I've ever written. So I use
the 200 number as kind of a rule of thumb.
And I just don't see that happening, I don't see getting enough people together in the right
places, to be able to do that. So I continue to watch what people are doing with Perl 6, I
continue compiling it every day, and I'd love for it to become extremely popular so I could go
back to that, and say I could continue my Perl heritage.
But, as I mentioned earlier, I think Dart has legs. Given that Google's behind it, given
that Google and a number of other companies are already deploying public-facing projects in it.
Given that it does compile down and work in all modern browsers, I easily see the need for like
rent a hotel room for a weekend and have 20, 50, 100 people show up to learn about it,
because single-page applications are all the rage right now, and Dart is a really solid
language for that, and Google is betting on that.
You may say, Where is Go in that equation? Go is great for server-side stuff, and
great for the kind of things they're doing on back-ends, and although Dart can also do back-end
stuff, essentially replacing Node.JS for that sort of thing, and have a single language for
both back-end and front-end. Dart's real win is in the front-end, being able to be transpiled
over to JavaScript and being able to scale to hundreds of thousands of lines of code for some
of their larger applications. I think that's got legs, I'm in on the groundfloor, like I was on
Perl, I'm already recognized among the Dart people as being someone who can put things
together. I did a one-hour long intro to Dart talk that was reviewed by some of the key people
in the Dart community, and they really like what I did with it, so I seem to have, again, that
knack for finding something complex and finding the simplest ends of it, and I'm already there
with Dart.
And also, the whole Fuchsia announcement a few weeks ago, where
Google's coming out with this language for real-time operating systems, and it has a strong
Dart component in it. I think that's another thing that says, say if they start putting that in
Google Glass , or if
they even put that as a replacement for the Android operating system, or for Google Chrome,
which some people are suspecting that this is all amalgamation of it.
Especially when somebody's looking at the source code the other day, and it has a lot of
files, not only from Android, but also from the old Be OS , which was sort of the predecessor of what
eventually became OS X, kind of interesting that that's part of that project as well.
So with Fuchsia on the horizon, with Dart already being deployed by numbers of people, with
me having a knack for understanding how Dart actually works, given that it was also built by
some of the key players in Smalltalk, which I go back 16 years with, I think this is probably
the right place for me to look at my future.
22:02 Gabor Szabo And I guess, FLOSS Weekly?
22:05 Randal Schwartz
FLOSS Weekly will continue.
In fact I just had a converstaion recently with Leo, we're one of the smaller shows on the
network, but he's absolutely committed to this show. He likes what I'm doing with it, he likes
the directions I'm taking it, he likes the team I've put together, who were able to pick up the
show, even when I was absent for six weeks, in the hospital recently, without notice
unfortunately, I guess that's always the way you end up in the hospital.
So my team picked up, and Aaron Newcomb did a great job of hosting while I was gone, but Leo
likes the team I've built and Leo likes the kinds of guests I'm getting on, the variety
especially. I've had a lot of people write in and say, I don't always want or understand the
thing you're talking about, but I listen to the way you interview them, and I listen to the
things you're able to pull out, like what's the governance model, how are you making money with
this, what got you started? These sorts of things are really sort of cross-project. You
know, you can learn that sort of stuff about anything you want to start, and like I said, I
learned a lot already by doing this show and so a lot of the audience is picking that up. And
we have a fun time.
I tell jokes sometimes and I have a bad way of making really bad puns. And that's kind of
the way it works but I really enjoy the show, I'm going to keep doing it. And I told Leo I
would just keep doing this as long as he let's me, and he goes, Well then, that makes two of
us. So we'll still be doing this in 20 years, if they let us. And I said, That sounds
like a great promise, Leo, thank you. So yeah, I'll be doing FLOSS Weekly for at least
awhile longer.
23:45 Gabor Szabo I'm happy to hear that and I hope to see a lot more of that. And I hope to
see you somewhere, I don't know, maybe at a Dart conference?
23:56 Randal Schwartz
Yeah, that'd be awesome!
And I think you come to OSCon , occasionally, or maybe, well I've got to get
out to a YAPC::Europe or a YAPC::Israel or
something at some point, but just haven't made those yet. I think it's partially because I need
to figure out what to pitch to the Perl conference.
Oh wait, I could just be press again! That's the other thing, is that FLOSS Weekly has
allowed me to apply as press for OSCon for the last few years, even though I don't have an
actual talk to give. And Red Hat actually
invited me to their conference, as press. And I thought, Well, that's the first time that's
happened. That really says I've made it. That really says that FLOSS Weekly is recognized as
legitimate press. So I'm wearing a whole 'nother hat, so my hat tree of all my hats,
hanging up in the corner, has gotten a whole 'nother rung.
#!/usr/bin/perl -w
#
# This script was developed by Robin Barker ([email protected]),
# from Larry Wall's original script eg/rename from the perl source.
#
# This script is free software; you can redistribute it and/or modify it
# under the same terms as Perl itself.
#
# Larry(?)'s RCS header:
# RCSfile: rename,v Revision: 4.1 Date: 92/08/07 17:20:30
#
# $RCSfile: rename,v $$Revision: 1.5 $$Date: 1998/12/18 16:16:31 $
#
# $Log: rename,v $
# Revision 1.5 1998/12/18 16:16:31 rmb1
# moved to perl/source
# changed man documentation to POD
#
# Revision 1.4 1997/02/27 17:19:26 rmb1
# corrected usage string
#
# Revision 1.3 1997/02/27 16:39:07 rmb1
# added -v
#
# Revision 1.2 1997/02/27 16:15:40 rmb1
# *** empty log message ***
#
# Revision 1.1 1997/02/27 15:48:51 rmb1
# Initial revision
#
use strict;
use Getopt::Long;
Getopt::Long::Configure('bundling');
my ($verbose, $no_act, $force, $op);
die "Usage: rename [-v] [-n] [-f] perlexpr [filenames]\n"
unless GetOptions(
'v|verbose' => \$verbose,
'n|no-act' => \$no_act,
'f|force' => \$force,
) and $op = shift;
$verbose++ if $no_act;
if (!@ARGV) {
print "reading filenames from STDIN\n" if $verbose;
@ARGV = ;
chop(@ARGV);
}
for (@ARGV) {
my $was = $_;
eval $op;
die $@ if $@;
next if $was eq $_; # ignore quietly
if (-e $_ and !$force)
{
warn "$was not renamed: $_ already exists\n";
}
elsif ($no_act or rename $was, $_)
{
print "$was renamed as $_\n" if $verbose;
}
else
{
warn "Can't rename $was $_: $!\n";
}
}
__END__
=head1 NAME
rename - renames multiple files
=head1 SYNOPSIS
B S ]> S ]> S ]> I S ]>
=head1 DESCRIPTION
C
renames the filenames supplied according to the rule specified as the
first argument.
The I
argument is a Perl expression which is expected to modify the C
string in Perl for at least some of the filenames specified.
If a given filename is not modified by the expression, it will not be
renamed.
If no filenames are given on the command line, filenames will be read
via standard input.
For example, to rename all files matching C to strip the extension,
you might say
rename 's/\.bak$//' *.bak
To translate uppercase names to lower, you'd use
rename 'y/A-Z/a-z/' *
=head1 OPTIONS
=over 8
=item B, B
Verbose: print names of files successfully renamed.
=item B, B
No Action: show what files would have been renamed.
=item B, B
Force: overwrite existing files.
=back
=head1 ENVIRONMENT
No environment variables are used.
=head1 AUTHOR
Larry Wall
=head1 SEE ALSO
mv(1), perl(1)
=head1 DIAGNOSTICS
If you give an invalid Perl expression you'll get a syntax error.
=head1 BUGS
The original C did not check for the existence of target filenames,
so had to be used with care. I hope I've fixed that (Robin Barker).
=cut
A module is a container which holds a group of variables and subroutines which can be used in a program. Every module has a public
interface, a set of functions and variables.
To use a module into your program, require or use statement can be used, although their semantics are slightly different.
The 'require' statement loads module at runtime to avoid redundant loading of module. The 'use' statement is like require with
two added properties, compile time loading and automatic importing.
Namespace is a container of a distinct set of identifiers (variables, functions). A namespace would be like name::variable .
Every piece of Perl code is in a namespace.
In the following code,
use strict;
use warnings;
my $x = "Hello" ;
$main ::x = "Bye" ;
print "$main::x\n" ; # Bye
print "$x\n" ; # Hello
Here are two different variables defined as x . the $main::x is a package variable and $x is a lexical variable. Mostly we use
lexical variable declared with my keyword and use namespace to separate functions.
In the above code, if we won't use use strict , we'll get a warning message as
Name "main::x" used only once: possible typo at line..
The main is the namespace of the current script and of current variable. We have not written anything and yet we are already in
the 'main' namespace.
By adding 'use strict', now we got the following error,
Global symbol "$x" requires explicit package name
In this error, we got a new word 'package'. It indicates that we forgot to use 'my' keyword before declaring variable but actually
it indicates that we should provide name of the package the variable resides in.
Perl Switching namespace using package keyword
Look at the following code,
use strict;
use warnings;
use 5.010;
sub hii {
return "main" ;
}
package two;
sub hii {
return "two" ;
}
say main::hii(); # main
say two::hii(); # two
say hii(); # two
package main;
say main::hii(); # main
say two::hii(); # two
say hii(); # main
Here we are using package keyword to switch from 'main' namespace to 'two' namespace.
Calling hii() with namespaces returns respective namespaces. Like , say main::hii(); returns 'main' and say two::hii(); returns
'two'.
Calling hii() without namespace prefix, returns the function that was local to the current namespace. In first time, we were in
'two' namespace. Hence it returned 'two'. In second time, we switched the namespace using package main. Hence it returns 'main'.
The older way is adding a -w flag on the sh-bang line. Usually looks like this as the first
line of your script:
#!/usr/bin/perl -w
There are certain differences, but as use warnings is available for 12 years now, there is
no reason to avoid it. In other words:
Always use warnings; !
Let's go back to the actual warning I wanted to explain.
A quick explanation
Use of uninitialized value $x in say at perl_warning_1.pl line 6.
This means the variable $x has no value (its value is the special value undef ). Either it
never got a value, or at some point undef was assigned to it.
You should look for the places where the variable got the last assignment, or you should try
to understand why that piece of code has never been executed.
A simple example
The following example will generate such warning.
use warnings ;
use strict ;
use 5.010 ;
my $x ;
say $x ;
Perl is very nice, tells us which file generated the warning and on which line.
Only a
warning
As I mentioned this is only a warning. If the script has more statements after that say
statement, they will be executed:
use warnings ;
use strict ;
use 5.010 ;
my $x ;
say $x ;
$x = 42 ;
say $x ;
This will print
Use of uninitialized value $x in say at perl_warning_1.pl line 6.
42
Confusing output order
Beware though, if your code has print statements before the line generating the warning,
like in this example:
use warnings ;
use strict ;
use 5.010 ;
print 'OK' ;
my $x ;
say $x ;
$x = 42 ;
say $x ;
the result might be confusing.
Use of uninitialized value $x in say at perl_warning_1.pl line 7.
OK
42
Here, 'OK', the result of the print is seen after the warning, even though it was
called before the code that generated the warning.
This strangeness is the result of IO buffering . By default Perl buffers STDOUT, the
standard output channel, while it does not buffer STDERR, the standard error channel.
So while the word 'OK' is waiting for the buffer to be flushed, the warning message already
arrives to the screen.
Turning off buffering
In order to avoid this you can turn off the buffering of STDOUT.
This is done by the following code: $| = 1; at the beginning of the script.
use warnings ;
use strict ;
use 5.010 ;
$ | = 1 ;
print 'OK' ;
my $x ;
say $x ;
$x = 42 ;
say $x ;
OKUse of uninitialized value $x in say at perl_warning_1.pl line 7.
42
(The warning is on the same line as the OK because we have not printed a newline \n
after the OK.)
The unwanted scope
use warnings ;
use strict ;
use 5.010 ;
my $x ;
my $y = 1 ;
if ( $y ) {
my $x = 42 ;
}
say $x ;
This code too produces Use of uninitialized value $x in say at perl_warning_1.pl line
11.
I have managed to make this mistake several times. Not paying attention I used my $x inside
the if block, which meant I have created another $x variable, assigned 42 to it just to let it
go out of the scope at the end of the block. (The $y = 1 is just a placeholder for some real
code and some real condition. It is there only to make this example a bit more realistic.)
There are of course cases when I need to declare a variable inside an if block, but not
always. When I do that by mistake it is painful to find the bug.
I use this scrub function to clean up output from other functions.
#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
my %h = (
a => 1,
b => 1
);
print scrub($h{c});
sub scrub {
my $a = shift;
return ($a eq '' or $a eq '~' or not defined $a) ? -1 : $a;
}
The problem occurs when I also would like to handle the case, where the key in a hash
doesn't exist, which is shown in the example with scrub($h{c}) .
What change should be make to scrub so it can handle this case?
You're checking whether $a eq '' before checking whether it's defined, hence the
warning "Use of uninitialized value in string eq". Simply change the order of things in the
conditional:
return (!defined($a) or $a eq '' or $a eq '~') ? -1 : $a;
As soon as anything in the chain of 'or's matches, Perl will stop processing the
conditional, thus avoiding the erroneous attempt to compare undef to a string.
In scrub it is too late to check, if the hash has an entry for key
key . scrub() only sees a scalar, which is undef , if
the hash key does not exist. But a hash could have an entry with the value undef
also, like this:
my %h = (
a => 1,
b => 1,
c => undef
);
So I suggest to check for hash entries with the exists function.
Perl doesn't offer a way to check whether or not a variable has been initialized.
However, scalar variables that haven't been explicitly initialized with some value happen
to have the value of undef by default. You are right about defined
being the right way to check whether or not a variable has a value of undef
.
There's several other ways tho. If you want to assign to the variable if it's
undef , which your example code seems to indicate, you could, for example, use
perl's defined-or operator:
It depends on what you're trying to do. The proper C way to do things is to
initialize variables when they are declared; however, Perl is not C , so one of
the following may be what you want:
1) $var = "foo" unless defined $var; # set default after the fact
2) $var = defined $var? $var : {...}; # ternary operation
3) {...} if !(defined $var); # another way to write 1)
4) $var = $var || "foo"; # set to $var unless it's falsy, in which case set to 'foo'
5) $var ||= "foo"; # retain value of $var unless it's falsy, in which case set to 'foo' (same as previous line)
6) $var = $var // "foo"; # set to $var unless it's undefined, in which case set to 'foo'
7) $var //= "foo"; # 5.10+ ; retain value of $var unless it's undefined, in which case set to 'foo' (same as previous line)
C way of doing things ( not recommended ):
# initialize the variable to a default value during declaration
# then test against that value when you want to see if it's been changed
my $var = "foo";
{...}
if ($var eq "foo"){
... # do something
} else {
... # do something else
}
Another long-winded way of doing this is to create a class and a flag when the variable's
been changed, which is unnecessary.
The multi dimensional
array is represented in the form of rows and columns, also called Matrix.
They can not hold arrays or hashes, they can only hold scalar values. They can contain
references to another arrays or hashes.
Perl Multidimensional Array Matrix Example
Here, we are printing a 3 dimensional matrix by combining three different arrays arr1 , arr2
and arr3 . These three arrays are merged to make a matrix array final .
Two for loops are used with two control variables $i and $j .
## Declaring arrays
my @arr1 = qw(0 10 0);
my @arr2 = qw(0 0 20);
my@arr3 = qw(30 0 0);
## Merging all the single dimensional arrays
my @final = (\@arr1, \@arr2, \@arr3);
print "Print Using Array Index\n" ;
for (my $i = 0; $i <= $#final; $i ++){
# $#final gives highest index from the array
for (my $j = 0; $j <= $#final ; $j ++){
print "$final[$i][$j] " ;
}
print "\n" ;
}
Output:
Print Using Array Index
0 10 0
0 0 20
30 0 0
Perl Multidimensional Array Initialization and Declaration Example
In this example we are initializing and declaring a three dimensional Perl array .
The hashes is the most essential and influential part of the perl language. A hash is a group of key-value pairs. The keys are
unique strings and values are scalar values.
Hashes are declared using my keyword. The variable name starts with a (%) sign.
Hashes are like arrays but there are two differences between them. First arrays are ordered but hashes are unordered. Second,
hash elements are accessed using its value while array elements are accessed using its index value.
No repeating keys are allowed in hashes which makes the key values unique inside a hash. Every key has its single value.
Syntax:
my %hashName = (
"key" => "value" ;
)
Perl Hash Accessing
To access single element of hash, ($) sign is used before the variable name. And then key element is written inside {} braces.
my %capitals = (
"India" => "New Delhi" ,
"South Korea" => "Seoul" ,
"USA" => "Washington, D.C." ,
"Australia" => "Canberra"
);
print "$capitals{'India'}\n" ;
print "$capitals{'South Korea'}\n" ;
print "$capitals{'USA'}\n" ;
print "$capitals{'Australia'}\n" ;
Output:
New Delhi
Seoul
Washington, D.C.
Canberra
Perl Hash Indexing
Hashes are indexed using $key and $value variables. All the hash values will be printed using a while loop. As the while loop
runs, values of each of these variables will be printed.
my %capitals = (
"India" => "New Delhi" ,
"South Korea" => "Seoul" ,
"USA" => "Washington, D.C." ,
"Australia" => "Canberra"
);
# LOOP THROUGH IT
while (( $key , $value ) = each(%capitals)){
print $key . ", " . $value . "\n" ;
}
Output:
Australia, Canberra
India, New Delhi
USA, Washington, D.C.
South Korea, Seoul
Perl sorting Hash by key
You can sort a hash using either its key element or value element. Perl provides a sort() function for this. In this example,
we'll sort the hash by its key elements.
my %capitals = (
"India" => "New Delhi" ,
"South Korea" => "Seoul" ,
"USA" => "Washington, D.C." ,
"Australia" => "Canberra"
);
# Foreach loop
foreach $key (sort keys %capitals) {
print "$key: $capitals{$key}\n" ;
}
Output:
Australia: Canberra
India: New Delhi
South Korea: Seoul
USA: Washington: D.C.
Look at the output, all the key elements are sorted alphabetically.
This title was published in hardcover in March 2005 by Apress, a relatively new member of
the technical publishing world. The publisher has a
Web page for the book that includes links to all of the source code in a Zip file, the
table of contents in PDF format, and a form for submitting errata. The book comprises 269
pages, the majority of which are organized into 16 chapters:
Introduction (not to be confused with the true Introduction immediately preceding
it),
Inspecting Variables and Getting Help, Controlling Program Execution, Debugging a
Simple Command Line Program, Tracing Execution, Debugging Modules, Debugging Object-Oriented
Perl, Using the Debugger As a Shell, Debugging a CGI Program, Perl Threads and Forked
Processes, Debugging Regular Expressions, Debugger Customization, Optimization and Performance
Hints and Tips, Command Line and GUI Debuggers, Comprehensive Command Reference, Book
References and URLs.
When debugging I emphasize the use of "warn" over "print". It's the same syntax, but the
warn statements don't get spooled and therefore their timing is quicker.
This is vital when you code just plain blows up. Using "print" means that a statement
which got executed before the disaster may not make it to console, thus leading you to
believe that it never got executed. "warn" avoids this problem and thus leads you to the
problem more accurately. It also makes it easy to globally comment out the warn statements
before going releasing the code.
[That's one freelance Perl programmer I'll have to remember never to hire.]
Seriously, I'm one of those people who use a debugger every day. Actually, when I write new
code in Perl, often the first thing I do is step through it in the debugger to make sure it
does what I think it should. Especially in Perl, it is very easy to accidentally do something
that's a little off. With the "wait until something goes wrong before I investigate" attitude
demonstrated here, you'll never know anything is amiss until some nasty bug crops up as a
result. Using the debugger to sanity check my code means that I catch most bugs before they
ever cause problems.
I'm sure I'm going to get some snide remarks about this approach, but really, I've been a
serious Perl programmer for about eight years now, and often write moderately complex Perl
programs that work perfectly the first time--run through the debugger or not. I can't say that
about any other language, and it's something most people can't say about any language, let
alone Perl ;)
#!/usr/bin/perl
use warnings;
use Net::Cisco;
################################### S
open( OUTPUTS, ">log_Success.txt" );
open( OUTPUTF, ">log_Fail.txt" );
################################### E
open( SWITCHIP, "ip.txt" ) or die "couldn't open ip.txt";
my $count = 0;
while (<SWITCHIP>) {
chomp($_);
my $switch = $_;
my $tl = 0;
my $t = Net::Telnet::Cisco->new(
Host => $switch,
Prompt =>
'/(?m:^(?:[\w.\/]+\:)?[\w.-]+\s?(?:\(config[^\)]*\))?\s?[\$#>]\s?(?:\(enable\))?\s*$)/',
Timeout => 5,
Errmode => 'return'
) or $tl = 1;
my @output = ();
################################### S
if ( $tl != 1 ) {
print "$switch Telnet success\n"; # for printing it in screen
print OUTPUTS "$switch Telnet success\n"; # it will print it in the log_Success.txt
}
else {
my $telnetstat = "Telnet Failed";
print "$switch $telnetstat\n"; # for printing it in screen
print OUTPUTF "$switch $telnetstat\n"; # it will print it in the log_Fail.txt
}
################################### E
$count++;
}
################################### S
close(SWITCHIP);
close(OUTPUTS);
close(OUTPUTF);
################################### E
In print statement after print just write the filehandle name which is OUTPUT in
your code:
print OUTPUT "$switch Telnet success\n";
and
print OUTPUT "$switch $telnetstat\n";
A side note: always use a lexical filehandle and three arguments with error handling to
open a file. This line open(OUTPUT, ">log.txt"); you can write like this:
But since you're opening a log.txt file with the handle OUTPUT ,
just change your two print statements to have OUTPUT as the first
argument and the string as the next (without a comma).
my $telnetstat;
if($tl != 1) {
$telnetstat = "Telnet success";
} else {
$telnetstat = "Telnet Failed";
}
print OUTPUT "$switch $telnetstat\n";
# Or the shorter ternary operator line for all the above:
print OUTPUT $swtich . (!$tl ? " Telnet success\n" : " Telnet failed\n");
cperl adds many more traditional compile-time optimizations: more and earlier constant
folding, type promotions, shaped arrays, usage of literal and typed constants, loop unrolling,
omit unnecessary array bounds checks, function inlining and conversion of static method calls
to functions.
Perl 5 only inlines constant function bodies with an explicit empty ()
prototype.
sub x() {1+2} # inlined in perl5
sub x {1+2} # inlined in cperl only
cperl inlines constant function bodies even without empty prototype declaration, has type
declarations for most internal ops, and optimizes these ops depending on the argument types;
currently for all arithmetic unops and binops, and the data-accessing ops padsv, svop, and
sassign. opnames.h stores PL_op_type_variants , all possible type
promotions for each op. opcode.h stores PL_op_type with the type
declarations of all ops.
Perl 11 is not (yet) an actual version of Perl; rather, Perl 11 is currently a philosophy
with 3 primary tenets:
1. Pluggability Of Perl On All Levels
2. Reunification Of Perl 5 & Perl 6
3. Runtime Performance Of C/C++ Or Faster
Perl 11 promotes ideas which will make Perl 5 pluggable at the following levels:
Runtime Virtual Machine
Compilation Unit Format / AST
Source Code Syntax / Compilers
This will open up the doors to many kinds of language / technology experimentation, without
endangering the existing Perl 5 / CPAN code bases that we depend on every day.
Pluggable VMs would be parrot, p2, JVM or .NET running Perl5 and Perl 6 code. 5 + 6 ==
11!
Perl 11 Projects
The following projects are important in reaching the vision of Perl 11:
RPerl
A Restricted Perl by Will Braswell which translates a medium-magic subset of Perl 5 into
C/C++ using Inline::C and Inline::CPP
cperl is an improved variant of perl5, running all of perl5 and CPAN code. With many perl6
features, just faster.
Faster than perl5 and perl6. It is stable and usable, but still in development with many more
features being added soon.
open my $fp, '<', $file or die $!;
while (<$fp>) {
my $line = $_;
if ($line =~ /$regex/) {
# How do I find out which line number this match happened at?
}
}
close $fp;
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (
http://creativecommons.org/licenses/by/4.0/ ), which permits
unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and
the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain
Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/
) applies to the data made available in this article, unless otherwise stated.
Go to:
AbstractBackground
To reproduce and report a bioinformatics analysis, it is important to be able to determine the environment in which a program
was run. It can also be valuable when trying to debug why different executions are giving unexpectedly different results.
Results
Log::ProgramInfo is a Perl module that writes a log file at the termination of execution of the enclosing program, to document
useful execution characteristics. This log file can be used to re-create the environment in order to reproduce an earlier execution.
It can also be used to compare the environments of two executions to determine whether there were any differences that might affect
(or explain) their operation.
Using Log::ProgramInfo in programs creating result data for publishable research, and including the Log::ProgramInfo output log
as part of the publication of that research is a valuable method to assist others to duplicate the programming environment as a precursor
to validating and/or extending that research. Keywords: Reproducibility, Log, Environment
Go to:Background
Reproducibility is a major concern in science as a whole, and computational biology in particular. For reproducibility, it is
not sufficient to provide access to the raw data -- it is ever more critical to also provide access to the program code used to analyse
those data [ 2 ]. But the program code
is a dynamic mixture of program text, command line arguments, libraries, and various other environmental aspects -- all of which
may need to be exactly reproduced to achieve the same results. So, simply providing access to the code used is not a complete solution.
It is necessary, but not sufficient.
The need for reproducibility is growing because our pipelines are getting increasingly complex: a typical sequencing pipeline
might involve a chain of a dozen unique tools [
3 ]. But reproducing these pipelines is fundamentally
very difficult, in part because it requires duplicating the versions of all dependent tools and libraries used in an analysis. Given
the rapid rate of release of updates to common tools (e.g. BWA had 7 updates during the course of 2014 [
4 ], this can be a significant challenge.
Among the best practices for scientific computing (e.g. [
5 ]) is listed the need to collect and publish:
Unique identifiers and version numbers for programs and libraries;
The values of parameters used to generate any given output; and
The names and version numbers of programs (however small) used to generate those outputs.
A large fraction of pipelines for bioinformatics are written in the Perl programming language (e.g. BioPerl [
6 ]). However, for logging the precise state
of a program at run-time, and capturing all the dependency versions and other key information, there are no automated choices available.
To resolve this issue, we introduce here the module Log::ProgramInfo to facilitate run-time logging of Perl-based pipelines, thereby
directly improving the reproducibility of modern bioinformatic analyses.
A further advantage to such tracking information is the ability to test an analsis using later versions of the component tools
to determine whether they provide different results (possibly more accurate if the later releases provide better resolution; possibly
identifying erroneous results in the original analysis if the tools have been updated with critical fixes to their operation).
Go to:
Related work
A search found some programs for related processes but nothing that served the same purposes.
There are some programs available to collect and document the computing process - by recording the steps invoved, including command
lines and arguments during the actual data processing. Such a program could work well together with the described module but addresses
a different aspect of the reproducibility issue. In our lab, when the workflow of the data analysis was sufficiently complex to require
such a description, we instead write a program to encapsulate that process, so there is no long list of manual processing steps to
document.
In particular, the program (ReproZip) [ 7
] was capable of discovering and bundling together all of the programs used during the execution of a process. That seems to have
different trade-offs. Such a bundle is only useful on similar hardware and it provides no possibility for assisting with script library
version info, or in allowing a later run to use selected variations on the programming environment (such as allowing updated versions
of programs that still have the same function but have had security problems fixed).
Go to:Implementation
The Log::ProgramInfo module Macdonald and Boutros, Log-ProgramInfo.
http://search.cpan.org/~boutroslb/Log-ProgramInfo/
is available as open source, and has been distributed on CPAN (the Comprehansive Perl Archive Network - used as the standard distribution
mechanism for the vast majority of open source Perl modules, and described in the Perl documentation with the command "perldoc perlmodinstall").
Log::ProgramInfo is enabled simply by being included with a Perl use statement. Since its effect is global to the program, it
should be enabled directly from the main program, or from a utility module that contains global configuration settings for a suite
of programs.
Any desired setting of non-default values for the options can be provided either through environment variables, or as "import"
list options.
When the module is used for the first time, the loading process carries out a number of actions for its operation:
- An END block is created. It will be executed when the program terminates, to write out the log information.
- Signal handlers are installed for catcheable signals - if one of them occurs, the log information will be printed out before
the program terminates.
- options are set to their default values
- any env variables to control options are saved
- a copy is made of the original command line arguments for eventual logging
- the start time is recorded for eventual logging
-... (numerous other system attributes are saved for eventual logging)
Every time the Log::ProgramInfo module is used, the import list is processed and any values in it are used to update the option
values. (The first time it is used, this processing happens after the initialization steps described above.)
That permits a common group of option settings be processed first, and then specific exceptions to that list over-ridden.
Any option settings provided in environent variables will over-ride the corresponding setting (whether a default or specified
by the program import lists). This allows changing the option settings for individual runs so that the log can be suppressed, enabled,
or redirected for a single run of the program.
The code that prints the log information ensures that it only executes once (in case multiple signals, or a signal during program
termination, would cause it to be called additional times).
If the main body of the program changes a signal handler after Log::ProgramInfo has set it up, that will usually not interfere
with Log::ProgramInfo. Usually, the program will catch signals and handle them in a way that allows it continue to operate, or to
terminate with an exception. It is only if the program resets a signal handler to its default (abort without normal termination processing)
that Log::ProgramInfo's log will not be written. That is not a problem for publication - if the program is being killed by some signal
then it is not yet running successfully, and thus not yet ready for publication. However, it does mean that the log might not be
available as a diagnostic aid in such situations.
For most cases, that is the only interaction between the program and Log::ProgramInfo.
The one additional interaction that might occur is if there is information unique to the program that is desired to be logged.
The function
Log::ProgramInfo::add_extra_logger can be called by the program to specify a callable function that will write additional information
to the log. (See the program documentation for precise details.)
Go to:Results and discussion
Parameters are available to control the logging process: whether (and if so, where) a log is to be written. Choosing the location
where the log is written allows collecting and managing this important information in a way that co-ordinates with the entire set
of computational activity carried out for a research project (or an entire organisation's collection of research projects). The default
name used for the log file includes the name of the program that is being reported upon as well as a time-stamp to distinguish separate
runs -- you might choose to override the name or directory path to provide more complete organisation of logged results. Suppressing
log output can be useful for runs that are not intended to generate reproducible results, such as while the software is being developed.
However, even in such cases, it might turn out to be useful to have this log output to assist diagnosing problems with system configuration
changes -- to confirm that the environment being used is the one that was intended and that updates have actually occurred, etc.
There is an additional parameter that permits the logged information to be sent to a separate logging mechanism, such as a Log4Perl
log. This would allow the information to be collected with the other logged information from the program. The output to such logs
is mixed with the other logged output from the program, and is also usually reformatted to some extent. Such logs cannot be processed
by the Log::ProgramInfo parser provided with the package; hence the normal action for Log::ProgramInfo is to still write its own
log file as well. Go to:Log output
The output created by Log::ProgramInfo contains the following information:
MODULE – Name, version, file location, and checksum for each perl library module used by the program.
INC – The search path used to find modules.
UNAME – Operating system information.
PROCn – Specific information for each processor (memory, cores, etc.)
PERL – The perl interpretor pathname.
PERLVer – The perl interpretor version.
PERLSum – Checksum of the perl interpretor binary.
libc – The version of libc used by the perl interpretor.
libcSUM – Checksum of the libc library used by the perl interpretor.
User – The user ID (and real user ID, if different) running the program.
Group – The group IDs (and real group IDs, if different) running the program.
ProgDir – The directory containing the program.
Program – The program name.
Version – The program's version.
ProgSUM – Checksum of the program file.
Args – The number and values of the command line arguments provided to the program.
Start – The time the program started running.
End – The time the program stopped running.
Elapsed – The elapsed time while the program was running.
EndStat – The program's exit status.
program-specified – Any additional info provided by program-specified callback functions.
The format of the log file is designed to be easily parsed. A parsing subroutine is provided in the package. You could call that
subroutine from a program that analyses logs according to your needs. See the program documentation for details. If you have written
the log info using a logging module such as Log4Perl, you will have to separately extract the bare ProgramInfo log information out
of that log, separating it from any other logging by the program, and removing any line decorations added by the log module.
Go to:Example
Here is an example of using Log::ProgramInfo. Assume a simple program, called simple.pl.
The first line is the expected output from the program, the second line comes from Log::ProgramInfo to tell you that a log file
was created, and where.
Now, take a look at the log file:
lines beginning with a plus sign are wrapped to fit the page width
lines wrapped in angle brackets describe text that has been omitted for brevity
Now that you have a log file, you still have to make use of it. Typically, you would treat this log file as one of the output
files of your processing activities. So, if you normally discard the output files (e.g. for a test run while developing the pipeline),
you will likely also discard the log. On the other hand, for significant runs, you would collect the log file along with the other
output files, labelling and storing them as appropriate for reference. The log file would be available as a synopsis of how the output
data was created, ready to be used for publication, or reproducing the process (either to validate the results, or to apply the same
process to additional data for subsequent research).
Go to:Limitations
The C environment is not well built for program introspection activities such as determining which static and/or dynamic libraries
have been linked into the program's executable image. This module lists the version of libc that was build into the perl binary -
but that information can be out of date. A future release may try to get info about other libraries beyond libc.
Another major problem is that even if a perl module is downloaded from CPAN (which would be one way of ensuring that other people
could get the same version), the install process that puts it into the library path for perl programs can be done in may ways, and
often is not even done on the same computer as the one that is running the perl program. So, it is not easy to do any sort of detailed
validation - the downloaded package bundle is not accessible in any determinable way (and possibly not at all) to the program itself
(and thus to Log::ProgramInfo). While it would be possible to compute checksums for every library module that has been loaded, that
would take a significant amount of time and is not currently being done. It may be added as an option that could request it explicitly.
Go to:Conclusion
Module Log::ProgramInfo provides a convenient way of logging information about the way a program is run. Adding it to existing
programs is as easy as adding one line to the program or any module the program already includes.
Log::ProgramInfo's output file can be easily included in the published results along with the actual source code (or references
to where it can be found). With this log output, other researchers have information necessary to any meaningful attempt to reproduce
the original research, either in the process of validating or extending that research.
Log::ProgramInfo is a good candidate for inclusion in modules intended to mandate standards, and may find use well beyond the
field of bioinformatics. Go to:Availability
and requirements
This study was conducted with the support of the Ontario Institute for Cancer Research to PCB through funding provided by the
Government of Ontario. This work was supported by Prostate Cancer Canada and is proudly funded by the Movember Foundation – Grant
#RS2014-01. Dr. Boutros was supported by a Terry Fox Research Institute New Investigator Award and a CIHR New Investigator Award.
This project was supported by Genome Canada through a Large-Scale Applied Project contract to PCB, Dr. Sohrab Shah and Dr. Ryan Morin.
Authors' contributions
The module was written by the authors. Both authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Paul C. Boutros, Email: [email protected] .
Go to:References 1. Macdonald J, Boutros
P. Log-ProgramInfo. module available from CPAN. http://search.cpan.org/~boutroslb/Log-ProgramInfo/
. 2. Nature-editorial. Code share. Nature. 2014;514. doi:10.1038/514536a. 3. Ewing A, Houlahan K, Hu Y, Ellrott K, Caloian C, Yamaguchi
T, Bare J, P'ng C, Waggott D, Sabelnykova V, ICGC-TCGA DREAM Somatic Mutation Calling Challenge participants. Kellen M, Norman T,
Haussler D, Friend S, Stolovitzky G, Margolin A, Stuart J, Boutros P. Combining accurate tumour genome simulation with crowd-sourcing
to benchmark somatic single nucleotide variant detection. Nat Methods. 2015; 514 :623–30. doi: 10.1038/nmeth.3407. [
PMC free article ] [
PubMed ] [
CrossRef ] [
Google Scholar ] 4. sourceforge-BWA-files. Sourceforge File Listing for BWA on 30 Apr 2015. hand counted from web page.
http://sourceforge.net/projects/bio-bwa/files/ . 5.
Wilson G, Aruliah DA, Brown CT, Hong NPC, Davis M, Guy RT, Haddock SHD, Huff KD, Mitchell IM, Plumbley MD, Waugh B, White EP, Wilson
P. Best practices for scientific computing. PLoS Biol. 2014;12(1). doi:10.1371/journal.pbio.1001745. [
PMC free article ] [
PubMed ] 6. Stajich J, Block D, Boulez K, Brenner SE,
Dagdigian C, Fuellen G, Gilbert JGR, Korf I, Lapp H, Lehväslaiho H, Matsalla C, Mungall CJ, Osborne BI, Popock MR, Schattner P, Senger
M, Stein L, Stupka E, Wilkinson MD, Birney E. The bioperl toolkit: Perl modules for the life sciences. Genome Res. 2002; 12 (10):1611–8.
doi: 10.1101/gr.361602. [ PMC free article ] [
PubMed ] [
CrossRef ] [
Google Scholar ] 7. Chirigati F, Shasha D, Freire J. Presented as Part of the 5th USENIX Workshop on the Theory and Practice
of Provenance. Berkeley: USENIX; 2013. Reprozip: Using provenance to support computational reproducibility. [
Google Scholar ]
These are available with the __LINE__ and __FILE__ tokens, as
documented in perldoc perldata under "Special
Literals":
The special literals __FILE__, __LINE__, and __PACKAGE__ represent the current filename,
line number, and package name at that point in your program. They may be used only as
separate tokens; they will not be interpolated into strings. If there is no current package
(due to an empty package; directive), __PACKAGE__ is the undefined value.
The caller function will do what you are looking for:
sub print_info {
my ($package, $filename, $line) = caller;
...
}
print_info(); # prints info about this line
This will get the information from where the sub is called, which is probably what you are
looking for. The __FILE__ and __LINE__ directives only apply to
where they are written, so you can not encapsulate their effect in a subroutine. (unless you
wanted a sub that only prints info about where it is defined)
In most of the cases we either want a variable to be accessible only from inside a small
scope, inside a function or even inside a loop. These variables get created when we enter the
function (or the scope created by a a block) and destroyed when we leave the scope.
In some cases, especially when we don't want to pay attention to our code, we want variables
to be global, to be accessible from anywhere in our script and be destroyed only when the
script ends. In General having such global variables is not a good practice.
In some cases we want a variable to stay alive between function calls, but still to be
private to that function. We want it to retain its value between calls.
In the C programming language one can designate a variable to be a static variable . This means it gets
initialized only once and it sticks around retaining its old value between function calls.
In Perl, the same can be achieved using the state variable which
is available starting from version 5.10, but there is a construct that will work in every
version of Perl 5. In a way it is even more powerful.
$counter is initialized to 0
only once, the first time we call counter() . In subsequent calls, the line state $counter = 0;
does not get executed and $counter has the same value as it had when we left the function the
last time.
showing that the state $counter = say "world"; line only gets executed once. In the first
call to count() say , which was also added in version
5.10 , will return 1 upon success.
static variables in the "traditional" way
use strict ;
use warnings ;
use 5.010 ;
{
my $counter = 0 ;
sub count {
$counter ++;
return $counter ;
}
}
say count ();
say count ();
say count ();
This provides the same result as the above version using state , except that this could work
in older versions of perl as well. (Especially if I did not want to use the say keyword, that
was also introduced in 5.10.)
This version works because functions declarations are global in perl - so count() is
accessible in the main body of the script even though it was declared inside a block. On the
other hand the variable $counter is not accessible from the outside world because it was
declared inside the block. Lastly, but probably most importantly, it does not get destroyed
when we leave the count() function (or when the execution is outside the block), because the
existing count() function still references it.
Thus $count is effectively a static variable.
First assignment time
use strict ;
use warnings ;
use 5.010 ;
say "hi" ;
{
my $counter = say "world" ;
sub count {
$counter ++;
return $counter ;
}
}
say "hello" ;
say count ();
say count ();
say count ();
hi
world
hello
2
3
4
This shows that in this case too, the declaration and the initial assignment my $counter =
say "world"; happens only once, but we can also see that the assignment happens before
the first call to count() as if the my $counter = say "world"; statement was part of the
control flow of the code outside of the block.
Shared static variable
This "traditional" or "home made" static variable has an extra feature. Because it does not
belong to the the count() subroutine, but to the block surrounding it, we can declare more than
one functions in that block and we can share this static variable between two or even more
functions.
For example we could add a reset_counter() function:
use strict ;
use warnings ;
use 5.010 ;
{
my $counter = 0 ;
sub count {
$counter ++;
return $counter ;
}
sub reset_counter {
$counter = 0 ;
}
}
say count ();
say count ();
say count ();
reset_counter ();
say count ();
say count ();
1
2
3
1
2
Now both functions can access the $counter variable, but still nothing outside the enclosing
block can access it.
Static arrays and hashes
As of now, you cannot use the state declaration in list context. This means you cannot write
state @y = (1, 1); . This limitation could be overcome by some extra coding. For example in
this implementation of the Fibonacci series, we checked if the array is empty and set the
default values:
use strict ;
use warnings ;
use 5.010 ;
sub fib {
state @y ;
@y = ( 1 , 1 ) if not @y ; # workaround initialization
push @y , $y [ 0 ]+ $y [ 1 ];
return shift @y ;
}
say fib ();
say fib ();
say fib ();
say fib ();
say fib ();
Alternatively we could use the "old-style" static variable with the enclosing block.
Here is the example generating the Fibonacci series:
If you want to do remote debug (for cgi or if you don't want to mess output with debug
command line) use this:
given test:
use v5.14;
say 1;
say 2;
say 3;
Start a listener on whatever host and port on terminal 1 (here localhost:12345):
$ nc -v -l localhost -p 12345
for readline support use rlwrap (you can use on perl
-d too):
$ rlwrap nc -v -l localhost -p 12345
And start the test on another terminal (say terminal 2):
$ PERLDB_OPTS="RemotePort=localhost:12345" perl -d test
Input/Output on terminal 1:
Connection from 127.0.0.1:42994
Loading DB routines from perl5db.pl version 1.49
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(test:2): say 1;
DB<1> n
main::(test:3): say 2;
DB<1> select $DB::OUT
DB<2> n
2
main::(test:4): say 3;
DB<2> n
3
Debugged program terminated. Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
DB<2>
Output on terminal 2:
1
Note the sentence if you want output on debug terminal
select $DB::OUT
If you are vim user, install this plugin: dbg.vim which provides basic support for perl
This is like "please can you give me an example how to drive a car" .
I have explained the basic commands that you will use most often. Beyond this you must
read the debugger's inline help and reread the perldebug documentation
The debugger starts by displaying the next line to be executed: usually the
first line in your program
Debugger commands are mostly single letters, possibly with parameters. The command will
be actioned as soon as you press Enter
You should concentrate on commands s and n to step through
the program. If the next statement is a subroutine (or method) call then s
will step into the subroutine while n will step over the call.
Otherwise s and n behave identically
Be careful using s when a single line of code contains multiple
subroutine calls. You may not be stepping into the subroutine that you expect
You can't step into a built-in function, or a subroutine not written in
Perl
Once you have executed a statement there is no going back. You must restart the
program to try something different
You can execute a line of Perl code just by typing it in and pressing Enter
. the code will be executed in the context of the current statement
You can examine or modify any variable this way
The p command is identical to print . The output from
p $var or p @arr will be the same as if you had typed p
$var or p @arr
You can use x to dump an expression in list context. The output
consists of numbered lines showing each element of the list
The commands dot . , hyphen - and v are useful
for looking at the source code. . and - will display the current
and previous source line respectively. v will display a window around the
current source line
To rapidly return to a specific line of code you can set a breakpoint and
continue execution until that line using the c command. For example c
13Enter will execute all code until line 13 and then stop
Breakpoints defined using c are temporary , so if you want to
continue to the same line again (in a loop) then you have to enter c 13Enter again
c without any parameters will run the rest of the program until it exits
or until a permanent breakpoint, defined using b , is reached
You can specify breakpoints with more complex conditions using the b
command. They can be deleted only with the corresponding B command, or B
* which will clear all breakpoints
h shows a list of the commands available, and h *command* ,
like h c , will show you detailed help on a single command
Finally, q will end the debug session and terminate the program
The debugger will do a lot more than this, but these are the basic commands that you need
to know. You should experiment with them and look at the contents of the help text to get
more proficient with the Perl debugger
I would like Perl to write to STDERR only if STDOUT is not the same. For example, if both
STDOUT and STDERR would redirect output to the Terminal, then I don't want STDERR to be
printed.
Consider the following example (outerr.pl):
#!/usr/bin/perl
use strict;
use warnings;
print STDOUT "Hello standard output!\n";
print STDERR "Hello standard error\n" if ($someMagicalFlag);
exit 0
Now consider this (this is what I would like to achieve):
bash $ outerr.pl
Hello standard output!
However, if I redirect out to a file, I'd like to get:
bash $ outerr.pl > /dev/null
Hello standard error
and similary the other way round:
bash $ outerr.pl 2> /dev/null
Hello standard output!
If I re-direct both out/err to the same file, then only stdout should be
displayed:
my @stat_err = stat STDERR;
my @stat_out = stat STDOUT;
my $stderr_is_not_stdout = (($stat_err[0] != $stat_out[0]) ||
($stat_err[1] != $stat_out[1]));
But that won't work on Windows, which doesn't have real inode numbers. It gives both false
positives (thinks they're different when they aren't) and false negatives (thinks they're the
same when they aren't).
EDIT: Solutions for the case that both STDERR and STDOUT are regular files:
Tom Christianson suggested to stat and compare the dev and ino fields. This will work in
UNIX, but, as @cjm pointed out, not in Windows.
If you can guarantee that no other program will write to the file, you could do the
following both in Windows and UNIX:
check the position the file descriptors for STDOUT and STDERR are at, if they are not
equal, you redirected one of them with >> to a nonempty file.
Otherwise, write 42 bytes to file descriptor 2
Seek to the end of file descriptor 1. If it is 42 more than before, chances are high
that both are redirected to the same file. If it is unchanged, files are different. If it
is changed, but not by 42, someone else is writing there, all bets are off (but then,
you're not in Windows, so the stat method will work).
I'm new to Perl and I'm writing a program where I want to force the user to enter a word. If
the user enters an empty string then the program should exit.
This is what I have so far:
print "Enter a word to look up: ";
chomp ($usrword = <STDIN>);
print "Enter a word to look up: ";
my $userword = <STDIN>; # I moved chomp to a new line to make it more readable
chomp $userword; # Get rid of newline character at the end
exit 0 if ($userword eq ""); # If empty string, exit.
File output is buffered by default. Since the prompt is so short, it is still sitting in
the output buffer. You can disable buffering on STDOUT by adding this line of code before
printing...
called on a hash in list context, returns a 2-element list consisting of the key and value
for the next element of a hash. In Perl 5.12 and later only, it will also return the index and
value for the next element of an array so that you can iterate over it; older Perls consider
this a syntax error. When called in scalar context, returns only the key (not the value) in a
hash, or the index in an array.
Hash entries are returned in an apparently random order. The actual random order is specific
to a given hash; the exact same series of operations on two hashes may result in a different
order for each hash. Any insertion into the hash may change the order, as will any deletion,
with the exception that the most recent key returned by each or keys may be deleted without
changing the order. So long as a given hash is unmodified you may rely on keys , values and each to repeatedly
return the same order as each other. See Algorithmic Complexity
Attacks in perlsec for details on why hash order is randomized. Aside from the guarantees
provided here the exact details of Perl's hash algorithm and the hash traversal order are
subject to change in any release of Perl.
After each has returned all entries from
the hash or array, the next call to each returns the empty list in list
context and undef in scalar context; the next
call following that one restarts iteration. Each hash or array has its own internal
iterator, accessed by each , keys , and values . The iterator is
implicitly reset when each has reached the end as just
described; it can be explicitly reset by calling keys or values on the hash or array. If
you add or delete a hash's elements while iterating over it, the effect on the iterator is
unspecified; for example, entries may be skipped or duplicated--so don't do that. Exception: It
is always safe to delete the item most recently returned by each , so the following code works
properly:
Starting with Perl 5.14, an experimental feature allowed each to take a scalar expression.
This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.
As of Perl 5.18 you can use a bare each in a while loop,
which will set $_ on
every iteration.
To avoid confusing would-be users of your code who are running earlier versions of Perl with
mysterious syntax errors, put this sort of thing at the top of your file to signal that your
code will work only on Perls of a recent vintage:
I'm surprised it works with cat but not with echo. cat should expect a file name as stdin,
not a char string. psql << EOF sounds logical, but not othewise. Works with cat but not
with echo. Strange behaviour. Any clue about that? – Alex
Mar 23 '15 at 23:31
Answering to myself: cat without parameters executes and replicates to the output whatever
send via input (stdin), hence using its output to fill the file via >. In fact a file name
read as a parameter is not a stdin stream. – Alex
Mar 23 '15 at 23:39
@Alex echo just prints it's command line arguments while cat reads stding(when
piped to it) or reads a file that corresponds to it's command line args – The-null-Pointer-
Jan 1 '18 at 18:03
This type of redirection instructs the shell to read input from the current source until
a line containing only word (with no trailing blanks) is seen.
All of the lines read up to that point are then used as the standard input for a
command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or pathname
expansion is performed on word . If any characters in word are quoted, the delimiter is the
result of quote removal on word , and the lines in the here-document are not expanded. If
word is unquoted, all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion. In the latter case, the character sequence
\<newline> is ignored, and \ must be used to quote the
characters \ , $ , and ` .
If the redirection operator is <<- , then all leading tab characters
are stripped from input lines and the line containing delimiter . This allows
here-documents within shell scripts to be indented in a natural fashion.
I was having the hardest time disabling variable/parameter expansion. All I needed to do was
use "double-quotes" and that fixed it! Thanks for the info! – Xeoncross
May 26 '11 at 22:51
Concerning <<- please note that only leading tab characters are
stripped -- not soft tab characters. This is one of those rare case when you actually need
the tab character. If the rest of your document uses soft tabs, make sure to show invisible
characters and (e.g.) copy and paste a tab character. If you do it right, your syntax
highlighting should correctly catch the ending delimiter. – trkoch
Nov 10 '15 at 17:23
I don't see how this answer is more helpful than the ones below. It merely regurgitates
information that can be found in other places (that have likely already been checked) –
BrDaHa
Jul 13 '17 at 19:01
The cat <<EOF syntax is very useful when working with multi-line text in
Bash, eg. when assigning multi-line string to a shell variable, file or a pipe. Examples
of cat <<EOF syntax usage in Bash:1. Assign multi-line string to a
shell variable
$ sql=$(cat <<EOF
SELECT foo, bar FROM db
WHERE foo='baz'
EOF
)
The $sql variable now holds the new-line characters too. You can verify
with echo -e "$sql" .
In your case, "EOF" is known as a "Here Tag". Basically <<Here tells the
shell that you are going to enter a multiline string until the "tag" Here . You
can name this tag as you want, it's often EOF or STOP .
Some rules about the Here tags:
The tag can be any string, uppercase or lowercase, though most people use uppercase by
convention.
The tag will not be considered as a Here tag if there are other words in that line. In
this case, it will merely be considered part of the string. The tag should be by itself on
a separate line, to be considered a tag.
The tag should have no leading or trailing spaces in that line to be considered a tag.
Otherwise it will be considered as part of the string.
example:
$ cat >> test <<HERE
> Hello world HERE <-- Not by itself on a separate line -> not considered end of string
> This is a test
> HERE <-- Leading space, so not considered end of string
> and a new line
> HERE <-- Now we have the end of the string
this is the best actual answer ... you define both and clearly state the primary purpose of
the use instead of related theory ... which is important but not necessary ... thanks - super
helpful – oemb1905
Feb 22 '17 at 7:17
The redirection operators "<<" and "<<-" both allow redirection of lines
contained in a shell input file, known as a "here-document", to the input of a command.
The here-document shall be treated as a single word that begins after the next and
continues until there is a line containing only the delimiter and a , with no characters in
between. Then the next here-document starts, if there is one. The format is as follows:
[n]<<word
here-document
delimiter
where the optional n represents the file descriptor number. If the number is omitted,
the here-document refers to standard input (file descriptor 0).
If any character in word is quoted, the delimiter shall be formed by performing quote
removal on word, and the here-document lines shall not be expanded. Otherwise, the
delimiter shall be the word itself.
If no characters in word are quoted, all lines of the here-document shall be expanded
for parameter expansion, command substitution, and arithmetic expansion. In this case, the
in the input behaves as the inside double-quotes (see Double-Quotes). However, the
double-quote character ( '"' ) shall not be treated specially within a here-document,
except when the double-quote appears within "$()", "``", or "${}".
If the redirection symbol is "<<-", all leading <tab>
characters shall be stripped from input lines and the line containing the trailing
delimiter. If more than one "<<" or "<<-" operator is specified on a line, the
here-document associated with the first operator shall be supplied first by the application
and shall be read first by the shell.
When a here-document is read from a terminal device and the shell is interactive, it
shall write the contents of the variable PS2, processed as described in Shell Variables, to
standard error before reading each line of input until the delimiter has been
recognized.
Examples
Some examples not yet given.
Quotes prevent parameter expansion
Without quotes:
a=0
cat <<EOF
$a
EOF
Output:
0
With quotes:
a=0
cat <<'EOF'
$a
EOF
or (ugly but valid):
a=0
cat <<E"O"F
$a
EOF
Outputs:
$a
Hyphen removes leading tabs
Without hyphen:
cat <<EOF
<tab>a
EOF
where <tab> is a literal tab, and can be inserted with Ctrl + V
<tab>
Output:
<tab>a
With hyphen:
cat <<-EOF
<tab>a
<tab>EOF
Output:
a
This exists of course so that you can indent your cat like the surrounding
code, which is easier to read and maintain. E.g.:
if true; then
cat <<-EOF
a
EOF
fi
Unfortunately, this does not work for space characters: POSIX favored tab
indentation here. Yikes.
In your last example discussing <<- and <tab>a , it
should be noted that the purpose was to allow normal indentation of code within the script
while allowing heredoc text presented to the receiving process to begin in column 0. It is a
not too commonly seen feature and a bit more context may prevent a good deal of
head-scratching... – David C. Rankin
Aug 12 '15 at 7:10
@JeanmichelCote I don't see a better option :-) With regular strings you can also consider
mixing up quotes like "$a"'$b'"$c" , but there is no analogue here AFAIK.
–
Ciro Santilli 新疆改造中心
六四事件 法轮功
Sep 23 '15 at 20:01
Not exactly as an answer to the original question, but I wanted to share this anyway: I
had the need to create a config file in a directory that required root rights.
That is all . (Ok, so
I realize some of you will need some more information. Brad Fitzpatrick, with Danga and now
SixApart, is pretty amazing when it comes to the software he's developed and released to the
public . These range from utilities to provide secure backups on hardware you don't own (
brackup ) distributed job
schedulers (The Schwartz) and others
I've written about . Note for you Perl-bashers that he did much of this in Perl.)
I don't do a lot of development work, but while learning Python I've found pycharm to be a
robust and helpful IDE. Other than that, I'm old school like Proksch and use vi.
MICHAEL BAKER
SYSTEM ADMINISTRATOR, IT MAIL SERVICES
Yes, I'm the same as @Proksch. For my development environment at Red Hat, vim is easiest to
use as I'm using Linux to pop in and out of files. Otherwise, I've had a lot of great
experiences with Visual Studio.
"... Editor's note: If you're looking for tips on how to write more efficient, robust, and maintainable Perl code, you'll want to check out Damien Conway's " Modern Perl Best Practices " video. ..."
The need to extract interesting bits of an HTML document comes up often enough that by now
we have all seen many ways of doing it wrong and some ways of doing it right for some values of
"right".
Let's say you want to check all the links on a page to identify stale ones, using regular
expressions:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
use strict ; use warnings ; use feature 'say' ; my $ re = qr /< as + href = [
"']([^"']+)["' ] / i ; my $ html = do { local $ / ; < DATA > }; # slurp _DATA_
section my @ links = ($ html =~ m { $ re } gx ); say for @ links ; __DATA__ < html
>< body > < p >< a href = "http://example.com/" > An Example </ a
></ p > <!-- < a href = "http://invalid.example.com/" > An Example </
a > --> </ body ></ html >
In this self-contained example, I put a small document in the __DATA__
section. This example corresponds to a situation where the maintainer of the page commented out
a previously broken link, and replaced it with the correct link.
It is surprisingly easy to fix using HTML::TokeParser::Simple . Just
replace the body of the script above with:
1 2 3 4 5 6 7 8
use HTML :: TokeParser :: Simple ; my $ parser = HTML :: TokeParser :: Simple -> new
( handle => * DATA ); while ( my $ anchor = $ parser -> get_tag ( 'a' )) { next
unless defined ( my $ href = $ anchor -> get_attr ( 'href' )); say $ href ; }
When run, this script correctly prints:
1 2
$ . / href http : //example.com/
And, it looks like we made it much more readable in the process!
Of course, interesting HTML parsing jobs involve more than just extracting links. While even
that task can be made ever-increasingly complex for the regular expression jockey by, say,
adding some interesting attributes between the a and the href , code
using HTML::TokeParser::Simple would not be affected.
Another specialized HTML parsing module is HTML::TableExtract . In most cases, it
makes going through tables on a page a breeze. For example, the State
Actions to Address Health Insurance Exchanges contains State Table 2: Snapshot of State
Actions and Figures. The contents of this page may change with new developments, so here is
a screenshot of the first few lines of the table:
Parsing this table using HTML::TableExtract is straightforward:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
use HTML :: TableExtract ; use Text :: Table ; my $ doc =
'state-actions-to-implement-the-health-benefit.aspx' ; my $ headers = [ 'State' , 'Selected
a Plan' ]; my $ table_extract = HTML :: TableExtract -> new ( headers => $ headers );
my $ table_output = Text :: Table -> new (@$ headers ); $ table_extract -> parse_file
($ doc ); my ($ table ) = $ table_extract -> tables ; for my $ row ($ table -> rows )
{ clean_up_spaces ($ row ); # not shown for brevity $ table_output -> load ($ row ); }
print $ table_output ;
Running this script yields:
1 2 3 4 5 6 7
$ . / te State Selected a Plan Alabama 624 Alaska 53 Arizona 739 Arkansas 250
Note that I did not even have to look at the underlying HTML code at all for this code to
work. If it hadn't, I would have had to delve into that mess to find the specific problem, but,
in this case, as in many others in my experience, HTML::TableExtract gave me just
what I wanted. So long as the substrings I picked continue to match the content, my script will
extract the desired columns even if some of the underlying HTML changes.
Both HTML::TokeParser::Simple (based on HTML::PullParser ) and
HTML::TableExtract (which subclasses HTML::Parser parse a stream rather than loading
the entire document to memory and building a tree. This made them performant enough for
whatever I was able to throw at them in the past.
With HTML::TokeParser::Simple , it is also easy to stop processing a file once
you have extracted what you need. That helps when you are dealing with thousands of documents,
each several megabytes in size where the interesting content is located towards the beginning.
With HTML::TablExtract , performance can be improved by switching to less robust
table identifiers such as depths and counts. However, in certain pathological conditions I seem
to run into a lot, you may need to play with regexes to first extract the exact region of the
HTML source that contains the content of interest.
In one case I had to process large sets of HTML files I had to process where each file was
about 8 Mb. The interesting table occurred about 3/4 through the HTML source, and it was
clearly separated from the rest of the page by <!-- interesting content here
--> style comments. In this particular case, slurping each file, extracting the
interesting bit, and passing the content to HTML::TableExtract helped. Throw a
little Parallel::ForkManager into the mix, and
a task that used to take a few hours went down to less than half an hour.
Sometimes, you just need to be able to extract the contents of the third span within the
sixth paragraph of the first content div on the right. Especially if you need to extract
multiple pieces of information depending on various parts of the document, creating a tree
structure will make that task simpler. It may have a huge performance cost, however, depending
on the size of the document. Building trees out of the smallest possible HTML fragments can
help here.
Once you have the tree structure, you can address each element or sets of elements.
XPath is a way of addressing those
elements. HTML::TreeBuilder builds a tree
representation of HTML documents. HTML::TreeBuilder::XPath adds the
ability to locate nodes in that representation using XPath expressions. So, if I wanted to get
the table of contents of the same document, I could have used something along the lines of:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
use HTML :: TreeBuilder :: XPath ; use Text :: Table ; my $ doc =
'state-actions-to-implement-the-health-benefit.aspx' ; my $ tree = HTML :: TreeBuilder ::
XPath -> new ; my $ toc_table = Text :: Table -> new ( 'Entry' , 'Link' ); $ tree
-> parse_file ($ doc ); my @ toc = $ tree -> findnodes (
'//table[@id="bookmark"]/tbody/*/*/*//li/a' ); for my $ el ( @ toc ) { $ toc_table ->
add ( $ el -> as_trimmed_text , $ el -> attr ( 'href' ), ); } print $ toc_table
;
Mojo::DOM is an
excellent module that uses JQuery style selectors
to address individual elements. It is extremely helpful when dealing with documents were HTML
elements, classes, and ids were used in intelligent ways.
XML::Twig will also work for
some HTML documents, but in general, using an XML parser to parse HTML documents found in the
wild is perilious. On the other hand, if you do have well-formed documents, or HTML::Tidy can make them nice,
XML::Twig is a joy to use. Unfortunately, it is depressingly too common to find
documents pretending to be HTML, using a mish-mash of XML and HTML styles, and doing all sorts
of things which browsers can accommodate, but XML parsers cannot.
And, if your purpose is just to clean some wild HTML document, use HTML::Tidy . It gives you an interface to the
command line utility tidyp . For really
convoluted HTML, it sometimes pays to pass through tidyp first before feeding it
into one of the higher level modules.
Thanks to others who have built on HTML::Parser , I have never had to write a line of
event handler code myself for real work. It is not that they are difficult to write. I do
recommend you study the examples bundled with the
distribution to see how the underlying machinery works. It is just that the modules others
have built on top of and beyond HTML::Parser make life so much easier that I never
had to worry much about going to the lowest possible level.
That's a good thing.
Editor's note: If you're looking for tips on how to write more efficient, robust, and
maintainable Perl code, you'll want to check out Damien Conway's " Modern Perl Best Practices " video.
For some reason, there exists a common misconception that there is no cross-platform,
built-in way in Perl to handle binary files. The copy_file code snippet below illustrates that
Perl handles such tasks quite well. The trick is to use "binmode" on both the input and output
files after opening them. "Binmode" switches files to binary mode, which for the input file
means it won't stop reading at the first "end of text file" character (^Z in win/dos); for the
output file binmode means it won't translate '\n' (LF) into '\r\n' (CRLF) when printing. In
this way the files get copied byte for byte.
sub copy_file {
my ($srcfile, $destfile) = @_;
my $buffer;
open INF, $srcfile
or die "\nCan't open $srcfile for reading: $!\n";
open OUTF, ">$destfile"
or die "\nCan't open $destfile for writing: $!\n";
binmode INF;
binmode OUTF;
while (
read (INF, $buffer, 65536) # read in (up to) 64k chunks, write
and print OUTF $buffer # exit if read or write fails
) {};
die "Problem copying: $!\n" if $!;
close OUTF
or die "Can't close $destfile: $!\n";
close INF
or die "Can't close $srcfile: $!\n";
}
jpk1292000
has asked for the wisdom of the Perl Monks
concerning the following question:
Hi monks, I'm new to the board and I've been struggling with this problem for some time now.
Hope someone can give me some suggestions... I am trying to read a binary file with the following format: The 4-byte
integer and (4 byte float) are in the native format of the machine.
*** First record (4 byte integer) - byte size of record (4*N) (f77 header) (4 byte float) ..
value 1 (4 byte float) .. value 2 ... (4 byte float) .. value N N = number of grid points in the field (4 byte
integer) .. byte size of record (4*N) (f77 trailer) **** Second record (4 byte integer) - byte size of record (4*N)
(f77 header) (4 byte float) .. value 1 (4 byte float) .. value 2 ... (4 byte float) .. value N N = number of grid
points in the field (4 byte integer) .. byte size of record (4*N) (f77 trailer)
[download]
The data is meteorological data (temperature in degrees K) on a 614 x 428 grid. I tried coding up a reader for this,
but am getting nonsensical results. Here is the code:
my $out_file = "/dicast2-papp/DICAST/smg_data/" . $gfn . ".bin"; #path
+
to binary file my $template = "if262792i"; #binary layout (integer 262792 floats
in
+
teger) as described in the format documentation
above (not sure if th
+
is is correct) my $record_length
= 4; #not sure what record_length is supposed to rep
+
resent
(number of values in 1st record, or should it be length of var
+
iable
[4 bytes]) my (@fields,$record); open (FH, $out_files ) || die "couldn't open $out_files\n"; until (eof(FH)) { my $val_of_read
= read (FH, $record, $record_length) == $record_
+
length
or die "short read\n"; @fields = unpack ($template, $record); print "field = $fields[0]\n"; }
[download]
The results I get when I print out the first field are non-sensical (negative numbers, etc). I think the issue is
that I'm not properly setting up my template and record length. Also, how do I find out what is "the native format of
the machine"?
You can find out more about how "read" works by reading
its documentation
.
From there, you'll find out that the third parameter (your $record_length) is the number of bytes to read
from the filehandle[1]. As your template is set up to handle all of the data for one record in one go, you'll
need to read one record's worth of data. That's 4 * (1 + 262792 + 1) bytes of data. Currently you're reading
four bytes, and the template is looking for a lot more.
If there are more pack codes or if the repeat count of a field or a group is larger than what the
remainder of the input string allows, the result is not well defined: in some cases, the repeat count is
decreased, or unpack() will produce null strings or zeroes, or terminate with an error. If the input string
is longer than one described by the TEMPLATE, the rest is ignored.
[1] Actually, the number of _characters_ but let's assume single byte characters for the time being.
Depending on your OS, another problem is the lack of
binmode
. Add
binmode(FH)
after the
open
so that Perl doesn't mess
with the data. Not all OSes require
binmode
, but it's safe to use
binmode
on all OSes.
Oh and I'd use
l
instead of
i
.
i
is not guaranteed to be 4 bytes.
jpk1292000
(Initiate)
on Nov 16, 2006 at 19:09 UTC
by
jpk1292000
(Initiate)
on Nov 16, 2006 at 19:09 UTC
Got it working. Thanks for help. My problem was two-fold. I wasn't using the correct record length, and I
wasn't using bin mode. Once I fixed these two issues, it worked.
Something like this should do it. See the docs and/or ask for anything you do not understand.
#! perl -slw use strict; my @grid; open my $fh, '<:raw', 'the file' or die $!; while( 1
) { my( $recSize, $dummy, $record ); sysread( $fh, $recSize, 4 ) or last; $recSize = unpack 'N', $recSize;
##(*) sysread( $fh, $record, $recSize ) == $recSize or die "truncated record"; sysread( $fh, $dummy, 4 ) == 4
and unpack( 'N', $dummy ) == $recSize ##(*) or die "missing or invalid trailer"; ## (*) You may need V
depending upon which platform your file was
+
created
on push @grid, [ unpack 'N*', $record ]; } close $fh; ## @grid should now contain your data ## Addressable in
the usual $grid[ X ][ Y ] manner. ## Though it might be $array[ Y ][ X ] ## I forget which order FORTRAN
writes arrays in?
[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
Why sysread over read? The only difference is that read is buffered, which is a good thing. I'd
replace sysread with read.
Partially habit. On my system, at least at some point in the past, the interaction between Perl
buffering and the OS caching was less productive that using the systems caching alone.
It bypasses buffered IO, so mixing this with other kinds of reads, print, write, seek, tell, or eof can
cause confusion because the perlio or stdio layers usually buffers data.
And since I used
'<:raw'
, which (as I understand it, bypasses PerlIO
layers), it seems prudent to avoid buffered IO calls.
N* for floats?
Mea culpa. The code is untested as I don't have a relevant data file, and could not mock one up because
I do not know what system it was written on.
Basically, the code I posted was intended as an example of how to proceed, not production ready
copy&paste.
I don't think a smaller than expected return value is an error. It simply means you need to call the read
function again.
I think that's true when reading from a stream device--terminal, socket or pipe--but for a disk file, if
you do not get the requested number of bytes, (I believe) it means end of file.
I'm open to correction on that, but I do not see the circumstances in which a disk read would fail to
return the requested number of bytes if they are available?
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
#!/usr/bin/perl -w use strict; open FILE, 'file.bin' or die "Couldn't open file: $!\n";
binmode FILE; my $record = 1; my $buffer = ''; while ( read( FILE, $buffer, 4 ) ) { my $record_length =
unpack 'N', $buffer; my $num_fields = $record_length / 4; printf "Record %d. Number of fields = %d\n",
$record, $num_fie
+
lds; for (1 .. $num_fields ) {
read( FILE, $buffer, 4 ); my $temperature = unpack 'f', $buffer; # Or if the above gives the wrong result try
this: #my $temperature = unpack 'f', reverse $buffer; print "\t", $temperature, "\n"; } # Read but ignore
record trailer. read( FILE, $buffer, 4 ); print "\n"; $record++; } __END__
[download]
If the number of fields is wrong subtitute
unpack 'V'
for
unpack 'N'
. If the float is wrong
try the
reverse
ed value that is commented out.
I'm having an issue with writing a Perl script to read a binary file.
My code is as the following
whereby the
$file
are files in binary format. I tried to search through the web and apply
in my code, tried to print it out, but it seems it doesn't work well.
Currently it only prints the '&&&&&&&&&&&" and ""ppppppppppp", but what I really want is it can
print out each of the
$line
, so that I can do some other post processing later. Also, I'm
not quite sure what the
$data
is as I see it is part of the code from sample in article,
stating suppose to be a scalar. I need somebody who can pin point me where the error goes wrong in my
code. Below is what I did.
my $tmp = "$basedir/$key";
opendir (TEMP1, "$tmp");
my @dirs = readdir(TEMP1);
closedir(TEMP1);
foreach my $dirs (@dirs) {
next if ($dirs eq "." || $dirs eq "..");
print "---->$dirs\n";
my $d = "$basedir/$key/$dirs";
if (-d "$d") {
opendir (TEMP2, $d) || die $!;
my @files = readdir (TEMP2); # This should read binary files
closedir (TEMP2);
#my $buffer = "";
#opendir (FILE, $d) || die $!;
#binmode (FILE);
#my @files = readdir (FILE, $buffer, 169108570);
#closedir (FILE);
foreach my $file (@files) {
next if ($file eq "." || $file eq "..");
my $f = "$d/$file";
print "==>$file\n";
open FILE, $file || die $!;
binmode FILE;
foreach ($line = read (FILE, $data, 169108570)) {
print "&&&&&&&&&&&$line\n";
print "ppppppppppp$data\n";
}
close FILE;
}
}
}
I have altered my code so that it goes like as below. Now I can read the $data. Thanks J-16 SDiZ for
pointing out that. I'm trying to push the info I got from the binary file to an array called "@array",
thinkking to grep data from the array for string whichever match "p04" but fail. Can someone point out
where is the error?
my $tmp = "$basedir/$key";
opendir (TEMP1, "$tmp");
my @dirs = readdir (TEMP1);
closedir (TEMP1);
foreach my $dirs (@dirs) {
next if ($dirs eq "." || $dirs eq "..");
print "---->$dirs\n";
my $d = "$basedir/$key/$dirs";
if (-d "$d") {
opendir (TEMP2, $d) || die $!;
my @files = readdir (TEMP2); #This should read binary files
closedir (TEMP2);
foreach my $file (@files) {
next if ($file eq "." || $file eq "..");
my $f = "$d/$file";
print "==>$file\n";
open FILE, $file || die $!;
binmode FILE;
foreach ($line = read (FILE, $data, 169108570)) {
print "&&&&&&&&&&&$line\n";
print "ppppppppppp$data\n";
push @array, $data;
}
close FILE;
}
}
}
foreach $item (@array) {
#print "==>$item<==\n"; # It prints out content of binary file without the ==> and <== if I uncomment this.. weird!
if ($item =~ /p04(.*)/) {
print "=>$item<===============\n"; # It prints "=><===============" according to the number of binary file I have. This is wrong that I aspect it to print the content of each binary file instead :(
next if ($item !~ /^w+/);
open (LOG, ">log") or die $!;
#print LOG $item;
close LOG;
}
}
Again, I changed my code as following, but it still doesn't work as it do not able to grep the "p04"
correctly by checking on the "log" file. It did grep the whole file including binary like this
"@^@^@^@^G^D^@^@^@^^@p04bbhi06^@^^@^@^@^@^@^@^@^@hh^R^@^@^@^^@^@^@p04lohhj09^@^@^@^^@@" . What I'm
aspecting is it do grep the anything with p04 only such as grepping p04bbhi06 and p04lohhj09. Here is
how my code goes:-
foreach my $file (@files) {
next if ($file eq "." || $file eq "..");
my $f = "$d/$file";
print "==>$file\n";
open FILE, $f || die $!;
binmode FILE;
my @lines = <FILE>;
close FILE;
foreach $cell (@lines) {
if ($cell =~ /b12/) {
push @array, $cell;
}
}
}
#my @matches = grep /p04/, @lines;
#foreach $item (@matches) {
foreach $item (@array) {
#print "-->$item<--";
open (LOG, ">log") or die $!;
print LOG $item;
close LOG;
}
There is no such thing as 'binary format'. Please be more precise.
What format are the files in? What characteristics do they have that cause you to call them 'in
binary format'?
–
reinierpost
Jan 30 '12 at 13:00
It is in .gds format. This file is able to read in Unix with strings
command. It was reaable in my Perl script but I am not able to grep the data I wanted (p04* here
in my code) .
–
Grace
Jan 31 '12 at 6:56
As already suggested, use File::Find or something to get your list of
files. For the rest, what do you really want? Output the whole file content if you found a match?
Or just the parts that match? And what do you want to match?
p04(.*)
matches
anything from "p04" up to the next newline. You then have that "anything" in
$1
.
Leave out all the clumsy directory stuff and concentrate first on what you want out of a single
file. How big are the files? You are only reading the first 170MB. And you keep overwriting the
"log" file, so it only contains the last item from the last file.
–
mivk
Nov 19 '13 at 13:16
@reinierpost the OP under the "binary file" probably mean the opposite
of the text files - e.g. same thing as is in the
perldoc's -X
documentation
see the
-B
explanation. (cite:
-B
File is a "binary"
file (opposite of -T).)
–
jm666
May 12 '15 at 6:44
The data is in
$data
; and
$line
is the number of bytes read.
my $f = "$d/$file" ;
print "==>$file\n" ;
open FILE, $file || die $! ;
I guess the full path is in
$f
, but you are opening
$file
. (In my
testing -- even
$f
is not the full path, but I guess you may have some other glue
code...)
If you just want to walk all the files in a directory, try
File::DirWalk
or
File::Find
.
Hi J-16 SDiZ, thanks for the reply. each of the $file is in binary
format, and what I want to do is to read eaxh of the file to grep some information in readable
format and dump into another file (which I consider here as post processing). I want to perform
something like "strings <filename> | grep <text synctax>" as in Unix. whereby the <filename> is
the $file here in my code. My problem here is cannot read the binary file so that I can proceed
with other stuff. Thanks.
–
Grace
Jan 19 '12 at 2:34
Hi Dinanoid, thanks for your answer, I tried it but it didn't work
well for me. I tried to edit my code as above (my own code, and it didn't work). Also, tried
code as below as you suggested, it didn't work for me either. Can you point out where I did
wrong? Thanks.
–
Grace
Jan 30 '12 at 4:30
I'm not sure I'll be able to answer the OP question exactly, but here are some notes that may be
related. (edit: this is the same approach as answer by @Dimanoid, but with more detail)
Say you
have a file, which is a mix of ASCII data, and binary. Here is an example in a
bash
terminal:
Note that byte
00
(specified as
\x00
) is a non-printable character, (and
in
C
, it also means "end of a string") - thereby, its presence makes
tester.txt
a binary file. The file has size of 13 bytes as seen by
du
, because of the trailing
\n
added by the
echo
(as it can be seen from
hexdump
).
Now, let's see what happens when we try to read it with
perl
's
<>
diamond operator (see also
What's the use of <>
in perl?
):
$ perl -e '
open IN, "<./tester.txt";
binmode(IN);
$data = <IN>; # does this slurp entire file in one go?
close(IN);
print "length is: " . length($data) . "\n";
print "data is: --$data--\n";
'
length is: 7
data is: --aa aa
--
Clearly, the entire file didn't get slurped - it broke at the line end
\n
(and not at
the binary
\x00
). That is because the diamond filehandle
<FH>
operator is
actually shortcut for
readline
(see
Perl
Cookbook: Chapter 8, File Contents
)
The same link tells that one should undef the input record separator,
\$
(which by
default is set to
\n
), in order to slurp the entire file. You may want to have this
change be only local, which is why the braces and
local
are used instead of
undef
(see
Perl Idioms
Explained - my $string = do { local $/; };
); so we have:
$ perl -e '
open IN, "<./tester.txt";
print "_$/_\n"; # check if $/ is \n
binmode(IN);
{
local $/; # undef $/; is global
$data = <IN>; # this should slurp one go now
};
print "_$/_\n"; # check again if $/ is \n
close(IN);
print "length is: " . length($data) . "\n";
print "data is: --$data--\n";
'
_
_
_
_
length is: 13
data is: --aa aa
bb bb
--
... and now we can see the file is slurped in its entirety.
Since binary data implies unprintable characters, you may want to inspect the actual contents of
$data
by printing via
sprintf
or
pack
/
unpack
instead.
I have an attribute (32 bits-long), that each bit responsible to specific functionality. Perl
script I'm writing should turn on 4th bit, but save previous definitions of other bits.
I use in my program:
Sub BitOperationOnAttr
{
my $a="";
MyGetFunc( $a);
$a |= 0x00000008;
MySetFunc( $a);
}
** MyGetFunc/ MySetFunc my own functions that know read/fix value.
Questions:
if usage of $a |= 0x00000008; is right ?
how extract hex value by Regular Expression from string I have : For example:
Your questions are not related; they should be posted separately. That makes it easier for
other people with similar questions to find them. – Michael CarmanJan
12 '11 at 16:13
I upvoted, but there is something very important missing: vec operates on a
string! If we use a number; say: $val=5; printf("b%08b",$val); (this gives
b00000101 ) -- then one can see that the "check bit" syntax, say:
for($ix=7;$ix>=0;$ix--) { print vec($val, $ix, 1); }; print "\n"; will not
work (it gives 00110101 , which is not the same number). The correct is to
convert the number to ASCII char, i.e. print vec(sprintf("%c", $val), $ix, 1); .
– sdaauJul
15 '14 at 5:01
One very common error is to use elseif instead of the correct elsif
keyword. As you program, you'll find that you consistently make certain kinds of errors. This
is okay. Everyone has his or her own little quirks. Mine is that I keep using the assignment
operator instead of the equality operator. Just remember what your particular blind spot is.
When errors occur, check for your personal common errors first.
This section shows some common syntax errors and the error messages that are generated as a
result. First, the error message is shown and then the script that generated it. After the
script, I'll cast some light as to why that particular message was generated.
Missing semiconon in one of the statements
Scalar found where operator expected at test.pl line 2, near "$bar"
(Missing semicolon on previous line?)
$foo = { } # this line is missing a semi-colon.
$bar = 5;
Perl sees the anonymous hash on the first line and is expecting either an operator or the
semicolon to follow it. The scalar variable that it finds, $bar , does not fit the syntax
of an expression because two variables can't be right after each other. In this case, even though
the error message indicates line 2, the problem is in line 1.
Missing quote
Bare word found where operator expected at
test.pl line 2, near "print("This"
(Might be a runaway multi-line "" string starting on line 1)
syntax error at test.pl line 2, near "print("This is "
String found where operator expected at test.pl line 3, near "print(""
(Might be a runaway multi-line "" string starting on line 2)
(Missing semicolon on previous line?)
Bare word found where operator expected at
test.pl line 3, near "print("This"
String found where operator expected at test.pl line 3, at end of line
(Missing operator before ");
?)
Can't find string terminator '"' anywhere before EOF at test.pl line 3.
print("This is a test.\n); # this line is missing a ending quote.
print("This is a test.\n");
print("This is a test.\n");
In this example, a missing end quote has generated 12 lines of error messages! You really
need to look only at the last one in order to find out that the problem is a missing string
terminator. While the last error message describes the problem, it does not tell you where the
problem is. For that piece of information, you need to look at the first line where it tells you to
look at line two. Of course, by this time you already know that if the error message says line 2,
the error is probably in line 1.
Unquoted literal
Can't call method "a" in empty package "test" at test.pl line 1.
print(This is a test.\n); # this line is missing a beginning quote.
The error being generated here is very cryptic and has little to do with the actual problem.
In order to understand why the message mentions methods and packages, you need to understand the
different, arcane ways you can invoke methods when programming with objects. You probably need to
add a beginning quote if you ever see this error message.
... ... ..
This list of syntax errors could go on for quite a while, but you probably understand the
basic concepts:
Errors are not always located on the line mentioned in the error message.
Errors frequently have nothing to do with the error message displayed.
This safer version of chop removes
any trailing string that corresponds to the current value of
$/ (also known as $INPUT_RECORD_SEPARATOR
in the English module). It returns the total number of characters
removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that
the final record may be missing its newline. When in paragraph mode ( $/ = '' ), it removes all trailing newlines
from the string. When in slurp mode ( $/ = undef
) or fixed-length record mode ( $/ is a reference to
an integer or the like; see perlvar ),
chomp won't remove anything. If VARIABLE
is omitted, it chomps $_ . Example:
To debug a Perl program, specify the -d option when you run the program. For
example, to debug a program named debugtest , specify the following command:
$ perl -d debugtest
You can supply other options along with -d if you want to.
When the Perl interpreter sees the -d option, it starts the Perl debugger. The
debugger begins by displaying a message similar to the following one on your screen:
Loading DB routines from $RCSfile: perldb.pl,v $$Revision: 4.0.1.3
$$Date: 92/06/08 13:43:57 $
Emacs support available.
Enter h for help.
main::(debugtest:3): $dircount = 0;
DB<1>
The first few lines display the date on which this version of the debugger was created. The
only lines of interest are the last two.
The second-to-last line in this display lists the line that the debugger is about to
execute. When the debugger starts, the first executable line of the program is displayed.
When the debugger displays a line that it is about to execute, it also provides the
following information about the line:
The package in which the line is contained (in this case, the default package, which is
main )
The name of the file containing the line (here, the file is named debugtest
)
The current line number (which, in this example, is 3)
The last line of the display prompts you for a debugging command. The number enclosed in
angle brackets indicates the command number; in this case, the number is 1 , because
you are about to specify the first debugging command.
Later today you will learn how to use the debugging command number to re-enter debugging
commands you have previously executed.
NOTE
To enter the debugger without supplying a program, supply the -e option with
the -d option:
$ perl -d -e "1;"
This line starts the debugger with a "program" consisting of the single statement
1;
(which is an expression that doesn't do anything meaningful).
Starting the debugger without a program enables you to examine the predefined system
variables or supply statements to be executed. You will learn how to perform both of
these tasks later in today's lesson
You can list any part of your program from within the debugger. The following sections
describe debugging commands that perform the display operations.
The simplest way to list part of your program is with the l command, which lists
the next few statements in your program:
DB<1> l
3: $dircount = 0;
4: $curdir = "";
5: while (1) {
6: # if we don't have a current directory, get one
7: if ($curdir eq "") {
8: print ("Enter directory to list:\n");
9: $curdir = <STDIN>;
10: $curdir =~ s/^\s+|\s+$//g;
11: $curdir = &followlink($curdir);
12: &readsubdirs($curdir);
The l command lists a window of executable statements, which is a group of
statements small enough to be easily displayed on your screen. A window usually consists of
about ten statements. The line number of each statement is displayed at the beginning of its
line.
NOTE
The statements displayed in today's lesson are taken from the program presented in
"Week 2 in Review."
Entering the l command again displays the window of statements immediately
following the last displayed line, which in this example is line 12:
DB<1> l
13: }
14: $curdir = &menudir($curdir);
15: }
16:
17:
18: # Find all subdirectories of the given directory,
19: # and store them in an associative array.
20: #
21: # The associative array subscripts and values are:
22: # <directory name>: 1
You can specify the lines displayed by the l command. If you specify a single line
number with the l command, it displays that line:
DB<1> l 10
10: $curdir =~ s/^\s+|\s+$//g;
To display a range of lines, specify the first and last line number, and separate them with
a hyphen:
To display a specified number of lines starting at a certain point, supply the starting line
number, a + character, and a number of additional lines to display:
You also can use the l command to specify a subroutine to display. To do this,
provide the name of the subroutine to display:
DB<1> l readsubdirs
26: sub readsubdirs {
27: local ($dirname) = @_;
28: local ($dirvar, $subdircount, $name, $index);
29:
30: # open the current directory;
31: # $dircount ensures that each file variable is unique
32: $dirvar = "DIR" . ++$dircount;
33: if (!opendir ($dirvar, $dirname)) {
34: warn ("Can't open $dirname\n");
35: return;
This command lists the statements in the subroutine. If the subroutine is too large to fit
in a single window, only the first few statements are listed; you can list subsequent
statements by entering l with no arguments.
You can display the lines immediately preceding the last displayed line by entering the
- command. For example, the following - command lists the window of lines
immediately preceding the subroutine readsubdirs .
DB<1> -
16:
17:
18: # Find all subdirectories of the given directory,
19: # and store them in an associative array.
20: #
21: # The associative array subscripts and values are:
22: # <directory name>: 1
23: # (indicates that directory has been read)
24: # <directory name>.<num> the <num>th subdirectory
25:
Subsequent - commands go back further in the file.
To list a window of lines containinga specified line, use the w command, and
specify the number of the line to be included:
DB<1> w 7
4: $curdir = "";
5: while (1) {
6: # if we don't have a current directory, get one
7: if ($curdir eq "") {
8: print ("Enter directory to list:\n");
9: $curdir = <STDIN>;
10: $curdir =~ s/^\s+|\s+$//g;
11: $curdir = &followlink($curdir);
12: &readsubdirs($curdir);
13: }
The w command displays the three lines before the specified line and fills the
window with the lines following it.
One of the most useful features of the Perl debugger is the capability to execute a program
one statement at a time. The following sections describe the statements that carry out this
action.
To execute a single statement of your program, use the s command:
DB<2> s
main::(debugtest:4): $curdir = "";
This command executes one statement of your program and then displays the next statement to
be executed. If the statement executed needs to read from the standard input file, the debugger
waits until the input is provided before displaying the next line to execute.
TIP
If you have forgotten which line is the next line to execute (because, for example,
you have displayed lines using the l command), you can list the next line to
execute using the L command:
DB<2> L
3: $dircount = 0;
The L command lists the last lines executed by the program. It also lists
any breakpoints and line actions that have been defined for particular lines.
Breakpoints and line actions are discussed later today.
If the statement executed by the s command calls a subroutine, the Perl debugger
enters the subroutine but does not execute any statements in it. Instead, it stops at the first
executable statement in the subroutine and displays it. For example, if the following is the
current line:
main::(debugtest:12): &readsubdirs($curdir);
specifying the s command tells the Perl debugger to enter readsubdirs and
display the following, which is the first executable line of readsubdirs :
main::readsubdirs(debugtest:27): local ($dirname) = @_;
The s command assumes that you want to debug the subroutine you have entered. If
you know that a particular subroutine works properly and you don't want to step through it one
statement at a time, use the n command, described in the following
section.
The n command, like the s command, executes one line of your program and
displays the next line to be executed:
DB<2> n
main::(debugtest:5): while (1) {
The n statement, however, does not enter any subroutines. If the statement executed
by n contains a subroutine call, the subroutine is executed in its entirety. After the
subroutine is executed, the debugger displays the line immediately following the call.
For example, if the current line is
main::(debugtest:12): &readsubdirs($curdir);
the n command tells the debugger to execute readsubdirs and then display
the next line in the program, which is
main::(debugtest:13:): }
Combining the use of s and n ensures that the debugger examines only the
subroutines you want to see.
NOTE
The Perl debugger does not enable you to enter any library functions. You can enter
only subroutines that you have created yourself or that have been created previously
and added to a subroutine library
The f command tells the Perl debugger to execute the remainder of the statements in
the current subroutine and then display the line immediately after the subroutine call. This is
useful when you are looking for a bug and have determined that the current subroutine does not
contain the problem.
If you are stepping through a program using s or n , you can save yourself
some typing by just pressing Enter when you want to execute another statement. When you press
Enter, the debugger repeats the last s or n command executed.
For example, to step from line 5 to line 7, you can use the s command as usual:
DB<3> s
main::(debugtest:7): if ($curdir eq "") {
(Line 6 is skipped because it contains no executable statements.) To execute line 7, you can
now just press Enter:
DB<2>
main::(debugtest:8): print ("Enter directory to list:\n");
NOTE
Pressing Enter has no effect if you have not specified any s or n
commands.
If you are inside a subroutine and decide that you no longer need to step through it, you
can tell the Perl debugger to finish executing the subroutine and return to the statement after
the subroutine call. To do this, use the r command:
DB<4> r
main::(debugtest:13:): }
The statement displayed by the debugger is the first statement following the call to the
subroutine.
Another powerful feature of the Perl debugger is the capability to display the value of any
variable at any time. The following sections describe the commands that perform this
action.
The X command displays variables in the current package (which is main if
no other package has been specified). If the X command is specified by itself, it
lists all the variables in the current package, including the system-defined variables and the
variables used by the Perl interpreter itself. Usually, you won't want to use the X
command by itself, because there are a lot of system-defined and internal variables known to
the Perl interpreter.
To print the value of a particular variable or variables, specify the variable name or names
with the X command:
DB<5> X dircount
$dircount = '0'
This capability often is useful when you are checking for errors in your program.
You must not supply the $ character with the variable name when you use the
X command. If you supply the $ character (or the @ or
% characters for arrays), the debugger displays nothing.
You can use X to display the values of array variables and associative array
variables.
Each command prints the subscripts of the array and their values. Regular arrays are printed
in order of subscript; associative arrays are printed in no particular order.
NOTE
If you have an array variable and a scalar variable with the same name, the
X command prints both variables:
DB<8> X var
$var = '0'
@var = (
0 'test1'
1 'test2'
)
There is no way to use X to display one variable but not the other.
The V command is identical to the X command except that it prints the
values of variables in any package. If you specify just a package name, as in the following,
this command displays the values of all variables in the package (including system-defined and
internal variables):
DB<9> V mypack
If you specify a package name and one or more variable names, as in the following, the
debugger prints the values of the variables (if they are defined in that package):
As you have seen, you can tell the Perl debugger to execute one statement at a time. Another
way of controlling program execution is to tell the debugger to execute up to a certain
specified point in the program, called a breakpoint .
The following sections describe the commands that create breakpoints, and the command that
executes until a breakpoint is detected.
To set a breakpoint in your program, use the b command. This command tells the
debugger to halt program execution whenever it is about to execute the specified line. For
example, the following command tells the debugger to halt when it is about to execute line
10:
DB<11> b 10
(If the line is not breakable, the debugger will return Line 10 is not breakable
.)
NOTE
You can have as many breakpoints in your program as you want. The debugger will halt
program execution if it is about to execute any of the statements at which a
breakpoint has been defined.
The b command also accepts subroutine names:
DB<12> b menudir
This sets a breakpoint at the first executable statement of the subroutine menudir
.
You can use the b command to tell the program to halt only when a specified
condition is true. For example, the following command tells the debugger to halt if it is about
to execute line 10 and the variable $curdir is equal to the null string:
DB<12> b 10 ($curdir eq "")
The condition specified with the b statement can be any legal Perl conditional
expression.
If a statement is longer than a single line, you can set a breakpoint only at the
first line of the statement:
71: print ("Test", 72: " here is more output");
Here, you can set a breakpoint at line 71, but not line 72.
After you have set a breakpoint, you can tell the debugger to execute until it reaches
either the breakpoint or the end of the program. To do this, use the c command:
DB<13> c
main::(debugtest:10): $curdir =~ s/^\s+|\s+$//g;
DB<14>
When the debugger detects that it is about to execute line 10-the line at which the
breakpoint was set-it halts and displays the line. (Recall that the debugger always displays
the line it is about to execute.)
The debugger now prompts you for another debugging command. This action enables you to start
executing one statement at a time using n or s , continue execution using
c , set more breakpoints using b , or perform any other debugging
operation.
You can specify a temporary (one-time-only) breakpoint with the c command by
supplying a line number:
DB<15> c 12
main::(debugtest:12): &readsubdirs($curdir);
The argument 12 supplied with the c command tells the debugger to define a
temporary breakpoint at line 12 and then resume execution. When the debugger reaches line 12,
it halts execution, displays the line, and deletes the breakpoint. (The line itself still
exists, of course.)
Using c to define a temporary breakpoint is useful if you want to skip a few lines
without wasting your time executing the program one statement at a time. Using c also
means that you don't have to bother defining a breakpoint using b and deleting it
using d (described in the following section).
TIP
If you intend to define breakpoints using c or b , it is a good
idea to ensure that each line of your program contains at most one statement. If you
are in the habit of writing lines that contain more than one statement, such as
$x++; $y++;
you won't get as much use out of the debugger, because it can't stop in the middle of
a line
To list all of your breakpoints, use the L command. This command lists the last few
lines executed, the current line, the breakpoints you have defined, and the conditions under
which the breakpoints go into effect.
DB<16> L
3: $dircount = 0;
4: $curdir = "";
5: while (1) {
7: if ($curdir eq "") {
10: $curdir =~ s/^\s+|\s+$//g;
break if (1)
Here, the program has executed lines 3-7, and a breakpoint is defined for line 10. (Line 6
is not listed because it is a comment.) You can distinguish breakpoints from executed lines by
looking for the breakpoint conditional expression, which immediately follows the breakpoint.
Here, the conditional expression is (1) , which indicates that the breakpoint is
always in effect.
When you run a program using the Perl debugger, you can tell it to display each line as it
is executed. When the debugger is doing this, it is said to be in trace mode .
To turn on trace mode, use the T command.
DB<18> t
Trace = on
When a statement is executed in trace mode, the statement is displayed. For example, if the
current line is line 5 and the command c 10 (which executes up to line 10) is entered,
the following is displayed:
DB<18> c 10
main::(debugtest:5): while (1) {
main::(debugtest:7): if ($curdir eq "") {
main::(debugtest:10): $curdir =~ s/^\s+|\s+$//g;
DB<19>
The debugger prints and executes line 5 and line 7, then displays line 10 and waits for
further instructions.
To turn off trace mode, specify the t command again.
DB<19> t
Trace = off
At this point, trace mode is turned off until another t command is
entered.
The Perl debugger enables you to specify one or more statements to be executed whenever the
program reaches a specified line. Such statements are known as line actions. The most common
line actions are printing the value of a variable and resetting a variable containing an
erroneous value to the value you want.
The following sections describe the debugging commands that define line
actions.
To specify a line action for a particular line, use the a command.
DB<19> a 10 print ("curdir is $curdir\n");
This command tells the debugger to execute the statement
print ("curdir is $curdir\n");
whenever it is about to execute line 10 of the program. The debugger performs the action
just after it displays the current line and before it asks for the next debugging command.
To create a line action containing more than one statement, just string the statements
together. If you need more than one line for the statements, put a backslash at the end of the
first line.
DB<20> a 10 print ("curdir is $curdir\n"); print \
("this is a long line action\n");
In this case, when the debugger reaches line 10, it executes the following statements:
print ("curdir is $curdir\n");
print ("this is a long line action\n");
To define a line action that is to be executed before the debugger executes any further
statements, use the > command.
DB<21> > print ("curdir before execution is $curdir\n");
This command tells the debugger to print the value of $curdir before
continuing.
Similarly, the < command defines a line action that is to be performed after the
debugger has finished executing statements and before it asks for another debugging
command:
DB<22> < print ("curdir after execution is $curdir\n");
This command tells the debugger to print the value of $curdir before halting
execution again.
The < and > commands are useful when you know that one of your
variables has the wrong value, but you don't know which statement assigned the wrong value to
the variable. By single-stepping through the program using s or n , and
printing the variable either before or after executing each statement, you can determine where
the variable was given its incorrect value.
NOTE
To delete a line action defined by the < command, enter another
< command with no line action defined.
DB<23> <
Similarly, the following command undoes the effects of a > command:
The L command prints any line actions you have defined using the a command
(as well as breakpoints and executed lines). For example, suppose that you have defined a line
action using the following command:
DB<25> a 10 print ("curdir is $curdir\n");
The L command then displays this line action as shown here:
main::(debugtest:10): $curdir =~ s/^\s+|\s+$//g;
action: print ("curdir is $curdir\n");
The line action is always displayed immediately after the line for which it is defined. This
method of display enables you to distinguish lines containing line actions from other lines
displayed by the L command.
The H (for "history") command lists the preceding few commands you have
entered.
DB<4> H
3: b 7
2: b 14
1: b 13
The commands are listed in reverse order, with the most recently executed command listed
first. Each command is preceded by its command number, which is used by the ! command
(described in the following section).
NOTE
The debugger saves only the commands that actually affect the debugging environment.
Commands such as l and s , which perform useful work but do not
change how the debugger behaves, are not listed by the H command.
This is not a significant limitation because you can enter the letter again if
needed.
Each command that is saved by the debugger and can be listed by the H command has a
command number. You can use this command number to repeat a previously executed command. For
example, to repeat command number 5, make the following entry:
DB <11> !5
b 8
DB <12>
The debugger displays command number 5-in this case, the command b 8 - and then
executes it.
If you omit the number, the debugger repeats the last command executed.
DB <12> $foo += $bar + 1
DB <13> !
$foo += $bar + 1
DB <14>
If you specify a negative number with ! , the debugger skips back that many
commands:
DB <14> $foo += $bar + 1
DB <15> $foo *= 2
DB <16> ! -2
$foo += $bar + 1
DB <17>
Here, the ! -2 command refers to the command $foo += $bar + 1 .
You can use ! only to repeat commands that are actually repeatable. Use the
H command to list the commands that the debugger has saved and that can be
repeated
The T command enables you to display a stack trace, which is a collection of all
the subroutines that have been called, listed in reverse order. Here is an example:
DB <16> T
$ = &main::sub2('hi') from file debug1 line 7
$ = &main::sub1('hi') from file debug1 line 3
Here, the T command indicates that the program is currently inside subroutine
sub2 , which was called from line 7 of your program; this subroutine is part of the
main package. The call to sub2 is passed the argument 'hi' .
The $ = preceding the subroutine name indicates that the subroutine call is
expecting a scalar return value. If the call is expecting a list to be returned, the characters
@ = appear in front of the subroutine name.
The next line of the displayed output tells you that sub2 was called by another
subroutine, sub1 . This subroutine was also passed the argument 'hi' , and it
was called by line 3 of the program. Because the stack trace lists no more subroutines, line 3
is part of your main program.
NOTE
The list of arguments passed to a subroutine that is displayed by the stack trace is
the list of actual values after variable substitution and expression evaluation are
performed. This procedure enables you to use the stack trace to check whether your
subroutines are being passed the values you expect.
If you find yourself repeatedly entering a long debugging command and you want to save
yourself some typing, you can define an alias for the long command by using the =
command. For example:
DB <15> = pc print ("curdir is $curdir\n");
= pc print ("curdir is $curdir\n");
The = command prints the alias you have just defined and then stores it in the
associative array %DB'alias (package DB , array name alias ) for
future reference. From here on, the command
DB <16> pc
is equivalent to the command
DB <16> print ("curdir is $curdir\n");
To list the aliases you have defined so far, enter the = command by itself:
DB <17> =
pc = print ("curdir is $curdir\n")
This command displays your defined aliases and their equivalent values.
You can define aliases that are to be created every time you enter the Perl debugger.
When the debugger starts, it first searches for a file named .perldb in your home
directory. If the debugger finds this file, it executes the statements contained there.
To create an alias, add it to the .perldb file. For example, to add the alias
= pc print ("curdir is $curdir\n");
add the following statement to your .perldb file:
$DB'alias{"pc"} = 's/^pc/print ("curdir is $curdir\n");/';
Here's how this works: when the Perl debugger creates an alias, it adds an element to the
$DB'alias associative array. The subscript for this element is the alias you are
defining, and the value is a substitution command that replaces the alias with the actual
command you want to use. In the preceding example, the substitution takes any command starting
with pc and replaces it with
print ("curdir is $curdir\n");
Be careful when you define aliases in this way. For example, your substitution should
match only the beginning of a command, as in /^pc/ . Otherwise, the alias
will replace any occurrence of the letters pc with your print command, which
is not what you want.
The h (for help) command provides a list of each of the debugger commands listed in
today's lesson, along with a one-line explanation of each. This is handy if you are in the
middle of debugging a program and forget the syntax of a particular command.
Is it possible to enter more than one debugging command at a time?
A:
No; however, there's no real need to do so. If you want to perform several single steps
at once, use the c command to skip ahead to a specified point. If you want to both
step ahead and print the value of a variable, use the < or >
command.
Q:
Is it possible to examine variables in one package while inside another?
A:
Yes. Use the V command or the standard Perl package/variable syntax.
Q:
If I discover that my program works and I want to turn off debugging, what do I
do?
A:
You cannot exit the debugger in the middle of a program. However, if you delete all
breakpoints and line actions and then enter the c command, the program begins
executing normally and is no longer under control of the debugger.
Q:
How can I convert to a reusable breakpoint a one-time breakpoint created using
c ?
A:
By default, the b command sets a breakpoint at the line that is about to be
executed. This is the line at which c has set its one-time breakpoint.
Q:
How can I execute other UNIX commands from inside the debugger?
A:
Enter a statement containing a call to the Perl system function. For example,
to display the contents of the current directory, enter the following command: DB <11> system ("ls"); To temporarily escape from the debugger to a UNIX
shell, enter the following command: DB <12> system ("sh"); When you are finished with the shell, enter the
command exit, and you will return to the debugger.
Q:
What special built-in variables can be accessed from inside the debugger?
Debugger commands are then stored in ~/.perldb.hist and accessible across
sessions.
I did the following:
1) Created ~/.perldb , which did not exist previously.
2) Added &parse_options("HistFile=$ENV{HOME}/.perldb.hist"); from mirod's
answer.
3) Added export PERLDB_OPTS=HistFile=$HOME/.perldb.history to ~/.bashrc from
mephinet's answer.
4) Ran source .bashrc
5) Ran perl -d my program.pl , and got this warning/error
perldb: Must not source insecure rcfile /home/ics/.perldb.
You or the superuser must be the owner, and it must not
be writable by anyone but its owner.
6) I protected ~/.perldb with owner rw chmod 700 ~/.perldb , and
the error went away.
There is one more variation of the list code command, l . It is the ability to list
the code of a subroutine, by typing l sub , where
sub is the subroutine name.
Running the code in Listing 2 returns:
Loading DB routines from perl5db.pl version 1
Emacs support available.
Enter h or h h for help.
main::(./p2.pl:3): require 5.001;
DB<1>
Entering l searchdir allows us to see the text of searchdir , which is the
meat of this program.
22 sub searchdir { # takes directory as argument
23: my($dir) = @_;
24: my(@files, @subdirs);
25
26: opendir(DIR,$dir) or die "Can't open \"
27: $dir\" for reading: $!\n";
28
29: while(defined($_ = readdir(DIR))) {
30: /^\./ and next; # if file begins with '.', skip
31
32 ### SUBTLE HINT ###
As you can see, I left a subtle hint. The bug is that I deleted an important line at
this point.
Setting Breakpoints
If we were to step through every line of code in a subroutine that is supposed to be
recursive, it would take all day. As I mentioned before, the code as in Listing 2 seems only to
list the files in the current directory, and it ignores the files in any subdirectories. Since
the code only prints the files in the current, initial directory, maybe the recursive calls
aren't working. Invoke the Listing 2 code under the debugger.
Now, set a breakpoint. A breakpoint is a way to tell the debugger that we want normal
execution of the program until it gets to a specific point in the code. To specify where the
debugger should stop, we insert a breakpoint. In the Perl debugger, there there are two basic
ways to insert a breakpoint. The first is by line number, with the syntax b linenum . If
linenum is omitted, the breakpoint is inserted at the next line about to be executed.
However, we can also specify breakpoints by subroutine, by typing b sub
, where sub is the subroutine name. Both forms of breakpointing take an
optional second argument, a Perl conditional. If when the flow of execution reached the
breakpoint the conditional evaluates to true, the debugger will stop at the breakpoint;
otherwise, it will continue. This gives greater control of execution.
For now we'll set a break at the searchdir subroutine with b searchdir . Once
the breakpoint is set, we'll just execute until we hit the subroutine. To do this, enter
c (for continue). Adding Actions
Looking at the code in Listing 2, we can see that the first call to searchdir comes
in the main code. This seems to works fine, or else nothing would be printed out. Press
c again to continue to the next invocation of searchdir , which occurs in the
searchdir routine.
We wish to know what is in the $dir variable, which represents the directory that
will be searched for files and subdirectories. Specifically, we want to know the contents of
this variable each time we cycle through the code. We can do this by setting an action. By
looking at the program listing, we see that by line 25, the variable $dir has been
assigned. So, set an action at line 25 in this way:
a 25 print "dir is $dir\n"
Now, whenever line 25 comes around, the print command will be executed. Note that for
the a command, the line number is optional and defaults to the next line to be
executed.
Pressing c will execute the code until we come across a breakpoint, executing action
points that are set along the way. In our example, pressing c continuously will yield
the following:
main::(../p2.pl:3): require 5.001;
DB<1> b searchdir
DB<2> a 25 print "dir is $dir\n"
DB<3> c
main::searchdir(../p2.pl:23): my($dir) = @_;
DB<3> c
dir is .
main::searchdir(../p2.pl:23): my($dir) = @_;
DB<3> c
dir is dir1.0
main::searchdir(../p2.pl:23): my($dir) = @_;
DB<3> c
dir is dir2.0
main::searchdir(../p2.pl:23): my($dir) = @_;
DB<3> c
dir is dir3.0
file1
file1
file1
file1
DB::fake::(/usr/lib/perl5/perl5db.pl:2043):
2043: "Debugged program terminated. Use `q' to quit or `R' to
restart.";
DB<3>
Note that older versions of the debugger don't output the last line as listed here, but
instead exit the debugger. This newer version is nice because when the program has finished it
still lets you have control so that you can restart the program.
It still seems that we aren't getting into any subdirectories. Enter D and A
to clear all breakpoints and actions, respectively, and enter R to restart. Or, in older
debugger versions, simply restart the program to begin again.
We now know that the searchdir subroutine isn't being called for any subdirectories
except the first level ones. Looking back at the text of the program, notice in lines 44
through 46 that the only time the searchdir subroutine is called recursively is when there is
something in the @subdirs list. Put an action at line 42 that will print the $dir
and @subdirs variables by entering:
a 42 print "in $dir is @subdirs \n"
Now, put a breakpoint at line 12 to prevent the program from outputting to our screen ( b
12 ), then enter c . This will tell us all the subdirectories that our program
thinks are in the directory.
main::(../p2.pl:3): require 5.001;
DB<1> a 42 print "in $dir is @subdirs \n"
DB<2> b 12
DB<3> c
in . is dir1.0 dir2.0 dir3.0
in dir1.0 is
in dir2.0 is
in dir3.0 is
main::(../p2.pl:12): foreach (@files) {
DB<3>
This program sees that there are directories in ".", but not in any of the subdirectories
within ".". Since we are printing out the value of @subdirs at line 42, we know that
@subdirs has no elements in it. (Notice that when listing line 42, there is the letter "a"
after the line number and a colon. This tells us that there is an action point here.) So, nothing
is being assigned to @subdirs in line 37, but should be if the current (as held in
$_ ) file is a directory. If it is, it should be pushed into the @subdirs list. This
is not happening.
One error I've committed (intentionally, of course) is on line 38. There is no catch-all
"else" statement. I should probably put an error statement here. Instead of doing this, let's
put in another action point. Reinitialize the program so that all points are cleared and enter
the following:
a 34 if( ! -f $_ and ! -d $_ ) { print "in $dir: $_ is
weird!\n" }
b 12"
c
which reveals:
main::(../p2.pl:3): require 5.001;
DB<1> a 34 if( ! -f $_ and ! -d $_ ) { print "in $dir:
$_ is weird!\n" }
DB<2> b 12
DB<3> c
in dir1.0: dir1.1 is weird!
in dir1.0: dir2.1 is weird!
in dir1.0: file2 is weird!
in dir1.0: file3 is weird!
in dir2.0: dir2.1 is weird!
in dir2.0: dir1.1 is weird!
in dir2.0: file2 is weird!
in dir2.0: file3 is weird!
main::(../p2.pl:12): foreach (@files) {
DB<3>
While the program can read (through the readdir call on line 29) that dir1.1 is a file
of some type in dir1.0, the file test (the -f construct) on dir1.1 says that it is not.
It would be nice to halt the execution at a point (line 34) where we have a problem. We can
use the conditional breakpoint that I mentioned earlier to do this. Reinitialize or restart the
debugger, and enter:
b 34 ( ! -f $_ and ! -d $_ )
c
p
p $dir
You'll get output that looks like this:
main::(../p2.pl:3): require 5.001;
DB<1> b 34 ( ! -f $_ and ! -d $_ )
DB<2> c
main::searchdir(../p2.pl:34): if( -f $_) { # if its a file...
DB<2> p
dir1.1
DB<2> p $dir
dir1.0
DB<3>
The first line sets the breakpoint, the next c executes the program until the break
point stops it. The p prints the contents of the variable $_ and the last command,
p $dir prints out $dir . So, dir1.1 is a file in dir1.0, but the file tests (
-d and -f ) don't admit that it exists, and therefore dir1.1 is not being inserted
into @subdirs (if it's a directory) or into @files (if it's a file).
Now that we are back at a prompt, we could inspect all sorts of variables, subroutines or
any other Perl construct. To save you from banging your heads against your monitors, and thus
saving both your heads and your monitors, I'll tell you what is wrong.
All programs have something known as the current working directory (CWD). By default, the
CWD is the directory where the program starts. Any and all file accesses (such as file tests or
file and directory openings) are made in reference from the CWD. At no time does our program
change its CWD. But the values returned by the readdir call on line 29 are simply file
names relative to the directory that readdir is reading (which is in $dir ). So,
when we do the readdir , $_ gets assigned a string representing a file (or
directory) within the directory in $dir (which is why it's called a subdirectory). But
when running the -f and -d file tests, they look for $_ in the context of
the CWD. But it isn't in the CWD, it's in the directory represented by $dir . The moral
of the story is that we should be working with $dir/$_ , not just $_ . So the
string
###SUBTLE HINT###
should be replaced by
$_ = "$dir/$_"; # make all path names absolute
That sums it up. Our problem was we were dealing with relative paths, not absolute (from the
CWD) paths.
Putting it back into our example, we need to check dir1.0/dir1.1 , not dir1.1
. To check to make sure that this is what we want, we can put in another action point. Try
typing:
a 34 $_ = "$dir/$_"
c
In effect this temporarily places the corrective measure into our code. Action points are
the first item on the line to be evaluated. You should now see the proper results of the
execution of the program:
DB<1> a 34 $_ = "$dir/$_"
DB<2> c
./file1
./dir1.0/file1
./dir1.0/file2
./dir1.0/file3
./dir1.0/dir1.1/file1
./dir1.0/dir1.1/file2
./dir1.0/dir1.1/file3
./dir2.0/file1
./dir2.0/file2
./dir2.0/file3
./dir2.0/dir2.1/file1
./dir2.0/dir2.1/file2
./dir3.0/file1
DB::fake::(/usr/lib/perl5/perl5db.pl:2043):
2043: "Debugged program terminated. Use `q' to quit or `R' to
restart.";
DB<2>
Stack Traces
Now that we've got the recursive call debugged, let's play with the calling stack a bit.
Giving the command T will display the current calling stack. The calling stack is a list
of the subroutines which have been called between the current point in execution and the
beginning of execution. In other words, if the main portion of the code executes subroutine
"a", which in turn executes subroutine "b", which calls "c", then pressing "T" while in the
middle of subroutine "c" outputs a list going from "c" all the way back to "main".
Start up the program and enter the following commands (omit the second one if you have fixed
the bug we discovered in the last section):
b 34 ( $_ =~ /file2$/)
a 34 $_ = "$dir/$_"
c
These commands set a breakpoint that will only stop execution if the value of the variable
$_ ends with the string file2 . Effectively, this code will halt execution at
arbitrary points in the program. Press T and you'll get this:
@ = main::searchdir('./dir1.0/file2') called from file '../p2.pl' line
45
@ = main::searchdir(.) called from file '../p2.pl' line 10
Enter c , then T again:
@ = main::searchdir('./dir1.0/dir1.1/file2') called from file
`../p2.pl' line 45
@ = main::searchdir(undef) called from file '../p2.pl' line 45
@ = main::searchdir(.) called from file '../p2.pl' line 10
Do it once more:
@ = main::searchdir('./dir2.0/file2') called from file '../p2.pl' line
45
@ = main::searchdir(.) called from file '../p2.pl' line 10
You can go on, if you so desire, but I think we have enough data from the arbitrary stack
dumps we've taken.
We see here which subroutines were called, the debugger's best guess of which arguments were
passed to the subroutine and which line of which file the subroutine was called from. Since the
lines begin with @ = , we know that searchdir will return a list. If it were
going to return a scalar value, we'd see $ = . For hashes (also known as associative
arrays), we would see % = .
I say "best guess of what arguments were passed" because in Perl, the arguments to
subroutines are placed into the @_ magic list. However, manipulating @_ (or $_ ) in the
body of the subroutine is allowed and even encouraged. When a T is entered, the stack
trace is printed out, and the current value of @_ is printed as the arguments to the
subroutine. So when @_ is changed, the trace doesn't reflect what was actually passed as
arguments to the subroutine.
The
= command is used to create command aliases. If you find yourself issuing the same
long command over and over again, you can create an alias for that command. For example, the
debugger command
= pFoo print("foo=$foo\n");
creates an alias called pFoo . After this command is issued, typing pFoo at
the debugger prompt produces the same results as typing print("foo=$foo\n"); .
You use the = command without any arguments when you want a list of the current
aliases.
If you want to set up some aliases that will always be defined, create a file called
.perldb and fill it with your alias definitions. Use the following line as a
template:
$DB::alias{'pFoo'} = 'print("foo=$foo\n");';
After you create this file and its alias definitions, the aliases will be available in every
debugging session.
The debugger reads commands from the files .perldb in the current and home directories, and
stops before the first run-time executable statement, displaying the line it is about to
execute and a prompt:
DB<1>
If you run code from the debugger and hit another breakpoint, the prompt will look like
DB"42". The numbers within the angle brackets are the command numbers, used when repeating
commands.
Any input to the debugger that is not recognized is executed as Perl code in the current
package.
Prefixing a command with ' | ' pipes the output to your current pager.
To see the values of certain variables in the program, use the V command. Used by itself, V lists all the variables
in scope at this time. Here's the syntax:
V [ package [ variable ]]
To look at values in your program, you'll want to look at the main package. For example, to print the value of $reply
, use this command:
V main reply
$reply = '1'
Note that the dollar sign before the variable specified to V is not supplied. Therefore, if you specify the command
V main $reply , you are actually asking for the value of $$reply and not $reply .
The trace option is available with the t toggle command. Issuing trace once turns it on, and issuing
it again turns it off. See Figure 30.4 for a sample use of the trace command on Listing 30.2. In this example, trace
is turned on, and then the c command is issued to run the debugger continuously. In trace mode, the debugger
prints out each line of code that executes.
The X command is helpful when displaying values of variables in the current package. Remember that the main
package is the default package for a Perl script. Issued by itself with no options, the X command displays all the variables
in the current package. Avoid issuing the X command by itself because it can generate a very long listing of all the variables
in the main package.
To see the value of a particular variable instead of all the variables, type the name of the variable after the X command.
For example, the following command
X fileNumber
will print the value of the fileNumber variable in the current package. If you have array variables and scalar
variables with the same name in the same package, the X command will display the values of both these variables. For example,
if you have a scalar variable called names and an array called names , the X command will show the values
of both variables:
You can place breakpoints at suspect locations in your code and run the program until one of the specified breakpoints is hit.
Breakpoints can be specified to be hit as soon as the line of code is about to be executed.
The c command is used to step forward until either the program stops or a specified breakpoint is hit. To specify a breakpoint
at the current line, use the b command without any parameters. To specify a specific line, use the command of the form:
b linenumber
Usually, you use trace statements to see statements between the current execution point and a breakpoint (refer to Figure
30.4). The program is run in continuous mode with the c command until it hits a breakpoint. There is a breakpoint in Listing
30.1 that causes the debugger to stop. The L command is issued in the example to list the breakpoints in the system.
Breakpoints can also be specified to occur at the first executable line of code within a subroutine. Simply use the b
command with the name of the subroutine as the first parameter. For example, to break at the first line of code in the xyc
subroutine, try this command:
b xyc
You can also ask the debugger to look at a condition when a line is hit with a breakpoint tag on it. If the breakpoint is specified
at a line and the condition is true, the debugger stops; otherwise, it keeps on going. For example, if you want the debugger to stop
in xyc only when the global $reply is 1 , use this command:
b xyc ($reply == '1')
To list all breakpoints defined during a debug session, use the L command. If you issue unconditional breakpoints, you'll
see breakpoints listed as this:
break if (1)
The L command will also list up to the last five executed lines of the program.
To remove a breakpoint, use the d command and specify the line number to delete. To remove all breakpoints, use the
D command. For example, to delete a breakpoint at line 12, you would issue the command d 12 .
The DB package uses the following sequence to hit breakpoints and evaluate code on each line of executable code:
Checks to see whether the breakpoint is defined at this line number. If there is no breakpoint defined for this line, it starts
to process the next line. If there is a break-
point at this line, the debugger prepares to stop. If the condition for the defined breakpoint is true, the debugger stops execution
and presents a prompt to the user.
Checks to see whether the line of code is printable. If so, it prints the entire line of code (including code spanning multiple
lines).
Checks to see whether there are any actions defined for this line and performs these actions. (An action is a set of
Perl commands to be executed.)
Checks to see whether the stop was due to a breakpoint. If the condition for the breakpoint is true and a breakpoint has been
marked in this location, the debugger stops and presents a prompt for user interaction.
Evaluates the line and gets ready to execute it. Gets user input if the user is stopping; otherwise, it executes the line
and returns to item 1 in order to process the next line.
You can specify actions to take when a certain line of code is executed. This step is very important when you want to print out
values as the program executes (see Figure 30.5). Notice how the value of reply is printed out when line 73 is reached.
The action is defined with this statement:
Notice that you did not have to terminate the action command with a semicolon. You need to use semicolons only if you have more
than one statement for an action. If you forget to supply the terminating semicolon, the debugger will supply it for you. In any
event, try to keep actions simple and short. Don't write lengthy actions unless absolutely necessary; otherwise, you'll slow down
the debugger and clutter up the output on your terminal.
Actions are not limited to displaying values. For instance, you can use an action to reset a variable to a known value while in
a loop, using a statement like this:
a 73 $reply = 1; print "forced reply to 1\n";
To execute statements within the debugged program's space, simply type the command at the prompt. For example, to explicitly create
and set the value of $kw to 2 in the code, use the following commands at the DB<> prompt:
DB<1> $kw = 2 ... nothing is printed here ... DB<1> print $kw
2
DB<1> V main kw
$kw = '2'
In this example, the variable $kw is created and defined in the program environment. You cannot modify the source code
in the original program, but you can add items to the name space.
In some cases, your program may have redirected its output to STDOUT and therefore whatever it is printing will not be
shown on the console. To evaluate an expression and print its value out to the console regardless of how STDOUT is redirected,
you can use the p command. The p command evaluates an expression in the current program's environment and prints
it out to the debugger console. Basically, the print command prints the output to wherever STDOUT is redirected,
whereas the p command is equivalent to the following print command:
print DB::OUT
The command above forces output from a print command to where the DB:: package prints its output.
To look for certain strings in the source code, you can use the forward slash command followed by the string to look for. Note
that there are no spaces between the / and the string you are looking for. The string can be specified between two slashes,
but the second slash is optional. Actually, you can search for regular expressions, just as in Perl.
To search forward in the file, use the / operator. To search backward, use the question mark operator ( ? ).
The history of the commands you have executed is tracked in the debugger. Only commands greater than one character long are listed
in this directory. To execute commands from the history list, use the bang operator ( ! ) followed by the index of the command.
To execute a command from the history, type ! and the index of the command to redo. This should be familiar to Bash and
C shell programmers.
To see the current history of commands in the buffer of commands in the debugger, type the H command. For example, in
the middle of a debug session, if you type in the H command at the DB<3> prompt, you should expect to see three
items listed in reverse order of execution:
To list all the subroutines currently in the system, use the S command. The output from the S command lists
all subroutines in any package that your code uses. For example, if you run the program in Listing 30.2 with the debugger, you will
see output as shown in Figure 30.6.
At any time in a debug session, you can do a "stack trace," which is a listing of the calling order of the functions called so
far. Be aware that if you are modifying the argument stack in any way, the values of the passed arguments might not be correct. The
T command will do a stack trace for you.
First of all, there is no way to restart the debugger if there is a problem. If you overstep something, you have to start all
over. This means getting out of the program and restarting the debugger.
Second, the debugger itself is not completely debugged yet. If you notice certain problems, such as your commands not being recognized,
it's probably because you typed too many characters at the prompt.
Table 30.1 lists the information about the available debugger commands. All information in this table is gleaned from the
perl5db.pl source file. Keep this table handy so that you don't have to go to the file to see what options are available.
Table 30.1. The commands available from the debugger.
Command
Description
a [ ln ] command
Sets an action to take before the line is executed.
b
Sets an unconditional breakpoint at the current line.
b [ ln ] [ cond ]
Sets a breakpoint if the condition is true at the specified line number.
b sname [ cond ]
Sets a breakpoint at the first line inside the subroutine sname() .
c
Continues until the next breakpoint or until the end of the program.
c line
Continues and stops at the specified line.
d [ line ]
Deletes the breakpoint at a given line.
D
Deletes all breakpoints.
f filename
Switches to the filename as the default.
H - number
Displays history of all commands longer than one character.
L
Lists all breakpoints and actions.
l min+incr
Lists incr+1 lines starting at line #min .
l min-max
Lists lines from min to max , inclusively.
l line
Lists one line of code at a specified line.
l
Lists the next 10 lines of code from the last location.
l name
Lists a subroutine by name.
n
Next code at the same level. Steps over subroutine calls.
p expr
Same as print DB::OUT expr in current package.
q or ^D
Quits. You cannot use quit .
r
Returns from current subroutine.
s
Single-step over code. Steps into subroutines.
S
Lists all known subroutine names in the current scope.
t
Toggles trace mode on and off.
T
Performs a stack trace.
V
Lists all variables in all used packages.
V pkg
List all variables in a given package.
V pkg var
Lists all variables in a package that have var in them.
w line
Lists five lines before and five lines after current line.
<CR>
Repeats last n or s .
-
Lists the previous window.
/ regexp /
Searches forward for a pattern using a regular expression.
? regexp ?
Searches backward for a pattern using a regular expression.
< command
Defines the command before the prompt.
> command
Defines the command after the prompt.
! number
Redoes a command (the default is the previous command).
! - number
Redoes number\'th to the last command.
= [ alias value ]
Starts a command alias.
=
Lists all the current aliases.
command
Executes as a Perl statement in the current package.
There are ways to customize your debugger environment. If you do not like the one-character commands that come with the debugger,
you can use different aliases. There is a hash in the DB:: package called %alias() that contains the command strings.
You can substitute your own commands in place of the existing ones using the = command. Since most of the time you'll want
to keep your changes consistent between debug sessions, you can edit a file called .perldb in the current working directory
and place the assignments there. Here's a sample .perldb file:
These two lines will substitute the value of p for every command ln you type, and the value of l for
every z command. Of course, you'll probably want to alias long commands into short one-character sequences to save yourself
some time.
Using the debugger should not be your only method for getting bugs out of the system. The -w switch is important if you
want Perl to do checking and warn you of error conditions while executing. The types of messages generated vary from warnings to
notifications of fatal errors that can cause the program to abort.
Reading the source file perl5db.pl gives you a few clues about how the debugger works and the commands that are available
during a debug session. Consult the perldebug.html page at www.metronet.com . This file contains the full list
of all the options in the debug environment. Review the perldiag.html page for a list of possible diagnostic values you
get from using the w switch.
Nothing really beats the use of well-placed print statements to do debugging. However, Perl does offer a simple yet powerful
debugging tool with the -d option. The interactive debugger lets you step through code, into or over subroutines, set breakpoints,
execute commands, and look at variables in a Perl program.
In October 2009 we ran a poll asking people Which editor(s) or IDE(s) are you using for
Perl development? . The poll was promoted via the blog of Gabor Szabo which is syndicated in several Perl related
sites such as the Iron Man
Challenge , Perlshpere and Planet Perl . It was also promoted via Twitter , the Perl group in Reddit , the Perl Mongers group in LinkedIn and the Perl Community Adserver to get more people to cast their vote.
Request was also sent to the Perl Monger group
leaders. Some of them have forwarded the request to their respective groups.
The list of editors was taken from the Perl Development Tools page on
Perlmonks and the "randomize answers" checkbox was clicked after filling in the data. No idea
if that really randomized the answers. During the poll people could mark other editors
and type in the name of and editor. Some of these editors were added to the list of possible
answers during the poll. In addition there were people who typed in the name of the editor in
the other field even though the name appeared on the list.
At the begining we set the poll to allow multiple choice with up to 3 answers per person but
later on we noticed that at one of the updates it became multiple choice unlimited answers.
Unfortunatelly the free polling system we used gave details only on the number of answers and
not the number of people who answered.
The poll ran between 21-24 October 2009 for about 72 hours. There were 3,234 answers when it
was closed.
On this page, I will post aides and tools that Perl provides which allow you to more
efficently debug your Perl code. I will post updates as we cover material necessary for
understanding the tools mentioned.
CGI::Dump
Dump is one of the functions exported in CGI.pm's :standard set. It's
functionality is similar to that of Data::Dumper . Rather than pretty-printing a
complex data structure, however, this module pretty-prints all of the parameters passed to
your CGI script. That is to say that when called, it generates an HTML list of each
parameter's name and value, so that you can see exactly what parameters were passed to your
script. Don't forget that you must print the return value of this function - it doesn't do
any printing on its own.
Analyzing two or more chunks of code to see how they compare time-wise is known as
"Benchmarking". Perl provides a standard module that will Benchmark your code for you. It
is named, unsurprisingly, Benchmark . Benchmark provides several helpful
subroutines, but the most common is called cmpthese() . This subroutine takes two
arguments: The number of iterations to run each method, and a hashref containing the code
blocks (subroutines) you want to compare, keyed by a label for each block. It will run each
subroutine the number of times specified, and then print out statistics telling you how
they compare.
For example, my solution to ICA5 contained three different
ways of creating a two dimensional array. Which one of these ways is "best"? Let's have
Benchmark tell us:
#!/usr/bin/perl
use strict;
use warnings;
use Benchmark 'cmpthese';
sub explicit {
my @two_d = ([ ('x') x 10 ],
[ ('x') x 10 ],
[ ('x') x 10 ],
[ ('x') x 10 ],
[ ('x') x 10 ]);
}
sub new_per_loop {
my @two_d;
for (0..4){
my @inner = ('x') x 10;
push @two_d, \@inner;
}
}
sub anon_ref_per_loop {
my @two_d;
for (0..4){
push @two_d, [ ('x') x 10 ];
}
}
sub nested {
my @two_d;
for my $i (0..4){
for my $j (0..9){
$two_d[$i][$j] = 'x';
}
}
}
cmpthese (10_000, {
'Explicit' => \&explicit,
'New Array Per Loop' => \&new_per_loop,
'Anon. Ref Per Loop' => \&anon_ref_per_loop,
'Nested Loops' => \&nested,
}
);
The above code will print out the following statistics (numbers may be slightly off, of
course):
Benchmark: timing 10000 iterations of Anon. Ref Per Loop, Explicit, Nested Loops, New Array Per Loop...
Anon. Ref Per Loop: 2 wallclock secs ( 1.53 usr + 0.00 sys = 1.53 CPU) @ 6535.95/s (n=10000)
Explicit: 1 wallclock secs ( 1.24 usr + 0.00 sys = 1.24 CPU) @ 8064.52/s (n=10000)
Nested Loops: 4 wallclock secs ( 4.01 usr + 0.00 sys = 4.01 CPU) @ 2493.77/s (n=10000)
New Array Per Loop: 2 wallclock secs ( 1.76 usr + 0.00 sys = 1.76 CPU) @ 5681.82/s (n=10000)
Rate Nested Loops New Array Per Loop Anon. Ref Per Loop Explicit
Nested Loops 2494/s -- -56% -62% -69%
New Array Per Loop 5682/s 128% -- -13% -30%
Anon. Ref Per Loop 6536/s 162% 15% -- -19%
Explicit 8065/s 223% 42% 23% --
The benchmark first tells us how many iterations of which subroutines it's running. It
then tells us how long each method took to run the given number of iterations. Finally, it
prints out the statistics table, sorted from slowest to fastest. The Rate column
tells us how many iterations each subroutine was able to perform per second. The remaining
colums tells us how fast each method was in comparison to each of the other methods. (For
example, 'Explicit' was 223% faster than 'Nested Loops', while 'New Array Per Loop' is 13%
slower than 'Anon. Ref Per Loop'). From the above, we can see that 'Explicit' is by far the
fastest of the four methods. It is, however, only 23% faster than 'Ref Per Loop', which
requires far less typing and is much more easily maintainable (if your boss suddenly tells
you he'd rather have the two-d array be 20x17, and each cell init'ed to 'X' rather than
'x', which of the two would you rather had been used?).
You can, of course, read more about this module, and see its other options, by reading:
perldoc Benchmark
Command-line options
Perl provides several command-line options which make it possible to write very quick and
very useful "one-liners". For more information on all the options available, refer to
perldoc perlrun
-e
This option takes a string and evaluates the Perl code within. This is the primary
means of executing a one-liner
perl -e'print qq{Hello World\n};'
(In windows, you may have to use double-quotes rather than single. Either way, it's probably
better to use q// and qq// within your one liner, rather than remembering to escape the quotes).
-l
This option has two distinct effects that work in conjunction. First, it sets $\ (the
output record terminator) to the current value of $/ (the input record separator). In
effect, this means that every print statement will automatically have a newline
appended. Secondly, it auto-chomps any input read via the <> operator, saving you
the typing necessary to do it.
perl -le 'while (<>){ $_ .= q{testing}; print; }'
The above would automatically chomp $_, and then add the newline back on at the print
statement, so that "testing" appears on the same line as the entered string.
-w
This is the standard way to enable warnings in your one liners. This saves you from
having to type use warnings;
This disturbingly powerful option wraps your entire one-liner in a while (<>)
{ ... } loop. That is, your one-liner will be executed once for each line of each
file specified on the command line, each time setting $_ to the current line and $. to
current line number.
perl -ne 'print if /^\d/' foo.txt beta.txt
The above one-line of code would loop through foo.txt and beta.txt, printing out all the
lines that start with a digit. ($_ is assigned via the implicit while (<>) loop, and
both print and m// operate on $_ if an explict argument isn't given).
-p
This is essentially the same thing as -n , except that it places a
continue { print; } block after the while (<>) { ... } loop in
which your code is wrapped. This is useful for reading through a list of files, making
some sort of modification, and printing the results.
perl -pe 's/Paul/John/' email.txt
Open the file email.txt, loop through each line, replacing any instance of "Paul" with
"John", and print every line (modified or not) to STDOUT
-i
This one sometimes astounds people that such a thing is possible with so little typing.
-i is used in conjunction with either -n or -p. It causes the files specified on the
command line to be edited "in-place", meaning that while you're looping through the
lines of the files, all print statements are directed back to the original files. (That
goes for both explicit print s, as well as the print in the continue
block added by -p.)
If you give -i a string, this string will be used to create a back-up copy of the
original file. Like so:
perl -pi.bkp -e's/Paul/John/' email.txt msg.txt
The above opens email.txt, replaces each line's instance of "Paul" with "John", and prints
the results back to email.txt. The original email.txt is saved as email.txt.bkp. The same is then
done for msg.txt
Remember that any of the command-line options listed here can also be given at the end
of the shebang in non-one-liners. (But please do not start using -w in your real programs -
use warnings; is still preferred because of its lexical scope and
configurability).
Data::Dumper
The standard Data::Dumper module is very useful for examining exactly what is contained in
your data structure (be it hash, array, or object (when we come to them) ). When you
use this module, it exports one function, named Dumper . This function
takes a reference to a data structure and returns a nicely formatted description of what
that structure contains.
#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;
my @foo = (5..10);
#add one element to the end of the array
#do you see the error?
$foo[@foo+1] = 'last';
print Dumper(\@foo);
When run, this program shows you exactly what is inside @foo:
$VAR1 = [
5,
6,
7,
8,
9,
10,
undef,
'last'
];
(I know we haven't covered references yet. For now, just accept my assertion that you
create a reference by prepending the variable name with a backslash...)
__DATA__ & <DATA>
Perl uses the __DATA__ marker as a pseudo-datafile. You can use this marker to write quick
tests which would involve finding a file name, opening that file, and reading from that
file. If you just want to test a piece of code that requires a file to be read (but don't
want to test the actual file opening and reading), place the data that would be in the
input file under the __DATA__ marker. You can then read from this pseudo-file using
<DATA>, without bothering to open an actual file:
#!/usr/bin/env perl
use strict;
use warnings;
while (my $line = <DATA>) {
chomp $line;
print "Size of line $.: ", length $line, "\n";
}
__DATA__
hello world
42
abcde
The above program would print:
Size of line 1: 11
Size of line 2: 2
Size of line 3: 5
$.
The $. variable keeps track of the line numbers of the file currently being
processed via a while (<$fh>) { ... } loop. More explicitly, it is the number
of the last line read of the last file read.
__FILE__ & __LINE__
These are two special markers that return, respectively, the name of the file Perl is
currently executing, and the Line number where it resides. These can be used in your own
debugging statements, to remind yourself where your outputs were in the source code:
print "On line " . __LINE__ . " of file " . __FILE__ . ", \$foo = $foo\n";
Note that neither of these markers are variables, so they cannot be interpolated in a
double-quoted string
warn() & die()
These are the most basic of all debugging techniques. warn() takes a list of
strings, and prints them to STDERR. If the last element of the list does not end in a
newline, warn() will also print the current filename and line number on which the
warning occurred. Execution then proceeds as normal.
die() is identical to warn() , with one major exception - the program
exits after printing the list of strings.
All debugging statements should make use of either warn() or die()
rather than print() . This will insure you see your debugging output even if
STDOUT has been redirected, and will give you the helpful clues of exactly where in your
code the warning occurred.
This section explains how to use Open Perl IDE for debugging.
Important: Open Perl IDE is not able to debug any scripts, if it does not know a path to
"perl.exe". If the PATH environment variable contains a valid location, then "perl.exe" will be
detected automatically. Otherwise it is necessary to enter a valid location into the
"Preferences | General | Directories | Path to perl.exe" field.
There are two methods to debug a script:
Set one or more breakpoints (as explained in section 5.1 Breakpoints) and run the script,
which is executed until a breakpoint is reached.
Choose Step Over from the Run Menu. After the script and all required modules are loaded
and initalized, the execution stops on the first line of non-initialization code.
After execution is stopped, it is possible to analyse the actual state of the script by
Viewing Console Output, see section 4.2 Compile and Run a script
Evaluating some variables, see section 5.2 Variable Evaluation
Viewing the list of loaded modules, see section 5.3 Other debug windows
Viewing the callstack, see section 5.3 Other debug windows
Furthermore, it is possible to set/delete breakpoints (see section 5.1 Breakpoints) or to
continue/abort the execution of the script. The following table shows the different navigation
possibilities:
Table: Debug Navigation
Name
Shortcut
Description
Run
F9
Start/Continue script execution until next breakpoint is reached.
Step Over
F8
Execute the current script line, not tracing into subroutines.
Step Into
F7
Execute the next command in the current script line, tracing into subroutines.
Abort
CTRL-F2
Request termination of debug session.
Force Termination
CTRL-ALT-F12
Immediately terminate debug session. You should only use "Force Termination" if you see no other way to stop script
execution. Dont't expect Open Perl IDE to work correctly after using forced termination
!
If script execution has finished, then Open Perl IDE automatically switches back from debug
mode to edit mode.
"Complexity is the enemy, and our aim is to kill it."-Jan Baan
One of Perl's greatest strengths is its expressiveness and extreme conciseness. Complexity
is the bane of software development: when a program grows beyond a certain size, it becomes
much harder to test, maintain, read, or extend. Unfortunately, today's problems mean this is
true for every program we need. Anything you can do to minimize the complexity of your program
will pay handsome dividends.
The complexity of a program is a function of several factors:
The number of distinct lexical tokens
The number of characters
The number of branches in which control can pass to a different point
The number of distinct program objects in scope at any time
Whenever a language allows you to change some code to reduce any of these factors, you
reduce complexity.
3.7.1 Lose the Temporary Variables
The poster child for complexity is the temporary variable. Any time a language intrudes
between you and the solution you visualize, it diminishes your ability to implement the
solution. All languages do this to some degree; Perl less than most. 13 In
most languages, you swap two variables a and b with the following
algorithm:
Declare temp to be of the same type as a and b
temp = a;
a = b;
b = temp;
But most languages are not Perl:
($b, $a) = ($a, $b);
Iterating over an array usually requires an index variable and a count of how many things
are currently stored in the array:
int i;
for (i = 0; i < count_lines; i++)
{
strcat (line[i], suffix);
}
Whereas in Perl, you have the foreach construct borrowed from the shell:
foreach my $line (@lines) { $line .= $suffix }
And if you feel put out by having to type foreach instead of just for ,
you're in luck, because they're synonyms for each other; so just type for if you want
(Perl can tell which one you mean).
Because functions can return lists, you no longer need to build special structures just to
return multivalued data. Because Perl does reference-counting garbage collection, you can
return variables from the subroutine in which they are created and know that they won't be
trampled on, yet their storage will be released later when they're no longer in use. And
because Perl doesn't have strong typing of scalars, you can fill a hierarchical data structure
with heterogeneous values without having to construct a union datatype and some kind of type
descriptor.
Because built-in functions take lists of arguments where it makes sense to do that, you can
pass them the results of other functions without having to construct an iterative loop:
unlink grep /~$/, readdir DIR;
And the map function lets you form a new list from an old one with no unnecessary
temporary variables:
open PASSWD, '/etc/passwd' or die "passwd: $!\n";
my @usernames = map /^([^:]+)/, <PASSWD>;
close PASSWD;
Because Perl's arrays grow and shrink automatically and there are simple operators for
inserting, modifying, or deleting array elements, you don't need to build linked lists and
worry if you've got the traversal termination conditions right. And because Perl has the hash
data type, you can quickly locate a particular chunk of information by key or find out whether
a member of a set exists.
3.7.2 Scope Out the Problem
Of course, sometimes temporary variables are unavoidable. Whenever you create one though, be
sure and do it in the innermost scope possible (in other words, within the most deeply nested
set of braces containing all references to the variable).
Create variables in the innermost
scope possible.
For example, let's say somewhere in my program I am traversing my Netscape history file and
want to save the URLs visited in the last 10 days in @URLs :
use Netscape::History;
my $history = new Netscape::History;
my (@URLs, $url);
while (defined($url = $history->next_url() ))
{
push @URLs, $url if
time - $url->last_visit_time < 10 * 24 * 3600;
}
This looks quite reasonable on the face of it, but what if later on in our program we create
a variable called $history or $url ? We'd get the message
"my" variable $url masks earlier declaration in same scope
which would cause us to search backward in the code to find exactly which one it's referring
to. Note the clause " in same scope " -- if in the meantime you created a variable
$url at a different scope, well, that may be the one you find when searching backward
with a text editor, but it won't be the right one. You may have to check your indentation level
to see the scope level.
This process could be time-consuming. And really, the problem is in the earlier code, which
created the variables $history or $url with far too wide a scope to begin
with. We can (as of perl 5.004) put the my declaration of $url right where it
is first used in the while statement and thereby limit its scope to the while
block. As for $history , we can wrap a bare block around all the code to limit the
scope of those variables:
use Netscape::History;
my @URLs;
{
my $history = new Netscape::History;
while (defined(my $url = $history->next_url() ))
{
push @URLs, $url
if time - $url->last_visit_time < 10 * 24 * 3600;
}
}
If you want to create a constant value to use in several places, use constant.pm to
make sure it can't be overwritten:
$PI = 3.1415926535897932384;
use constant PI => 3.1415926535897932384;
my $volume = 4/3 * PI * $radius ** 3;
$PI = 3.0; # The 'Indiana maneuver' works!
PI = 3.0; # But this does not
In response to the last statement, Perl returns the error message, " Can't modify
constant item in scalar assignment ."
constant.pm creates a subroutine of that name which returns the value you've
assigned to it, so trying to overwrite it is like trying to assign a value to a subroutine
call. Although the absurdity of that may sound like sufficient explanation for how use
constant works, in fact, the latest version of perl allows you to assign a value to a
subroutine call, provided the result of the subroutine is a place where you could store the
value. For example, the subroutine could return a scalar variable. The term for this feature is
lvaluable subroutine . But since the results of the subroutines created by use
constant aren't lvalues, lvaluable subroutines won't cause problems for them.
Bugs are as inevitable as death and taxes. Nevertheless, the following material should help you avoid the pitfalls of bugs.
... ... ...
First
let's simply make sure the bug is repeatable. We'll set an action on line 8 to print $line
where the error occurred, and run the program.
perl -d ./buggy.pl buggy.pl
use Data::Dumpe
a 8 print 'The line variable is now ', Dumper $line
The Data::Dumper module loads so that the autoaction can use a nice output format. The
autoaction is set to do a print statement every time line 8 is reached. Now let's watch the
show.
Earlier we discussed
the basics of how to write and execute a perl program using
Perl Hello World Example .
In this article, Let us review how to debug a perl program / script using Perl debugger ,
which is similar to the gdb tool for debugging C
code .
To debug a perl program, invoke the perl debugger using "perl -d" as shown below.
# perl -d ./perl_debugger.pl
To understand the perl debugger commands in detail, let us create the following sample perl
program (perl_debugger.pl).
$ cat perl_debugger.pl
#!/usr/bin/perl -w
# Script to list out the filenames (in the pwd) that contains specific pattern.
#Enabling slurp mode
$/=undef;
# Function : get_pattern
# Description : to get the pattern to be matched in files.
sub get_pattern
{
my $pattern;
print "Enter search string: ";
chomp ($pattern = <> );
return $pattern;
}
# Function : find_files
# Description : to get list of filenames that contains the input pattern.
sub find_files
{
my $pattern = shift;
my (@files,@list,$file);
# using glob, obtaining the filenames,
@files = <./*>;
# taking out the filenames that contains pattern.
@list = grep {
$file = $_;
open $FH,"$file";
@lines = <$FH>;
$count = grep { /$pattern/ } @lines;
$file if($count);
} @files;
return @list;
}
# to obtain the pattern from STDIN
$pattern = get_pattern();
# to find-out the list of filenames which has the input pattern.
@list = find_files($pattern);
print join "\n",@list;
1. Enter Perl Debugger
# perl -d ./perl_debugger.pl
it prompts,
DB<1>
2. View specific lines or subroutine statements using (l)
3. Set the breakpoint on get_pattern function using (b)
DB<3> b find_files
4. Set the breakpoint on specific line using (b)
DB<4> b 44
5. View the breakpoints using (L)
DB<5> L
./perl_debugger.pl:
22: my $pattern = shift;
break if (1)
44: print join "\n",@list;
break if (1)
6. step by step execution using (s and n)
DB<5> s
main::(./perl_debugger.pl:39): $pattern = get_pattern();
DB<5> s
main::get_pattern(./perl_debugger.pl:12):
12: my $pattern;
Option s and n does step by step execution of each statements. Option s steps into the
subroutine. Option n executes the subroutine in a single step (stepping over it).
The s option does stepping into the subroutine but while n option which would execute the
subroutine(stepping over it).
7. Continue till next breakpoint (or line number, or
subroutine) using (c)
DB<5> c
Enter search string: perl
main::find_files(./perl_debugger.pl:22):
22: my $pattern = shift;
8. Continue down to the specific line number using (c)
DB<5> c 36
main::find_files(./perl_debugger.pl:36):
36: return @list;
9. Print the value in the specific variable using (p)
DB<6> p $pattern
perl
DB<7> c
main::(./perl_debugger.pl:44): print join "\n",@list;
DB<7> c
./perl_debugger.pl
Debugged program terminated. Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
After the last continue operation, the output gets printed on the stdout as
"./perl_debugger.pl" since it matches the pattern "perl".
10. Get debug commands from the
file (source)
Perl debugger can get the debug command from the file and execute it. For example, create
the file called "debug_cmds" with the perl debug commands as,
c
p $pattern
q
Note that R is used to restart the operation(no need quit and start debugger again).
DB<7> R
DB<7> source debug_cmds
>> c
Enter search string: perl
./perl_debugger.pl
Debugged program terminated. Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
>> p $pattern
perl
>> q
Not for the fainthearted, if you want to see how a regular expression runs when used in a
match or substitution, use the core re pragma with its debug
option:
% perl -Mstrict -Mwarnings
use re qw(debug);
$_ = "cats=purr, dog=bark";
my %sound = /(\w+)=(\w+)/g;
^D
Compiling REx `(\w+)=(\w+)'
size 15 first at 4
1: OPEN1(3)
3: PLUS(5)
4: ALNUM(0)
5: CLOSE1(7)
7: EXACT <=>(9)
9: OPEN2(11)
11: PLUS(13)
12: ALNUM(0)
13: CLOSE2(15)
15: END(0)
floating `=' at 1..2147483647 (checking floating) stclass `ALNUM' plus
minlen 3
Guessing start of match, REx `(\w+)=(\w+)' against `cats=purr,
dog=bark'...
Found floating substr `=' at offset 4...
Does not contradict STCLASS...
Guessed: match at offset 0
Matching REx `(\w+)=(\w+)' against `cats=purr, dog=bark'
Setting an EVAL scope, savestack=3
0 <> <cats=purr, d> | 1: OPEN1
0 <> <cats=purr, d> | 3: PLUS
ALNUM can match 4 times out of 32767...
Setting an EVAL scope, savestack=3
4 <cats> <=purr, d> | 5: CLOSE1
4 <cats> <=purr, d> | 7: EXACT <=>
5 <cats=> <purr, d> | 9: OPEN2
5 <cats=> <purr, d> | 11: PLUS
Setting an EVAL scope, savestack=3
9 <=purr> <, dog=b> | 13: CLOSE2
9 <=purr> <, dog=b> | 15: END
Match successful!
Guessing start of match, REx `(\w+)=(\w+)' against `, dog=bark'...
Found floating substr `=' at offset 5...
By STCLASS: moving 0 --> 2
Guessed: match at offset 2
Matching REx `(\w+)=(\w+)' against `dog=bark'
Setting an EVAL scope, savestack=3
11 <urr, > <dog=bar> | 1: OPEN1
11 <urr, > <dog=bar> | 3: PLUS
ALNUM can match 3 times out of 32767...
Setting an EVAL scope, savestack=3
14 <rr, dog> <=bark> | 5: CLOSE1
14 <rr, dog> <=bark> | 7: EXACT <=>
15 <rr, dog=> <bark> | 9: OPEN2
15 <rr, dog=> <bark> | 11: PLUS
ALNUM can match 4 times out of 32767...
Setting an EVAL scope, savestack=3
19 <rr, dog=bark> <> | 13: CLOSE2
19 <rr, dog=bark> <> | 15: END
Match successful!
Freeing REx: `(\w+)=(\w+)'
debugcolor option instead of debug , you'll get some form of
highlighting or coloring in the output that'll make it prettier, if not more understandable
Actually, just writing the tests is often a damn fine way of finding bugs. No exactly what OP is after at the moment, but something
that is at the forefront of my mind because I'm in the middle of writing a set of tests (in Perl :) for some XML processing C++ code
and turning up a pile of bugs as I go.
However it does suggest another test avenue: write test harnesses for modules so that you can exercise them in isolation and better
understand how they work. If the test harness ends up part of a regression test system so much the better.
converter has asked for the wisdom of the Perl Monks concerning
the following question:
For the past several months I've been busy rewriting the horrible Perl code left behind by my predecessor. His approach to
development was "Write some code. If the code runs without revealing any of the damage it's done, ship it. If not, write some
more code."
This code is so bad that when co-workers ask me what I'm working on, I tell them "The Madman's Diary." Yes, it would have been
cheaper and faster to throw this code away and start over, but I wasn't given that option.
My latest assignment is the repair of a tangled mess of a show-stopper that was discovered in a product that was supposed to
ship today. After adding an open() override that logs the arguments to open() and some quality time with the watch(1) utility
observing changes to the files containing the data that are causing the problem, I've narrowed the list of suspects down to a
couple in-house scripts and a few (probably altered) webmin modules.
Now that I know where to look, I'd like to identify as quickly as possible which details can be safely ignored. I plan to use
Devel::DProf to produce an execution graph for reference and Tie::Watch to watch variables, but I wonder if there are other tools
that I should look at. A utility or module that would allow me to incrementally build a profile with persistent notes would be
wonderful.
Debugging this code is a whole different game, and I'd really appreciate some input from other monks who've dealt with this
type of problem.
I agree with adrianh . If a component is not broken, don't
rewrite it. Rewrite a component when you find a number of bugs in it. But first write a regression test suite for the component.
I've seen many folks over the years throw out old code, rewrite it ... and introduce a heap of new bugs in the process. If you
come into a new company and introduce a swag of new bugs in previously working code, you will start to smell very badly.
Actually, just writing the tests is often a damn fine way of finding bugs. No exactly what OP is after at the moment, but something
that is at the forefront of my mind because I'm in the middle of writing a set of tests (in Perl :) for some XML processing C++
code and turning up a pile of bugs as I go.
However it does suggest another test avenue: write test harnesses for modules so that you can exercise them in isolation and
better understand how they work. If the test harness ends up part of a regression test system so much the better.
I would not spend any time fixing the code if it's not breaking (assuming you're not being paid to review/fix the code). However
evil it may be - if it's doing it's job leave it alone.
Instead - every time you need to fix a bug or add some new functionality just test/refactor the bits of the evil code that
are touched by the changes. I've found incrementally adding tests and refactoring to be much more effective than any sort of "big
bang" fixing things for the sake of them approach :-)
If you are being paid to do a review/fix then
Perl::Critic might give you some
useful places to look.
Definitely agree about the approach of sorting things out with gradual refactoring and tests as the need arises. The problem with
the "Big Bang" approach is that you have the potential for a very long stretch of time where there are two forks of the code:
ugly shipping code that will need to be fixed and refactored as bugs are reported, and pretty nonfunctioning code that will need
to incorporate those fixes as they are uncovered, resulting in a perpetual loop of "it's not quite ready yet."
What tools are you using already and on what platform? For a large range of "detail" debugging there is nothing like as good
as an IDE with a good integrated debugger. For a higher level view of where things are going
Devel::TraceCalls may be handy, although it's
output can be rather voluminous.
1) Ensure there is sufficient functional and technical design documentation against which the routines can be tested.
2) (updated) Make sure there is a sufficiently detailed project plan to include tasks for: systems analysis, functional and
technical design, test planning, test script writing (e.g. using
Expect ), developing, unit-, integrated and
functional testing, rework and implementation, to include a GANTT chart of the work done so far and by who to what % of completion,
to avoid getting the blame for not meeting poorly conceived targets over which you had no control.
In response to formal testing against the plan, I find it a useful aid to bug-fixing to monitor execution with perl -d, setting
breakpoints and examining variables to hunt down which line of code causes each failure.
The Doxygen perl extension creates docs that are great for seeing what classes re-implement what methods etc. Also the
UML::Sequence sounds intriguing
- it pupports to generate a sequence diagram by monitoring code execution.
Tie::File represents a regular text file as a Perl array. Each element in the
array corresponds to a record in the file. The first line of the file is element 0 of the
array; the second line is element 1, and so on.
The file is not loaded into memory, so this will work even for gigantic files.
Changes to the array are reflected in the file immediately.
Lazy people and beginners may now stop reading the manual.
recsep
What is a 'record'? By default, the meaning is the same as for the <...>
operator: It's a string terminated by $/ , which is probably "\n" .
(Minor exception: on DOS and Win32 systems, a 'record' is a string terminated by
"\r\n" .) You may change the definition of "record" by supplying the
recsep option in the tie call:
This says that records are delimited by the string es . If the file contained
the following data:
Curse these pesky flies !\
then the @array would appear to have four elements:
"Curse th"
"e p"
"ky fli"
"!\n"
An undefined value is not permitted as a record separator. Perl's special "paragraph mode"
semantics (à la $/ = "" ) are not emulated.
Records read from the tied array do not have the record separator string on the end; this is
to allow
$array 17 ] .= "extra"
to work as expected.
(See autochomp
, below.) Records stored into the array will have the record separator string appended before
they are written to the file, if they don't have one already. For example, if the record
separator string is "\n" , then the following two lines do exactly the same
thing:
$array 17 ] = "Cherry pie"
$array 17 ] = "Cherry pie\n"
The result is that the contents of line 17 of the file will be replaced with "Cherry pie"; a
newline character will separate line 17 from line 18. This means that this code will do
nothing:
Because the chomp ed value will have
the separator reattached when it is written back to the file. There is no way to create a file
whose trailing record separator string is missing.
Inserting records that contain the record separator string is not supported by this
module. It will probably produce a reasonable result, but what this result will be may change
in a future version. Use 'splice' to insert records or to replace one record with
several.
autochomp
Normally, array elements have the record separator removed, so that if the file contains the
text
Gold
Frankincense
Myrrh
the tied array will appear to contain "Gold" "Frankincense" "Myrrh" . If you
set autochomp to a false value, the record separator will not be removed. If the
file above was tied with
then the array @gifts would appear to contain "Gold\n" "Frankincense\n"
"Myrrh\n" , or (on Win32 systems) "Gold\r\n" "Frankincense\r\n" "Myrrh\r\n"
.
mode
Normally, the specified file will be opened for read and write access, and will be created
if it does not exist. (That is, the flags O_RDWR | O_CREAT are supplied in the
open call.) If you want
to change this, you may supply alternative flags in the mode option. See
Fcntl for a listing of
available flags. For example:
# open the file if it exists, but fail if it does not exist
Opening the data file in write-only or append mode is not
supported.
memory
This is an upper limit on the amount of memory that Tie::File will consume at
any time while managing the file. This is used for two things: managing the read cache
and managing the deferred write buffer .
Records read in from the file are cached, to avoid having to re-read them repeatedly. If you
read the same record twice, the first time it will be stored in memory, and the second time it
will be fetched from the read cache . The amount of data in the read cache will not
exceed the value you specified for memory . If Tie::File wants to
cache a new record, but the read cache is full, it will make room by expiring the
least-recently visited records from the read cache.
The default memory limit is 2Mib. You can adjust the maximum read cache size by supplying
the memory option. The argument is the desired cache size, in bytes.
# I have a lot of memory, so use a large cache to speed up access
Setting the memory limit to 0 will inhibit caching; records will be fetched from disk every
time you examine them.
The memory value is not an absolute or exact limit on the memory used.
Tie::File objects contains some structures besides the read cache and the deferred
write buffer, whose sizes are not charged against memory .
The cache itself consumes about 310 bytes per cached record, so if your file has many short
records, you may want to decrease the cache memory limit, or else the cache overhead may exceed
the size of the cached data.
dw_size
(This is an advanced feature. Skip this section on first reading.)
If you use deferred writing (See Deferred
Writing , below) then data you write into the array will not be written directly to the
file; instead, it will be saved in the deferred write buffer to be written out later.
Data in the deferred write buffer is also charged against the memory limit you set with the
memory option.
You may set the dw_size option to limit the amount of data that can be saved in
the deferred write buffer. This limit may not exceed the total memory limit. For example, if
you set dw_size to 1000 and memory to 2500, that means that no more
than 1000 bytes of deferred writes will be saved up. The space available for the read cache
will vary, but it will always be at least 1500 bytes (if the deferred write buffer is full) and
it could grow as large as 2500 bytes (if the deferred write buffer is empty.)
If you don't specify a dw_size , it defaults to the entire memory
limit.
Option Format
- mode is a synonym for mode . - recsep is a synonym
for recsep . - memory is a synonym for memory . You get
the idea.
Public Methods
The tie call returns an
object, say $o . You may call
$rec = $o->FETCH $n
$o->STORE $n $rec
to fetch or store the record at line $n , respectively; similarly the other
tied array methods. (See perltie for details.)
You may also call the following methods on this object:
will lock the tied file. MODE has the same meaning as the second argument to
the Perl built-in flock function; for
example LOCK_SH or LOCK_EX | LOCK_NB . (These constants are provided
by the use Fcntl ':flock'
declaration.)
MODE is optional; the default is LOCK_EX .
Tie::File maintains an internal table of the byte offset of each record it has
seen in the file.
When you use flock to lock the file,
Tie::File assumes that the read cache is no longer trustworthy, because another
process might have modified the file since the last time it was read. Therefore, a successful
call to flock discards the
contents of the read cache and the internal record offset table.
Tie::File promises that the following sequence of operations will be safe:
In particular, Tie::File will not read or write the file during the
tie call. (Exception:
Using mode => O_TRUNC will, of course, erase the file during the
tie call. If you want to
do this safely, then open the file without O_TRUNC , lock the file, and use
@array = () .)
The best way to unlock a file is to discard the object and untie the array. It is probably
unsafe to unlock the file without also untying it, because if you do, changes may remain
unwritten inside the object. That is why there is no shortcut for unlocking. If you really want
to unlock the file prematurely, you know what to do; if you don't know what to do, then don't
do it.
All the usual warnings about file locking apply here. In particular, note that file locking
in Perl is advisory , which means that holding a lock will not prevent anyone else from
reading, writing, or erasing the file; it only prevents them from getting another lock at the
same time. Locks are analogous to green traffic lights: If you have a green light, that does
not prevent the idiot coming the other way from plowing into you sideways; it merely guarantees
to you that the idiot does not also have a green light at the same
time.
autochomp
my $old_value =
$o->autochomp # disable autochomp option
my $old_value =
$o->autochomp # enable autochomp option
Handles that were opened write-only won't work. Handles that were opened read-only will work
as long as you don't try to modify the array. Handles must be attached to seekable sources of
data---that means no pipes or sockets. If Tie::File can detect that you supplied a
non-seekable handle, the tie call will throw an
exception. (On Unix systems, it can detect this.)
Note that Tie::File will only close any filehandles that it opened internally. If you passed
it a filehandle as above, you "own" the filehandle, and are responsible for closing it after
you have untied the @array.
Deferred Writing
(This is an advanced feature. Skip this section on first reading.)
Normally, modifying a Tie::File array writes to the underlying file
immediately. Every assignment like $a ] = ... rewrites as much of the file as is
necessary; typically, everything from line 3 through the end will need to be rewritten. This is
the simplest and most transparent behavior. Performance even for large files is reasonably
good.
However, under some circumstances, this behavior may be excessively slow. For example,
suppose you have a million-record file, and you want to do:
for @FILE
$_ = "> $_"
The first time through the loop, you will rewrite the entire file, from line 0 through the
end. The second time through the loop, you will rewrite the entire file from line 1 through the
end. The third time through the loop, you will rewrite the entire file from line 2 to the end.
And so on.
If the performance in such cases is unacceptable, you may defer the actual writing, and then
have it done all at once. The following loop will perform much better for large files:
If Tie::File 's memory limit is large enough, all the writing will done in
memory. Then, when you call ->flush , the entire file will be rewritten in a
single pass.
(Actually, the preceding discussion is something of a fib. You don't need to enable deferred
writing to get good performance for this common case, because Tie::File will do it
for you automatically unless you specifically tell it not to. See autodeferring
, below.)
Calling ->flush returns the array to immediate-write mode. If you wish to
discard the deferred writes, you may call ->discard instead of
->flush . Note that in some cases, some of the data will have been written
already, and it will be too late for ->discard to discard all the changes.
Support for ->discard may be withdrawn in a future version of
Tie::File .
Deferred writes are cached in memory up to the limit specified by the dw_size
option (see above). If the deferred-write buffer is full and you try to write still more
deferred data, the buffer will be flushed. All buffered data will be written immediately, the
buffer will be emptied, and the now-empty space will be used for future deferred writes.
If the deferred-write buffer isn't yet full, but the total size of the buffer and the read
cache would exceed the memory limit, the oldest records will be expired from the
read cache until the total size is under the limit.
push , pop ,
shift , unshift ,
and splice cannot be
deferred. When you perform one of these operations, any deferred data is written to the file
and the operation is performed immediately. This may change in a future version.
If you resize the array with deferred writing enabled, the file will be resized immediately,
but deferred records will not be written. This has a surprising consequence: @a =
... erases the file immediately, but the writing of the actual data is deferred. This
might be a bug. If it is a bug, it will be fixed in a future version.
Autodeferring
Tie::File tries to guess when deferred writing might be helpful, and to turn it
on and off automatically.
for @a
$_ = "> $_"
In this example, only the first two assignments will be done immediately; after this, all
the changes to the file will be deferred up to the user-specified memory limit.
You should usually be able to ignore this and just use the module without thinking about
deferring. However, special applications may require fine control over which writes are
deferred, or may require that all writes be immediate. To disable the autodeferment feature,
use
Similarly, ->autodefer re-enables autodeferment, and ->autodefer
() recovers the current value of the autodefer setting.
CONCURRENT ACCESS TO
FILES
Caching and deferred writing are inappropriate if you want the same file to be accessed
simultaneously from more than one process. Other optimizations performed internally by this
module are also incompatible with concurrent access. A future version of this module will
support a concurrent => option that enables safe concurrent access.
Previous versions of this documentation suggested using memory => for safe
concurrent access. This was mistaken. Tie::File will not support safe concurrent access before
version 0.96.
CAVEATS
(That's Latin for 'warnings'.)
Reasonable effort was made to make this module efficient. Nevertheless, changing the size
of a record in the middle of a large file will always be fairly slow, because everything
after the new record must be moved.
The behavior of tied arrays is not precisely the same as for regular arrays. For example:
undef -ing a
Tie::File array element just blanks out the corresponding record in the file.
When you read it back again, you'll get the empty string, so the supposedly-
undef 'ed value will
be defined. Similarly, if you have autochomp disabled, then
# This DOES print "How unusual!" if 'autochomp' is disabled
Because when autochomp is disabled, $a 10 will read back as
"\n" (or whatever the record separator string is.)
There are other minor differences, particularly regarding exists and
delete , but in
general, the correspondence is extremely close.
I have supposed that since this module is concerned with file I/O, almost all normal use
of it will be heavily I/O bound. This means that the time to maintain complicated data
structures inside the module will be dominated by the time to actually perform the I/O. When
there was an opportunity to spend CPU time to avoid doing I/O, I usually tried to take
it.
You might be tempted to think that deferred writing is like transactions, with
flush as commit and discard as rollback ,
but it isn't, so don't.
There is a large memory overhead for each record offset and for each cache entry: about
310 bytes per cached data record, and about 21 bytes per offset table entry.
The per-record overhead will limit the maximum number of records you can access per
file. Note that accessing the length of the array via $x = scalar
@tied_file accesses all records and stores their offsets. The same for
foreach @tied_file , even if you exit the loop early.
SUBCLASSING
This version promises absolutely nothing about the internals, which may change without
notice. A future version of the module will have a well-defined and stable subclassing
API.
WHAT ABOUT DB_File ?
People sometimes point out that DB_File will do
something similar, and ask why Tie::File module is necessary.
To contact the author, send email to: mjd perl tiefile @plover com
To receive an announcement whenever a new version of this module is released, send a blank
email message to mjd perl tiefile subscribe @plover com .
The most recent version of this module, including documentation and any news of importance,
will be available at
http://perl.plover.com/TieFile/
LICENSE
Tie::File version 0.96 is copyright (C) 2003 Mark Jason Dominus.
This library is free software; you may redistribute it and/or modify it under the same terms
as Perl itself.
These terms are your choice of any of (1) the Perl Artistic Licence, or (2) version 2 of the
GNU General Public License as published by the Free Software Foundation, or (3) any later
version of the GNU General Public License.
This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this library
program; it should be in the file COPYING . If not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
For licensing inquiries, contact the author at:
Mark Jason Dominus
255 S. Warnock St.
Philadelphia, PA 19107
WARRANTY
Tie::File version 0.98 comes with ABSOLUTELY NO WARRANTY. For details, see the
license.
the script is very compilcated(12000 LOC). If I will comment out any of regex then the execution will fail..Also if I provide
dummy values, then it will give unexpected results :( –
AnonGeek
Jun 20 '12 at 20:55
I have this problem: I need to control the perl-debugger from an external script. By research I found out about various solutions,
but I don't understand them. I failed to properly set up the RemotePort option (editing ".perldb"), which was the first I tried,
and found no useful information on providing a filehandle from which the debugger would get its input (by somehow setting @cmdfhs)
I found both options over here: http://search.cpan.org/~nwclark/perl-5.8.6/lib/perl5db.pl
It would be nice if you could tell me how to provide the filehandle from which the debugger gets its input, or if you know
a link where this is explained?
Here's a simple example setting it up using RemotePort , which seemed easier to me:
The trick to using RemotePort is that you have to have someone listening on the remote end BEFORE you launch the
script to be debugged.
As soon as you launch your script with -d Perl will attempt to connect to RemotePort . So you have
to make sure the initial connection succeeds by having someone listening there beforehand.
Here I assume some Linux/Unix variant, which has the netcat utility installed. We use netcat to wait
for incoming connections in this example, but you can use anything else you wish too which is able to create a service port and
shuffle data between that and the current TTY:
In terminal 1
# Use netcat to listen for incoming connections on port 9999
> nc -l -p 9999
In terminal 2
# Start perl with -d and request a RemotePort connection
> PERLDB_OPTS=RemotePort=127.0.0.1:9999 perl -d my_script.pl
As soon as you do that in terminal 1 you will see something like this:
Loading DB routines from perl5db.pl version 1.39_10
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(my_script.pl:4):
DB<1>
There you go..debug away.
Devel::Trepan is a gdb-like debugger. Although it has remote
control, you can also run it at the outset with the option --command which will "source" (in the gdb-sense) or run
a series of debugger commands.
To go into remote control, either start the debugger using the --server option or inside the debugger use the
" server " command once inside
the debugger.
See Options for a list of options you can give
at the outset.
@char_array now contains all the characters of the $s1 string
and it's possible to manipulate it, iterate over it or do whatever to it just like with any
other array.
You can you splice to insert elements at a given position of the array:
echo -e 'hello\ndisk\ncaller' | perl -F'' -ane ' splice (@F,2,0," "); splice(@F,4,0," "); foreach(@F){print}'
he l lo
di s k
ca l ler
You can use Data::Dumper for better visualization when working with arrays:
@char_array now contains all the characters of the $s1 string
and it's possible to manipulate it, iterate over it or do whatever to it just like with any
other array.
You can you splice to insert elements at a given position of the array:
echo -e 'hello\ndisk\ncaller' | perl -F'' -ane ' splice (@F,2,0," "); splice(@F,4,0," "); foreach(@F){print}'
he l lo
di s k
ca l ler
You can use Data::Dumper for better visualization when working with arrays:
"... Smart::Comments++ When used with the -ENV switch, it's a fantastic tool for this sort of thing. Much better than having to strip all the tests out before going to production, as someone else suggested. ..."
Smart::Comments++ When used with the -ENV switch, it's a fantastic tool for this sort of
thing. Much better than having to strip all the tests out before going to production, as
someone else suggested.
$var_to_check =~ /sometest/ or die "bad variable!";
I tend to throw things like this in my code, and later use a find and replace to get rid
of them (in production code).
Also, ' eval '
can be used to run a section of code and capture errors and can be used to create exception
handling functionality. If you are asserting that a value is not 0, perhaps you want to throw
an exception and handle that case in a special way?
> ,
if ( $next_sunrise_time > 24*60*60 ) { warn( "assertion failed" ); } # Assert that the sun must rise in the next 24 hours.
You can do this if you do not have access to Perl 5.9 which is required for Carp::Assert .
Smart::Comments++ When used with the -ENV switch, it's a fantastic tool for this sort of
thing. Much better than having to strip all the tests out before going to production, as
someone else suggested.
$var_to_check =~ /sometest/ or die "bad variable!";
I tend to throw things like this in my code, and later use a find and replace to get rid
of them (in production code).
Also, ' eval '
can be used to run a section of code and capture errors and can be used to create exception
handling functionality. If you are asserting that a value is not 0, perhaps you want to throw
an exception and handle that case in a special way?
> ,
if ( $next_sunrise_time > 24*60*60 ) { warn( "assertion failed" ); } # Assert that the sun must rise in the next 24 hours.
You can do this if you do not have access to Perl 5.9 which is required for Carp::Assert .
Perl array FAQ: How can I test to see if a Perl array already
contains a given value? (Also written as, How do I search an array with the Perl grep
function?)
I use the Perl grep function to see if a Perl array contains a given entry. For
instance, in this Perl code:
if ( grep { $_ eq $clientAddress} @ip_addresses ) {
# the array already contains this ip address; skip it this time
next;
} else {
# the array does not yet contain this ip address; add it
push @ip_addresses, $clientAddress;
}
I'm testing to see if the Perl array "@ip_addresses" contains an entry given by the variable
"$clientAddress".
Just use this Perl array search technique in an "if" clause, as shown, and then add whatever
logic you want within your if and else statements. In this case, if the current IP address is
not already in the array, I add it to the array in the "else" clause, but of course your logic
will be unique.
An easier "Perl array contains" example
If it's easier to read without a variable in there, here's another example of this "Perl
array contains" code:
if ( grep { $_ eq '192.168.1.100'} @ip_addresses )
if you'd like more details, I didn't realize it, but I have another good example out here in
my " Perl
grep array tutorial ." (It's pretty bad when you can't find things on your own
website.)
I've been reading up on dispatch tables and I get the general idea of how they work, but I'm
having some trouble taking what I see online and applying the concept to some code I
originally wrote as an ugly mess of if-elsif-else statements.
I have options parsing configured by using GetOpt::Long , and in turn, those
options set a value in the %OPTIONS hash, depending on the option used.
Taking the below code as an example... ( UPDATED WITH MORE DETAIL
use 5.008008;
use strict;
use warnings;
use File::Basename qw(basename);
use Getopt::Long qw(HelpMessage VersionMessage :config posix_default require_order no_ignore_case auto_version auto_help);
my $EMPTY => q{};
sub usage
{
my $PROG = basename($0);
print {*STDERR} $_ for @_;
print {*STDERR} "Try $PROG --help for more information.\n";
exit(1);
}
sub process_args
{
my %OPTIONS;
$OPTIONS{host} = $EMPTY;
$OPTIONS{bash} = 0;
$OPTIONS{nic} = 0;
$OPTIONS{nicName} = $EMPTY;
$OPTIONS{console} = 0;
$OPTIONS{virtual} = 0;
$OPTIONS{cmdb} = 0;
$OPTIONS{policyid} = 0;
$OPTIONS{showcompliant} = 0;
$OPTIONS{backup} = 0;
$OPTIONS{backuphistory} = 0;
$OPTIONS{page} = $EMPTY;
GetOptions
(
'host|h=s' => \$OPTIONS{host} ,
'use-bash-script' => \$OPTIONS{bash} ,
'remote-console|r!' => \$OPTIONS{console} ,
'virtual-console|v!' => \$OPTIONS{virtual} ,
'nic|n!' => \$OPTIONS{nic} ,
'nic-name|m=s' => \$OPTIONS{nicName} ,
'cmdb|d!' => \$OPTIONS{cmdb} ,
'policy|p=i' => \$OPTIONS{policyid} ,
'show-compliant|c!' => \$OPTIONS{showcompliant} ,
'backup|b!' => \$OPTIONS{backup} ,
'backup-history|s!' => \$OPTIONS{backuphistory} ,
'page|g=s' => \$OPTIONS{page} ,
'help' => sub { HelpMessage(-exitval => 0, -verbose ->1) },
'version' => sub { VersionMessage() },
) or usage;
if ($OPTIONS{host} eq $EMPTY)
{
print {*STDERR} "ERROR: Must specify a host with -h flag\n";
HelpMessage;
}
sanity_check_options(\%OPTIONS);
# Parse anything else on the command line and throw usage
for (@ARGV)
{
warn "Unknown argument: $_\n";
HelpMessage;
}
return {%OPTIONS};
}
sub sanity_check_options
{
my $OPTIONS = shift;
if (($OPTIONS->{console}) and ($OPTIONS->{virtual}))
{
print "ERROR: Cannot use flags -r and -v together\n";
HelpMessage;
}
elsif (($OPTIONS->{console}) and ($OPTIONS->{cmdb}))
{
print "ERROR: Cannot use flags -r and -d together\n";
HelpMessage;
}
elsif (($OPTIONS->{console}) and ($OPTIONS->{backup}))
{
print "ERROR: Cannot use flags -r and -b together\n";
HelpMessage;
}
elsif (($OPTIONS->{console}) and ($OPTIONS->{nic}))
{
print "ERROR: Cannot use flags -r and -n together\n";
HelpMessage;
}
if (($OPTIONS->{virtual}) and ($OPTIONS->{backup}))
{
print "ERROR: Cannot use flags -v and -b together\n";
HelpMessage;
}
elsif (($OPTIONS->{virtual}) and ($OPTIONS->{cmdb}))
{
print "ERROR: Cannot use flags -v and -d together\n";
HelpMessage;
}
elsif (($OPTIONS->{virtual}) and ($OPTIONS->{nic}))
{
print "ERROR: Cannot use flags -v and -n together\n";
HelpMessage;
}
if (($OPTIONS->{backup}) and ($OPTIONS->{cmdb}))
{
print "ERROR: Cannot use flags -b and -d together\n";
HelpMessage;
}
elsif (($OPTIONS->{backup}) and ($OPTIONS->{nic}))
{
print "ERROR: Cannot use flags -b and -n together\n";
HelpMessage;
}
if (($OPTIONS->{nic}) and ($OPTIONS->{cmdb}))
{
print "ERROR: Cannot use flags -n and -d together\n";
HelpMessage;
}
if (($OPTIONS->{policyid} != 0) and not ($OPTIONS->{cmdb}))
{
print "ERROR: Cannot use flag -p without also specifying -d\n";
HelpMessage;
}
if (($OPTIONS->{showcompliant}) and not ($OPTIONS->{cmdb}))
{
print "ERROR: Cannot use flag -c without also specifying -d\n";
HelpMessage;
}
if (($OPTIONS->{backuphistory}) and not ($OPTIONS->{backup}))
{
print "ERROR: Cannot use flag -s without also specifying -b\n";
HelpMessage;
}
if (($OPTIONS->{nicName}) and not ($OPTIONS->{nic}))
{
print "ERROR: Cannot use flag -m without also specifying -n\n";
HelpMessage;
}
return %{$OPTIONS};
}
I'd like to turn the above code into a dispatch table, but can't figure out how to do
it.
Are the sets of conflicting options always pairs? Can you have situations where options
a , b , and c cannot occur together but any two are
OK? Before you can pick a representation you need to be sure your model can handle the logic
you need in a general way. This is not an easy problem. – Jim Garrison
2 days ago
@JimGarrison -- you are correct. The if-elsif-else does not explicitly account for 3 options
that conflict (though it does account for that implicitly) As an example, using
-h is required with all of the other options. But, using -h ,
-r , v , all together is not allowed, while -h ,
-r , and -d is allowed. – Speeddymon
yesterday
I am not sure how a dispatch table would help since you need to go through pair-wise
combinations of specific possibilities, and thus cannot trigger a suitable action by one
lookup.
Here is another way to organize it
use List::MoreUtils 'firstval';
sub sanity_check_options
{
my ($OPTIONS, $opt_excl) = @_;
# Check each of 'opt_excl' against all other for ConFLict
my @excl = sort keys %$opt_excl;
while (my $eo = shift @excl)
{
if (my $cfl = firstval { $OPTIONS->{$eo} and $OPTIONS->{$_} } @excl)
{
say "Can't use -$opt_excl->{$eo} and -$opt_excl->{$cfl} together";
HelpMessage();
last;
}
}
# Go through specific checks on
# policyid, showcompliant, backuphistory, and nicName
...
return 1; # or some measure of whether there were errors
}
# Mutually exclusive options
my %opt_excl = (
console => 'r', virtual => 'v', cmdb => 'c', backup => 'b', nic => 'n'
);
sanity_check_options(\%OPTIONS, \%opt_excl);
This checks all options listed in %opt_excl against each other for conflict,
removing the segments of elsif involving the (five) options that are mutually
exclusive. It uses
List::MoreUtils::firstval . The few other specific invocations are best checked one by
one.
There is no use of returning $OPTIONS since it is passed as reference so any
changes apply to the original structure (while it's not meant to be changed either). Perhaps
you can keep track of whether there were errors and return that if it can be used in the
caller, or just return 1 .
This addresses the long elsif chain as asked, and doesn't go into the rest of
code. Here is one comment though: There is no need for {%OPTIONS} , which copies
the hash in order to create an anonymous one; just use return \%OPTIONS;
Comment on possible multiple conflicting options
This answer as it stands does not print all conflicting options that have been
used if there are more than two, as raised by ikegami in comments; it does catch any
conflicts so that the run is aborted.
The code is readily adjusted for this. Instead of the code in the if block
either
set a flag as a conflict is detected and break out of the loop, then print the list of
those that must not be used with each other ( values %opt_excl ) or point at
the following usage message
collect the conflicts as they are observed; print them after the loop
However, one is expected to know of allowed invocations and any listing of conflicts is a
courtesy to the forgetful user (or a debugging aid); a usage message is printed as well
anyway.
Given the high number of conflicting options the usage message should contain a prominent
note on this. Also consider that so many conflicting options may indicate a design flaw.
Finally, this code fully relies on the fact that this processing goes once per
run and operates with a handful of options; thus it is not concerned with efficiency and
freely uses ancillary data structures.
@Speeddymon Thank you, updated. This brings together checks of those five options which can't
go one with another. The remaining few I leave to be checked one by one; "encoding" one or
two possibilities in some all-encompassing system would just increase complexity (and may end
up less readable). – zdim
yesterday
@Speeddymon, Apparently, it's not clear as you think since you didn't realize if doesn't
work. It doesn't mention the error of using -r and -c together if
-b is also provided. And why is a hash being used at all? Wasteful and
needlessly complex. – ikegami
23 hours ago
You can use a dispatch table if there are a lot of options. I would build that table
programmatically. It might not be the best option here, but it works and the configuration is
more readable than your elsif construct.
use strict;
use warnings;
use Ref::Util::XS 'is_arrayref'; # or Ref::Util
sub create_key {
my $input = shift;
# this would come from somewhere else, probably the Getopt config
my @opts = qw( host bash nic nicName console virtual cmdb
policyid showcompliant backup backuphistory page );
# this is to cover the configuration with easier syntax
$input = { map { $_ => 1 } @{$input} }
if is_arrayref($input);
# options are always prefilled with false values
return join q{}, map { $input->{$_} ? 1 : 0 }
sort @opts;
}
my %forbidden_combinations = (
map { create_key( $_->[0] ) => $_->[1] } (
[ [qw( console virtual )] => q{Cannot use flags -r and -v together} ],
[ [qw( console cmdb )] => q{Cannot use flags -r and -d together} ],
[ [qw( console backup )] => q{Cannot use flags -r and -b together} ],
[ [qw( console nic )] => q{Cannot use flags -r and -n together} ],
)
);
p %forbidden_combinations; # from Data::Printer
The output of the p function is the dispatch table.
{
00101 "Cannot use flags -r and -v together",
00110 "Cannot use flags -r and -n together",
01100 "Cannot use flags -r and -d together",
10100 "Cannot use flags -r and -b together"
}
As you can see, we've sorted all the options ascii-betically to use them as keys. That
way, you could in theory build all kinds of combinations like exclusive options.
We use a list of array references. Each entry is on one line and contains two pieces of
information. Using the fat comma => makes it easy to read. The first part,
which is much like a key in a hash, is the combination. It's a list of fields that
should not occur together. The second element in the array ref is the error message. I've
removed all the recurring elements, like the newline, to make it easier to change how and
where the error can be displayed.
The map around this list of combination configuration runs the options
through our create_key function, which translates it to a simple bitmap-style
string. We assign all of it to a hash of that map and the error message.
Inside create_key , we check if it was called with an array reference as its
argument. If that's the case, the call was for building the table, and we convert it to a
hash reference so we have a proper map to look stuff up in. We know that the
%OPTIONS always contains all the keys that exist, and that those are pre-filled
with values that all evaluate to false . We can harness that convert the truthiness
of those values to 1 or 0 , which then builds our key.
We will see in a moment why that is useful.
Now how do we use this?
sub HelpMessage { exit; }; # as a placeholder
# set up OPTIONS
my %OPTIONS = (
host => q{},
bash => 0,
nic => 0,
nicName => q{},
console => 0,
virtual => 0,
cmdb => 0,
policyid => 0,
showcompliant => 0,
backup => 0,
backuphistory => 0,
page => q{},
);
# read options with Getopt::Long ...
$OPTIONS{console} = $OPTIONS{virtual} = 1;
# ... and check for wrong invocations
if ( exists $forbidden_combinations{ my $key = create_key($OPTIONS) } ) {
warn "ERROR: $forbidden_combinations{$key}\n";
HelpMessage;
}
All we need to do now is get the $OPTIONS hash reference from Getopt::Long,
and pass it through our create_key function to turn it into the map string. Then
we can simply see if that key exists in our
%forbidden_combinations dispatch table and show the corresponding error
message.
Advantages of this approach
If you want to add more parameters, all you need to do is include them in
@opts . In a full implementation that would probably be auto-generated from the
config for the Getopt call. The keys will change under the hood, but since that is abstracted
away you don't have to care.
Furthermore, this is easy to read. The create_key aside, the actual dispatch
table syntax is quite concise and even has documentary character.
Disadvantages of this approach
There is a lot of programmatic generation going on for just a single call. It's certainly
not the most efficient way to do it.
To take this further, you can write functions that auto-generate entries for certain
scenarios.
Thank you for the detailed answer. I've updated the question to help clarify how the
$OPTIONS hash is setup. Can your example work within the bounds of what I have
already, or should I rewrite the whole thing from scratch? – Speeddymon
yesterday
@Speeddymon yeah, that should work. I see you've got %OPTIONS , and it is always
pre-set with values. That's going to be interesting. Let me try. – simbabque
yesterday
Speaking of the HOP book... That was actually what I was using to try to learn and where I
was having trouble in applying the concept to my code. :-) I couldn't find a PDF version
before, so thank you for the link! – Speeddymon
yesterday
@Speeddymon I've updated the answer and changed it to match your updated code. I suggest you
read the diff first. What I don't like about it yet is that the possible keys are there
twice, but that can be solved with some more trickery. I think that would blow up the answer
even more, so I didn't do that. – simbabque
yesterday
You shouldn't be using elsif here because multiple condition could be true. And since
multiple conditions could be true, a dispatch table can't be used. Your code can still be
simplified greatly.
my @errors;
push @errors, "ERROR: Host must be provided\n"
if !defined($OPTIONS{host});
my @conflicting =
map { my ($opt, $flag) = @$_; $OPTIONS->{$opt} ? $flag : () }
[ 'console', '-r' ],
[ 'virtual', '-v' ],
[ 'cmdb', '-d' ],
[ 'backup', '-b' ],
[ 'nic', '-n' ];
push @errors, "ERROR: Can only use one the following flags at a time: @conflicting\n"
if @conflicting > 1;
push @errors, "ERROR: Can't use flag -p without also specifying -d\n"
if defined($OPTIONS->{policyid}) && !$OPTIONS->{cmdb};
push @errors, "ERROR: Can't use flag -c without also specifying -d\n"
if $OPTIONS->{showcompliant} && !$OPTIONS->{cmdb};
push @errors, "ERROR: Can't use flag -s without also specifying -b\n"
if $OPTIONS->{backuphistory} && !$OPTIONS->{backup};
push @errors, "ERROR: Can't use flag -m without also specifying -n\n"
if defined($OPTIONS->{nicName}) && !$OPTIONS->{nic};
push @errors, "ERROR: Incorrect number of arguments\n"
if @ARGV;
usage(@errors) if @errors;
Note that the above fixes numerous errors in your code.
Help vs Usage Error
--help should provide the requested help to STDOUT, and shouldn't result
in an error exit code.
Usage errors should be printed to STDERR, and should result in an error exit code.
Calling HelpMessage indifferently in both situations is therefore
incorrect.
Create the following sub named usage to use (without arguments) when
GetOptions returns false, and with an error message when some other usage error
occurs:
use File::Basename qw( basename );
sub usage {
my $prog = basename($0);
print STDERR $_ for @_;
print STDERR "Try '$prog --help' for more information.\n";
exit(1);
}
Keep using HelpMessage in response to --help , but the defaults
for the arguments are not appropriate for --help . You should use the
following:
I wondered if it would be impossible because of multiple conditions being true, but based on
other answers, it seems that it is possible to still build a table and compare... –
Speeddymon
yesterday
What are you talking about? No answer used a dispatch table. All the answers (including mine)
used a ( for or map ) loop that performs as many checks as there
are conditions. The points of a dispatch table is to do a single check no matter how many
conditions there are. Since all conditions can be true, you need to check all conditions, so
a dispatch table is impossible by definition. (And that's without even mentioning that the
value of a dispatch table should be a code reference or similar (something to dispatch to).)
– ikegami
yesterday
The difference between mine and the others is that mine avoids using an inefficient unordered
hash and uses an efficient ordered list instead. (You could place the list in an array if you
prefer.) – ikegami
yesterday
Updated to match updated question. That fact that none of the other answers can be extended
for your updated question proves my pointthat trying to put everything into one loop or table
just makes things less flexible, longer and more complex. – ikegami
yesterday
Just to add a minor suggestion here, to the full cover reply of fellow monk Discipulus . It will assist you a
lot also to read Simple Module
Tutorial
Update: Direct answer to your question can be found here How to add a relative
directory to @INC with multiple possible solutions. I would strongly recommend to go
through all the articles that all monks proposed.
Hope this helps, BR.
Seeking for Perl wisdom...on the process of learning...not
there...yet!
This is a monastery - a place of quite contemplation. The louder you shout the less wisdom
shall you receive.
The error message Can't locate dog.pm in @INC is pretty explicit. Either your
module file is not called dog.pm in which case, change it or else your file
dog.pm is not in any of the directories listed in @INC in which case either move it
to one of those directories or else change @INC with use lib .
I also see, despite the lack of formatting in your post that your module doesn't use any
namespace. You should probably address that. Perhaps a solid read through Simple Module Tutorial would be a good
idea?
Welcome to the language ... and, to the Monastery. The "simple module tutorial" listed
above is a very good place to start. Like all languages of its kind, Perl looks at runtime
for external modules in a prescribed list of places, in a specified order. You can affect
this in several ways, as the tutorials describe. Please read them carefully.
In the Perl(-5) language, this list is stored in a pre-defined array variable called
@INC and it is populated from a variety of sources: a base-list that is
compiled directly into the Perl interpreter, the PERL5LIB
environment-variable, use lib statements, and even direct modification of the
variable itself. Perl searches this list from beginning to end and processes (only) the first
matching file that it finds.
(Note that, in Perl, the use statement is actually a pragma, or
declaration to the compiler, and as such it has many "uses" and a somewhat complicated
syntax.)
(Note that, in Perl, the use statement is actually a pragma, or declaration to the
compiler, and as such it has many "uses" and a somewhat complicated syntax.)
Please no.
The word "pragma" has a special meaning in Perl, and it is highly confusing to claim that
a Perl "keyword" would be a "pragma". use certainly is a keyword and nothing
else.
If you mean to say something different, please describe in more words what you want to
say.
Perl grep array FAQ - How to search an array/list of strings By Alvin Alexander. Last
updated: June 3 2016 Perl "grep array" FAQ: Can you demonstrate a Perl grep array
example? (Related: Can you demonstrate how to search a Perl array?)
A very cool thing about Perl is that you can search lists (arrays) with the Perl
grep function. This makes it very easy to find things in large lists -- without having
to write your own Perl for/foreach loops.
A simple Perl grep array example (Perl array
search)
Here's a simple Perl array grep example. First I create a small string array (pizza
toppings), and then search the Perl array for the string "pepper":
# create a perl list/array of strings
@pizzas = qw(cheese pepperoni veggie sausage spinach garlic);
# use the perl grep function to search the @pizzas list for the string "pepper"
@results = grep /pepper/, @pizzas;
# print the results
print "@results\n";
As you might guess from looking at the code, my @results Perl array prints the
following output:
pepperoni
Perl grep array - case-insensitive searching
If you're familiar with Perl regular expressions, you might also guess that it's very easy
to make this Perl array search example case-insensitive using the standard i
operator at the end of my search string.
Here's what our Perl grep array example looks like with this change:
@results = grep /pepper/i, @pizzas;
Perl grep array and regular expressions (regex)
You can also use more complex Perl regular expressions (regex) in your array search. For
instance, if for some reason you wanted to find all strings in your array that contain at least
eight consecutive word characters, you could use this search pattern:
@results = grep /\w{8}/, @pizzas;
That example results in the following output:
pepperoni
Perl grep array - Summary
I hope this Perl grep array example (Perl array search example) has been helpful. For
related Perl examples, see the Related block on this web page, or use the search form on this
website. If you have any questions, or better yet, more Perl array search examples, feel free
to use the Comments section below.
Given an array @A we want to check if the element $B is in it. One
way is to say this:
Foreach $element (@A){
if($element eq $B){
print "$B is in array A";
}
}
However when it gets to Perl, I am thinking always about the most elegant way. And this is
what I am thinking: Is there a way to find out if array A contains B if we convert A to a
variable string and use
The List::Util::first() example is (potentially) subtly incorrect when searching for false
values, since $found will also evaluate false. ( die unless $found
... oops!) List::MoreUtils::any does the right
thing here. – pilcrow
May 2 '12 at 19:56
You have to be very careful with this because this distributes the match over the elements.
If @A has an array reference element that contains $B, this will still match even though $B
isn't a top level element of @A. The smart match is fundamentally broken for this and many
other reasons. – brian d foy
Apr 20 '12 at 13:07
Beware that first can also return a false value if it finds, e.g., "0", which
would confound the example given in this answer. any has the desired semantics.
– pilcrow
May 3 '12 at 1:38
Bitwise
operator works on bits and perform bit by bit operation. Assume if $a = 60; and $b = 13; Now in
binary format they will be as follows − $a = 0011 1100 $b = 0000 1101 -----------------
$a&$b = 0000 1100 $a|$b = 0011 1101 $a^$b = 0011 0001 ~$a = 1100 0011 There are following
Bitwise operators supported by Perl language, assume if $a = 60; and $b = 13
S.No.
Operator & Description
1
&
Binary AND Operator copies a bit to the result if it exists in both operands.
Example − ($a & $b) will give 12 which is 0000 1100
2
|
Binary OR Operator copies a bit if it exists in eather operand.
Example − ($a | $b) will give 61 which is 0011 1101
3
^
Binary XOR Operator copies the bit if it is set in one operand but not both.
Example − ($a ^ $b) will give 49 which is 0011 0001
4
~
Binary Ones Complement Operator is unary and has the efect of 'flipping' bits.
Example − (~$a ) will give -61 which is 1100 0011 in 2's complement form
due to a signed binary number.
5
<<
Binary Left Shift Operator. The left operands value is moved left by the number of
bits specified by the right operand.
Example − $a << 2 will give 240 which is 1111 0000
6
>>
Binary Right Shift Operator. The left operands value is moved right by the number of
bits specified by the right operand.
Example − $a >> 2 will give 15 which is 0000 1111
Example
Try the following example to understand all the bitwise operators available in Perl. Copy
and paste the following Perl program in test.pl file and execute this program.
#!/usr/local/bin/perl
use integer;
$a = 60;
$b = 13;
print "Value of \$a = $a and value of \$b = $b\n";
$c = $a & $b;
print "Value of \$a & \$b = $c\n";
$c = $a | $b;
print "Value of \$a | \$b = $c\n";
$c = $a ^ $b;
print "Value of \$a ^ \$b = $c\n";
$c = ~$a;
print "Value of ~\$a = $c\n";
$c = $a << 2;
print "Value of \$a << 2 = $c\n";
$c = $a >> 2;
print "Value of \$a >> 2 = $c\n";
When the above code is executed, it produces the following result −
Value of $a = 60 and value of $b = 13
Value of $a & $b = 12
Value of $a | $b = 61
Value of $a ^ $b = 49
Value of ~$a = -61
Value of $a << 2 = 240
Value of $a >> 2 = 15
Bitwise
operator works on bits and perform bit by bit operation. Assume if $a = 60; and $b = 13; Now in
binary format they will be as follows − $a = 0011 1100 $b = 0000 1101 -----------------
$a&$b = 0000 1100 $a|$b = 0011 1101 $a^$b = 0011 0001 ~$a = 1100 0011 There are following
Bitwise operators supported by Perl language, assume if $a = 60; and $b = 13
S.No.
Operator & Description
1
&
Binary AND Operator copies a bit to the result if it exists in both operands.
Example − ($a & $b) will give 12 which is 0000 1100
2
|
Binary OR Operator copies a bit if it exists in eather operand.
Example − ($a | $b) will give 61 which is 0011 1101
3
^
Binary XOR Operator copies the bit if it is set in one operand but not both.
Example − ($a ^ $b) will give 49 which is 0011 0001
4
~
Binary Ones Complement Operator is unary and has the efect of 'flipping' bits.
Example − (~$a ) will give -61 which is 1100 0011 in 2's complement form
due to a signed binary number.
5
<<
Binary Left Shift Operator. The left operands value is moved left by the number of
bits specified by the right operand.
Example − $a << 2 will give 240 which is 1111 0000
6
>>
Binary Right Shift Operator. The left operands value is moved right by the number of
bits specified by the right operand.
Example − $a >> 2 will give 15 which is 0000 1111
Example
Try the following example to understand all the bitwise operators available in Perl. Copy
and paste the following Perl program in test.pl file and execute this program.
#!/usr/local/bin/perl
use integer;
$a = 60;
$b = 13;
print "Value of \$a = $a and value of \$b = $b\n";
$c = $a & $b;
print "Value of \$a & \$b = $c\n";
$c = $a | $b;
print "Value of \$a | \$b = $c\n";
$c = $a ^ $b;
print "Value of \$a ^ \$b = $c\n";
$c = ~$a;
print "Value of ~\$a = $c\n";
$c = $a << 2;
print "Value of \$a << 2 = $c\n";
$c = $a >> 2;
print "Value of \$a >> 2 = $c\n";
When the above code is executed, it produces the following result −
Value of $a = 60 and value of $b = 13
Value of $a & $b = 12
Value of $a | $b = 61
Value of $a ^ $b = 49
Value of ~$a = -61
Value of $a << 2 = 240
Value of $a >> 2 = 15
The command line debugger that comes with perl is very powerful.
Not only does it allow us to debug script but it can be used as a REPL - a Read Eval Print Loop to explore the capabilities of the
language. There are a few basic examples in this screencast.
nikmit has asked for
the wisdom of the Perl Monks concerning the following question:
Dear monks,
I came across this behaviour in perl which I find unintuitive, was wondering what the use
case scenario for it is or whether I have done something wrong to bring it about...
I had a statement checking for the existence of data like so return 0 unless keys
%{$hashref->{$key}} and I failed to realise that $key may not always exist.
I would have expected to see an error if $href->{$key} is undefined and
therefore not a reference, but instead $key was just added to the hash.
Example:
#!/usr/bin/perl -w #perl-5.22.3 use strict; my $href = { cat => {milk
=> 1}, dog => {bone => 1} }; if (keys %{$href->{cow}}) { print "noop\n"; } else {
if (exists $href->{cow}) { print "holy cow\n"; } else { print "no cow\n"; } }[download]
This prints 'holy cow'
Discipulus (Monsignor)
on Nov 17, 2017 at 09:32 UTC
This is "autovivification" and was just discussed the other day, see the replies in the
thread Array dereference in
foreach() , including the ones deeper down in the thread.
Use exists to
check if a hash key exists. As described in its documentation, if you have multi-level data
structures (hashes of hashes), you need to check every level. Update:Discipulus just updated to show an example.
FYI, while keys %hash returns the number of keys in scalar context, you can also
use the hash itself, the value will be false if the hash is empty and true otherwise
(actually 0 when empty, and information on the content otherwise). So if (exists
$href->{cow} and %{ $href->{cow} }) . Unlike the keys version, scalar %{ $href->{cow}
} will not create a new hash (autovivify) if the cow key doesn't exist, but die instead (at least if you
forgot to check if the key exists, you'll get an error in the right place).
TOPIC: FAST!! Random Access Indexed, Relational Flat File Databases, Indexed by external
Perl SDBM databases of key/value pairs tied to program "in memory" hash tables, where the Key
in the Key/Value Pair is one or more fields and/or partial fields concatenated together
(separated by a delimiter such as a pipe "|") and contained within the Flat File records for
you to arbitrarily seek to a single record or a sorted/related group of records within your
database.
Since it has been over 2 years ago since I first posted about this TOPIC I discovered, I
wanted to alert the Perl community to the original thread where you can find Perl source code
now for examples of how to implement Joint Database Technology/Methodology. Inparticular the
King James Bible Navigator software DEMO I posted which used FlatFile/SDBM for its database.
I have made this a native Windows GUI application (TreeView/RichEdit COMBO interface) to
demonstrate how to show your end-users a summary of the information of the data contained
within a database, and allow them to drill down to a small amount of specific information
(e.g. verses within a single book/chapter) for actual viewing (and retrieving from the
database). The TreeView Double Click Event was originally written to random access the first
verse within a chapter, then sequentially access the remaining verses within a chapter -
performing a READ for each verse. I posted a separate modified TreeView Double Click Event
for you to insert into the Application which reads an entire chapter in one (1) giant READ,
breaking out the individual verses (into an array) using the UNPACK statement. -- Eric
Consider this code that constructs a set and tests for membership:
my $num_set = set( < 1 2 3 4 > );
say "set: ", $num_set.perl;
say "4 is in set: ", 4 ∈ $num_set;
say "IntStr 4 is in set: ", IntStr.new(4, "Four") ∈ $num_set;
say "IntStr(4,...) is 4: ", IntStr.new(4, "Four") == 4;
say "5 is in set: ", 5 ∈ $num_set;
A straight 4 is not in the set, but the IntStr version is:
set: set(IntStr.new(4, "4"),IntStr.new(1, "1"),IntStr.new(2, "2"),IntStr.new(3, "3"))
4 is in set: False
IntStr 4 is in set: True
IntStr(4,...) is 4: True
5 is in set: False
I think most people aren't going to expect this, but the ∈ docs doesn't
say anything about how this might work. I don't have this problem if I don't use the quote
words (i.e. set( 1, 2, 3, 4) ).
You took a wrong turn in the middle. The important part is what nqp::existskey
is called with: the k.WHICH . This method is there for value types, i.e.
immutable types where the value - rather than identity - defines if two things are supposed
to be the same thing (even if created twice). It returns a string representation of an
object's value that is equal for two things that are supposed to be equal. For
<1>.WHICH you get IntStr|1 and for 1.WHICH you
get just Int|1 .
That's not really clear from that statement. That's talking about which elements are in the
set. Beyond that, even if you choose to compare with ===, you have to know how other things
are stored. This is the sort of info that should show up next to the Set operators. –
brian d foy
Nov 26 '16 at 21:28
Indeed, I think I've found a bug. The qw docs says this should be true: < a b 137
> eqv ( 'a', 'b', '137' ) , but in the same version of Rakudo Star I get false.
It's different object types on each side. – brian d foy
Nov 26 '16 at 23:09
Despite all this, your answer was the A-ha! moment that led me to look at the right thing.
Thanks for all of your help. – brian d foy
Nov 26 '16 at 23:16
As you mention in your answer, your code works if you write your numbers as a simple comma
separated list rather than using the <...> construct.
Here's why:
4 ∈ set 1, 2, 3, 4 # True
A bare numeric literal in code like the 4 to the left of ∈
constructs a single value with a numeric type. (In this case the type is Int, an integer.) If
a set constructor receives a list of similar literals on the right then
everything works out fine.
The various <...>"quote words" constructs turn
the list of whitespace separated literal elements within the angle brackets into an output
list of values.
The foundational variant ( qw<...> ) outputs nothing but strings. Using
it for your use case doesn't work:
4 ∈ set qw<1 2 3 4> # False
The 4 on the left constructs a single numeric value, type Int .
In the meantime the set constructor receives a list of strings, type
Str : ('1','2','3','4') . The ∈ operator doesn't
find an Int in the set because all the values are Str s so returns
False .
Moving along, the huffmanized <...> variant outputs Str s
unless an element is recognized as a number. If an element is recognized as a number then the
output value is a "dual value". For example a 1 becomes an IntStr .
According to the doc "an IntStr can be used interchangeably where one might use a Str or
an Int". But can it?
Your scenario is a case in point. While 1 ∈ set 1,2,3 and
<1> ∈ set <1 2 3> both work, 1 ∈ set <1 2
3> and <1> ∈ set 1, 2, 3 both return False
.
So it seems the ∈ operator isn't living up to the quoted doc's claim of
dual value interchangeability
This may already be recognized as a bug in the ∈ set operation and/or
other operations. Even if not, this sharp "dual value" edge of the <...>
list constructor may eventually be viewed as sufficiently painful that Perl 6 needs to
change.
I think this is a bug, but not in the set stuff. The other answers were very helpful in
sorting out what was important and what wasn't.
I used the angle-brackets form of the quote
words . The quote words form is supposed to be equivalent to the quoting version (that
is, True under eqv ). Here's the doc example:
<a b c> eqv ('a', 'b', 'c')
But, when I try this with a word that is all digits, this is broken:
$ perl6
> < a b 137 > eqv ( 'a', 'b', '137' )
False
But, the other forms work:
> qw/ a b 137 / eqv ( 'a', 'b', '137' )
True
> Q:w/ a b 137 / eqv ( 'a', 'b', '137' )
True
You typically see these sorts of errors when there are two code paths to get to a final
result instead of shared code that converges to one path very early. That's what I would look
for if I wanted to track this down (but, I need to work on the book!)
This does highlight, though, that you have to be very careful about sets. Even if this bug
was fixed, there are other, non-buggy ways that eqv can fail. I would have still
failed because 4 as Int is not
"4" as Str . I think this level
of attention to data types in unperly in it's DWIMery. It's certainly something I'd have to
explain very carefully in a classroom and still watch everyone mess up on it.
For what it's worth, I think the results of gist tend to be misleading in
their oversimplification, and sometimes the results of perl aren't rich enough
(e.g. hiding Str which forces me to .WHAT ). The more I use those,
the less useful I find them.
But, knowing that I messed up before I even started would have saved me from that code
spelunking that ended up meaning nothing!
Could you clarify what you consider the bug to be? As far as I can tell, this is all by
design: (a) <...> goes through &val , which returns allomorphs if
possible (b) set membership is defined in terms of identity, which distinguishes between
allomorphs and their corresponding value types; so I would not classify it as a bug, but
'broken' by design; or phrased another way, it's just the WAT that comes with this particular DWIM – Christoph
Nov 26 '16 at 23:55
This was intentionally added, and is
part of the
testsuite . ( I can't seem to find anywhere that tests for < > being
equivalent to q:w:v< > and << >> / "
" being equivalent to qq:ww:v<< >> ) – Brad Gilbert
Nov 26 '16 at 23:59
The documentation seems to be just wrong here, <...> does not correspond
to qw(...) , but qw:v(...) . Cf S02 for the description of the
adverb and this
test that Brad was <del>looking for</del> already linked to –
Christoph
Nov 27 '16 at 0:17
or perhaps not outright wrong, but rather 'just' misleading: <...> is
indeed a:w form, and the given example code does compare equal according
to eqv – Christoph
Nov 27 '16 at 0:45
Just to add to the other answers and point out a consistancy here between sets and object
hashes .
An object hash is declared as my %object-hash{Any} . This effectively hashes
on objects .WHICH method, which is similar to how sets distinguish individual
members.
Substituting the set with an object hash:
my %obj-hash{Any};
%obj-hash< 1 2 3 4 > = Any;
say "hash: ", %obj-hash.keys.perl;
say "4 is in hash: ", %obj-hash{4}:exists;
say "IntStr 4 is in hash: ", %obj-hash{ IntStr.new(4, "Four") }:exists;
say "IntStr(4,...) is 4: ", IntStr.new(4, "Four") == 4;
say "5 is in hash: ", %obj-hash{5}:exists;
gives similar results to your original example:
hash: (IntStr.new(4, "4"), IntStr.new(1, "1"), IntStr.new(2, "2"), IntStr.new(3, "3")).Seq
4 is in hash: False
IntStr 4 is in hash: True
IntStr(4,...) is 4: True
5 is in hash: False
In which I detail the Perl 6 elements that have most changed my Perl 5 coding, and share the
Perl 5 techniques I have adopted.
I eat, sleep, live, and breathe Perl!
Consultant and Contract Programmer Frequent PerlMongers speaker Dedicated Shakespeare
theater-goer Armchair Mathematician Author of Blue_Tiger, a tool for modernizing Perl.
36 years coding 22 years Perl 16 years Married 15 YAPCs 7 Hackathons 3 PerlWhirls Perl
interests: Refactoring, Perl Idioms / Micropatterns, RosettaCode, and Perl 6.
I have an attribute (32 bits-long), that each bit responsible to specific functionality. Perl
script I'm writing should turn on 4th bit, but save previous definitions of other bits.
I use in my program:
Sub BitOperationOnAttr
{
my $a="";
MyGetFunc( $a);
$a |= 0x00000008;
MySetFunc( $a);
}
** MyGetFunc/ MySetFunc my own functions that know read/fix value.
Questions:
if usage of $a |= 0x00000008; is right ?
how extract hex value by Regular Expression from string I have : For example:
Your questions are not related; they should be posted separately. That makes it easier for
other people with similar questions to find them. – Michael CarmanJan
12 '11 at 16:13
We have to use localtime to convert in time from epoch seconds and gmtime to convert in time
from normal seconds i got it now....Thanks!! – confused
yesterday
Exactly. To expound, localtime() takes the epoch and returns a string (or date
parts array) representing the time in your local timezone; gmtime() takes the
epoch and returns a string (or date parts array) representing the time in UTC. –
mwp
4 hours ago
,
I would recommend using Time::Piece for this job - it's core in perl.
#!/usr/bin/env perl
use strict;
use warnings;
use Time::Piece;
my $t = localtime ( 1510652305 );
print $t;
It'll print default format, or you can use formatted using strftime .
Dealing with data that comes from webpages can be really complicated. There is likely to be a
combination of ASCII, UTF-8, and wide characters in the data returned and you cannot depend
on the website to tell you what type of content is being returned. The routines safeString,
safeSubstr, testString, and trueLength can be used to easily manipulate these strings. Pass
any string to safeString and you will never get a wide character warning from print. Use
safeSubstr to extract complete UTF-8 characters sequences from a string. Use testString to
tell you what's really in the string. Use trueLength to find out how many characters wide the
output will be. # This string has a mixture of ASCII, UTF-8, 2 byte wide, and 4 byte #
wide characters my $crazy = "Hello\x{26c4}".encode("utf-8","\x{26f0}"). "\x{10102}\x{2fa1b}";
# Now the string only has ASCII and UTF-8 characters my $sane = safeString($crazy); #
testString($crazy) returns 7 # testString($sane) returns 3 # length($sane) returns 19 #
trueLength($sane) returns 9 my $snowman = safeSubstr($crazy,5,1);
######################################## # safeString($string) # return a safe version of the
string sub safeString { my ($string) = @_; return "" unless defined($string); my $t =
testString($string); return $string if $t <= 3; return encode("utf-8",$string) if $t <=
5; # The string has both UTF-8 and wide characters so it needs # tender-loving care my @s =
unpack('C*',$string); my @r; for (my $i = 0; $i < scalar(@s);) { if ($s[$i] < 128) {
push @r, $s[$i]; $i++; } elsif ($s[$i] > 255) { # encode a wide character push
@r,unpack("C*",encode("utf-8",chr($s[$i]))); $i++; } else { # copy all the utf-8 bytes $n =
_charBytes($i,@s) - 1; map { push @r, $s[$i+$_] } 0..$n; $i += $n + 1; } } return
pack("C*",@r); } ######################################## # safeSubstr($string,$pos,$n) #
return a safe substring (treats utf-8 sequences as a single # character) sub safeSubstr { my
($string,$pos,$n) = @_; $s = safeString($string); my $p = 0; my $rPos = 0; my $rEnd = -1; my
@s = unpack('C*',$s); for (my $i = 0; $i < scalar(@s);) { if ($s[$i] < 128) { $i++; }
elsif ($s[$i] > 255) { $i++; } else { $i += _charBytes($i,@s); } $p++; $rPos = $i if $p ==
$pos; $rEnd = $i-1 if $p == $pos + $n; } $rEnd = scalar(@s) - 1 if $rEnd < 0; return "" if
$rPos > $rEnd; my @r; map { push @r, $s[$_] } $rPos..$rEnd; return pack("C*",@r); }
######################################## # testString($string) # returns information about
the characters in the string # # The 1, 2, and 4 bits of the result are for ASCII, UTF-8, and
# wide characters respectively. If multiple bits are set, # characters of each type appear in
the string. If the result is: # <= 1 simple ASCII string # <= 3 simple UTF-8 string #
>3 && <= 5 mixed ASCII & wide characters # >= 6 mixed UTF-8 & wide
characters sub testString { my ($s) = @_; return undef unless defined($s); my $r = 0; my @s =
unpack('C*',$s); for (my $i = 0; $i < scalar(@s);) { if ($s[$i] < 128) { $r |= 1; $i++;
} elsif ($s[$i] > 255) { $r |= 4; $i++; } else { $r |= 2; $i += _charBytes($i,@s); } }
return $r; } ######################################## # trueLength($string) # returns the
number of UTF-8 characters in a string sub trueLength { my ($s) = @_; return unless
defined($s); my $len = 0; my @s = unpack('C*',$s); for (my $i = 0; $i < scalar(@s);) { if
($s[$i] < 128) { $i++; } elsif ($s[$i] > 255) { $i++; } else { $i += _charBytes($i,@s);
} $len++; } return $len; } ######################################## # String support routines
sub _charBytes { my $n = shift(@_); my $len = scalar(@_); if ($_[$n] < 128) { return 1; }
elsif ($_[$n] > 65535) { return 4; } elsif ($_[$n] > 255) { return 2; } elsif (($_[$n]
& 0xFC) == 0xFC) { return min(6,$len); } elsif (($_[$n] & 0xF8) == 0xF8) { return
min(5,$len); } elsif (($_[$n] & 0xF0) == 0xF0) { return min(4,$len); } elsif (($_[$n]
& 0xE0) == 0xE0) { return min(3,$len); } elsif (($_[$n] & 0xC0) == 0xC0) { return
min(2,$len); } else { return 1; } }
TOPIC: FAST!! Random Access Indexed, Relational Flat File Databases, Indexed by external
Perl SDBM databases of key/value pairs tied to program "in memory" hash tables, where the Key
in the Key/Value Pair is one or more fields and/or partial fields concatenated together
(separated by a delimiter such as a pipe "|") and contained within the Flat File records for
you to arbitrarily seek to a single record or a sorted/related group of records within your
database.
Since it has been over 2 years ago since I first posted about this TOPIC I discovered, I
wanted to alert the Perl community to the original thread where you can find Perl source code
now for examples of how to implement Joint Database Technology/Methodology. Inparticular the
King James Bible Navigator software DEMO I posted which used FlatFile/SDBM for its database.
I have made this a native Windows GUI application (TreeView/RichEdit COMBO interface) to
demonstrate how to show your end-users a summary of the information of the data contained
within a database, and allow them to drill down to a small amount of specific information
(e.g. verses within a single book/chapter) for actual viewing (and retrieving from the
database). The TreeView Double Click Event was originally written to random access the first
verse within a chapter, then sequentially access the remaining verses within a chapter -
performing a READ for each verse. I posted a separate modified TreeView Double Click Event
for you to insert into the Application which reads an entire chapter in one (1) giant READ,
breaking out the individual verses (into an array) using the UNPACK statement. -- Eric
Consider this code that constructs a set and tests for membership:
my $num_set = set( < 1 2 3 4 > );
say "set: ", $num_set.perl;
say "4 is in set: ", 4 ∈ $num_set;
say "IntStr 4 is in set: ", IntStr.new(4, "Four") ∈ $num_set;
say "IntStr(4,...) is 4: ", IntStr.new(4, "Four") == 4;
say "5 is in set: ", 5 ∈ $num_set;
A straight 4 is not in the set, but the IntStr version is:
set: set(IntStr.new(4, "4"),IntStr.new(1, "1"),IntStr.new(2, "2"),IntStr.new(3, "3"))
4 is in set: False
IntStr 4 is in set: True
IntStr(4,...) is 4: True
5 is in set: False
I think most people aren't going to expect this, but the ∈ docs doesn't
say anything about how this might work. I don't have this problem if I don't use the quote
words (i.e. set( 1, 2, 3, 4) ).
You took a wrong turn in the middle. The important part is what nqp::existskey
is called with: the k.WHICH . This method is there for value types, i.e.
immutable types where the value - rather than identity - defines if two things are supposed
to be the same thing (even if created twice). It returns a string representation of an
object's value that is equal for two things that are supposed to be equal. For
<1>.WHICH you get IntStr|1 and for 1.WHICH you
get just Int|1 .
That's not really clear from that statement. That's talking about which elements are in the
set. Beyond that, even if you choose to compare with ===, you have to know how other things
are stored. This is the sort of info that should show up next to the Set operators. –
brian d foy
Nov 26 '16 at 21:28
Indeed, I think I've found a bug. The qw docs says this should be true: < a b 137
> eqv ( 'a', 'b', '137' ) , but in the same version of Rakudo Star I get false.
It's different object types on each side. – brian d foy
Nov 26 '16 at 23:09
Despite all this, your answer was the A-ha! moment that led me to look at the right thing.
Thanks for all of your help. – brian d foy
Nov 26 '16 at 23:16
As you mention in your answer, your code works if you write your numbers as a simple comma
separated list rather than using the <...> construct.
Here's why:
4 ∈ set 1, 2, 3, 4 # True
A bare numeric literal in code like the 4 to the left of ∈
constructs a single value with a numeric type. (In this case the type is Int, an integer.) If
a set constructor receives a list of similar literals on the right then
everything works out fine.
The various <...>"quote words" constructs turn
the list of whitespace separated literal elements within the angle brackets into an output
list of values.
The foundational variant ( qw<...> ) outputs nothing but strings. Using
it for your use case doesn't work:
4 ∈ set qw<1 2 3 4> # False
The 4 on the left constructs a single numeric value, type Int .
In the meantime the set constructor receives a list of strings, type
Str : ('1','2','3','4') . The ∈ operator doesn't
find an Int in the set because all the values are Str s so returns
False .
Moving along, the huffmanized <...> variant outputs Str s
unless an element is recognized as a number. If an element is recognized as a number then the
output value is a "dual value". For example a 1 becomes an IntStr .
According to the doc "an IntStr can be used interchangeably where one might use a Str or
an Int". But can it?
Your scenario is a case in point. While 1 ∈ set 1,2,3 and
<1> ∈ set <1 2 3> both work, 1 ∈ set <1 2
3> and <1> ∈ set 1, 2, 3 both return False
.
So it seems the ∈ operator isn't living up to the quoted doc's claim of
dual value interchangeability
This may already be recognized as a bug in the ∈ set operation and/or
other operations. Even if not, this sharp "dual value" edge of the <...>
list constructor may eventually be viewed as sufficiently painful that Perl 6 needs to
change.
I think this is a bug, but not in the set stuff. The other answers were very helpful in
sorting out what was important and what wasn't.
I used the angle-brackets form of the quote
words . The quote words form is supposed to be equivalent to the quoting version (that
is, True under eqv ). Here's the doc example:
<a b c> eqv ('a', 'b', 'c')
But, when I try this with a word that is all digits, this is broken:
$ perl6
> < a b 137 > eqv ( 'a', 'b', '137' )
False
But, the other forms work:
> qw/ a b 137 / eqv ( 'a', 'b', '137' )
True
> Q:w/ a b 137 / eqv ( 'a', 'b', '137' )
True
You typically see these sorts of errors when there are two code paths to get to a final
result instead of shared code that converges to one path very early. That's what I would look
for if I wanted to track this down (but, I need to work on the book!)
This does highlight, though, that you have to be very careful about sets. Even if this bug
was fixed, there are other, non-buggy ways that eqv can fail. I would have still
failed because 4 as Int is not
"4" as Str . I think this level
of attention to data types in unperly in it's DWIMery. It's certainly something I'd have to
explain very carefully in a classroom and still watch everyone mess up on it.
For what it's worth, I think the results of gist tend to be misleading in
their oversimplification, and sometimes the results of perl aren't rich enough
(e.g. hiding Str which forces me to .WHAT ). The more I use those,
the less useful I find them.
But, knowing that I messed up before I even started would have saved me from that code
spelunking that ended up meaning nothing!
Could you clarify what you consider the bug to be? As far as I can tell, this is all by
design: (a) <...> goes through &val , which returns allomorphs if
possible (b) set membership is defined in terms of identity, which distinguishes between
allomorphs and their corresponding value types; so I would not classify it as a bug, but
'broken' by design; or phrased another way, it's just the WAT that comes with this particular DWIM – Christoph
Nov 26 '16 at 23:55
This was intentionally added, and is
part of the
testsuite . ( I can't seem to find anywhere that tests for < > being
equivalent to q:w:v< > and << >> / "
" being equivalent to qq:ww:v<< >> ) – Brad Gilbert
Nov 26 '16 at 23:59
The documentation seems to be just wrong here, <...> does not correspond
to qw(...) , but qw:v(...) . Cf S02 for the description of the
adverb and this
test that Brad was <del>looking for</del> already linked to –
Christoph
Nov 27 '16 at 0:17
or perhaps not outright wrong, but rather 'just' misleading: <...> is
indeed a:w form, and the given example code does compare equal according
to eqv – Christoph
Nov 27 '16 at 0:45
Just to add to the other answers and point out a consistancy here between sets and object
hashes .
An object hash is declared as my %object-hash{Any} . This effectively hashes
on objects .WHICH method, which is similar to how sets distinguish individual
members.
Substituting the set with an object hash:
my %obj-hash{Any};
%obj-hash< 1 2 3 4 > = Any;
say "hash: ", %obj-hash.keys.perl;
say "4 is in hash: ", %obj-hash{4}:exists;
say "IntStr 4 is in hash: ", %obj-hash{ IntStr.new(4, "Four") }:exists;
say "IntStr(4,...) is 4: ", IntStr.new(4, "Four") == 4;
say "5 is in hash: ", %obj-hash{5}:exists;
gives similar results to your original example:
hash: (IntStr.new(4, "4"), IntStr.new(1, "1"), IntStr.new(2, "2"), IntStr.new(3, "3")).Seq
4 is in hash: False
IntStr 4 is in hash: True
IntStr(4,...) is 4: True
5 is in hash: False
In which I detail the Perl 6 elements that have most changed my Perl 5 coding, and share the
Perl 5 techniques I have adopted.
I eat, sleep, live, and breathe Perl!
Consultant and Contract Programmer Frequent PerlMongers speaker Dedicated Shakespeare
theater-goer Armchair Mathematician Author of Blue_Tiger, a tool for modernizing Perl.
36 years coding 22 years Perl 16 years Married 15 YAPCs 7 Hackathons 3 PerlWhirls Perl
interests: Refactoring, Perl Idioms / Micropatterns, RosettaCode, and Perl 6.
I have an attribute (32 bits-long), that each bit responsible to specific functionality. Perl
script I'm writing should turn on 4th bit, but save previous definitions of other bits.
I use in my program:
Sub BitOperationOnAttr
{
my $a="";
MyGetFunc( $a);
$a |= 0x00000008;
MySetFunc( $a);
}
** MyGetFunc/ MySetFunc my own functions that know read/fix value.
Questions:
if usage of $a |= 0x00000008; is right ?
how extract hex value by Regular Expression from string I have : For example:
Your questions are not related; they should be posted separately. That makes it easier for
other people with similar questions to find them. – Michael CarmanJan
12 '11 at 16:13
We have to use localtime to convert in time from epoch seconds and gmtime to convert in time
from normal seconds i got it now....Thanks!! – confused
yesterday
Exactly. To expound, localtime() takes the epoch and returns a string (or date
parts array) representing the time in your local timezone; gmtime() takes the
epoch and returns a string (or date parts array) representing the time in UTC. –
mwp
4 hours ago
,
I would recommend using Time::Piece for this job - it's core in perl.
#!/usr/bin/env perl
use strict;
use warnings;
use Time::Piece;
my $t = localtime ( 1510652305 );
print $t;
It'll print default format, or you can use formatted using strftime .
I come to you because of a mystery I'
d like to unravel: The module import code doesn'
t work as I expected. So, as I'
m thinking that it probably is a problem with my chair-keyboard interface, rather than with the language, I need your help.
So, there are these modules I have, the first one goes like this:
use utf8;
use Date::Manip;
use LogsMarcoPolo;
package LibOutils;
BEGIN { require Exporter;
# set the version for version checking our $VERSION = 1.00;
# Inherit from Exporter to export functions and variables our @ISA = qw(Exporter);
# Functions and variables which are exported by default our @EXPORT = qw(getDateDuJour getHeureActuelle getInfosSemaine ge tTailleRepertoire
getInfosPartition getHashInfosContenuRepertoire dor mir);
# Functions and variables which can be optionally exported our @EXPORT_OK = qw();
} # Under this line are definitions of local variables, and the subs.
[download]
I also have another module, which goes like that:
use utf8;
use strict;
use warnings;
use Cwd;
# Module "
CORE"
use Encode;
use LibOutils qw(getHeureActuelle);
package LogsMarcoPolo;
BEGIN { require Exporter;
# set the version for version checking our $VERSION = 1.00;
# Inherit from Exporter to export functions and variables our @ISA = qw(Exporter);
# Functions and variables which are exported by default our @EXPORT = qw(setNomProgramme ouvreFichierPourLog assigneFluxPo urLog
pushFlux popFlux init printAndLog);
# Functions and variables which can be optionally exported our @EXPORT_OK = qw();
} # Here are other definitions of variables and subs, which I removed fo r the sake of clarity sub init { my ($nomDuProgramme, $pathLogGeneral,
$pathLogErreurs) = @_;
my $date = LibOutils::getDateDuJour();
# La date de l'
appel à init() my $time = LibOutils::getHeureActuelle();
# L'
heure de l'
appel à init() $nomProgramme = $nomDuProgramme;
# Ouverture du flux pour STDOUT: my $stdout = assigneFluxPourLog(*STDOUT);
# On l'
ajoute à la liste de flux '
OUT'
: pushFlux('
OUT'
, $stdout);
# Ouverture du flux pour STDERR: my $stderr = assigneFluxPourLog(*STDERR);
# On l'
ajoute à la liste de flux '
ERR'
, et à la liste '
DUO'
: pushFlux('
ERR'
, $stderr);
pushFlux('
DUO'
, $stderr);
if (defined $pathLogGeneral) { my $plg = $pathLogGeneral;
$plg =~ s/<
DATE>
/$date/g;
$plg =~ s/<
TIME>
/$time/g;
my $logG = ouvreFichierPourLog($plg);
pushFlux('
OUT'
, $logG);
pushFlux('
DUO'
, $logG);
} if (defined $pathLogErreurs) { my $ple = $pathLogErreurs;
$ple =~ s/<
DATE>
/$date/g;
$ple =~ s/<
TIME>
/$time/g;
my $logE = ouvreFichierPourLog($ple);
pushFlux('
ERR'
, $logE);
pushFlux('
DUO'
, $logE);
} }
[download]
Now, look at the second module: When, in the "
init"
sub, I call the getDateDuJour() and getHeureActuelle() functions with an explicit namespace, it works fine.
If I remove the prefix, it doesn'
t work, even for the function whose name I put in the "
qw(...)"
chain after the use.
By putting package after the
use clauses, you are importing all the functions to the
"
main"
namespace, not into your package'
s namespace. Moving the package declaration up should help.
I wonder, could it have something to do with loop-including ?
Circular dependencies don'
t automatically cause a problem, it also depends on what the module does in its body (which you haven'
t shown). If you think there is a problem, a short piece of example code that reproduces the problem would help, see http://sscce.org/
when dumping an array, do Data::Dumper::Dumper(\@array), not ...(@array). if passed a list,
Dumper dumps each element individually, which is not what you want here – ysth
yesterday
,
I would say "don't" and instead:
my %fin_nodes;
$fin_nodes{$node} = [$hindex, $e->{$hip}->{FREQ}]);
And then you can simply if ($fin_nodes{$node}) {
Failing that though - you don't need to grep every element, as your node name is always
first.
eq is probably a better choice than =~ here, because the latter
will substring match. (And worse, can potentially do some quite unexpected things if you've
metacharacters in there, since you're not quoting or escaping them)
E.g. in your example - if you look for a node called "node" you'll get
multiple hits.
Note - if you're only looking for one match, you can do something like:
This will just get you the first result, and the rest will be discarded. (Which
isn't too efficient, because grep will continue to iterate the whole list).
Your last statement was on point, I only needed one match. Then before pushing a node onto
fin_nodes this was enough: "if (!$first_match)" – Taranasaur
yesterday
@Taranasaur: I think you missed the point of Sobrique's answer. A hash is by far the
better choice for this, and you can simply write $fin_nodes{$node} //= [ $hindex,
$e->{$hip}{FREQ} ] and avoid the need for any explicit test altogether. –
Borodin
yesterday
@Borodin, no I do get Sobrique's point. The fin_nodes array is being used for a simple list
function that another method is already using quite happily in my program. I will at some
point go back and create a hash as there might be more attributes I'll need to include in
that array/hash – Taranasaur
yesterday
use Net::FTP;
my $ftp = Net::FTP->new("example.com", Debug => 1) or die "Cannot connect to example.com: $@";
$ftp->login("username",'xxxx') or die "Cannot login ", $ftp->message;
$ftp->pasv();
$ftp->binary();
$ftp->cwd("/web/example.com/public_html/cgi-bin/links/admin/IMPORT") or die "Cannot change working directory ", $ftp->message;
print "Currently in: " . $ftp->pwd(). "\n";
$ftp->put("/home/chambres/web/example.com/public_html/cgi-bin/links/admin/org.csv") or die "Cannot upload ", $ftp->message;
$ftp->quit;
However, when I run it I get:
Net::FTP>>> Net::FTP(3.05)
Net::FTP>>> Exporter(5.72)
Net::FTP>>> Net::Cmd(3.05)
Net::FTP>>> IO::Socket::SSL(2.024)
Net::FTP>>> IO::Socket::IP(0.37)
Net::FTP>>> IO::Socket(1.38)
Net::FTP>>> IO::Handle(1.35)
Net::FTP=GLOB(0x182e348)<<< 220 (vsFTPd 3.0.3)
Net::FTP=GLOB(0x182e348)>>> USER username
Net::FTP=GLOB(0x182e348)<<< 331 Please specify the password.
Net::FTP=GLOB(0x182e348)>>> PASS ....
Net::FTP=GLOB(0x182e348)<<< 230 Login successful.
Net::FTP=GLOB(0x182e348)>>> EPSV
Net::FTP=GLOB(0x182e348)<<< 229 Entering Extended Passive Mode (|||12065|)
Net::FTP=GLOB(0x182e348)>>> TYPE I
Net::FTP=GLOB(0x182e348)<<< 200 Switching to Binary mode.
Net::FTP=GLOB(0x182e348)>>> CWD /web/example.com/public_html/cgi-bin/links/admin/IMPORT
Net::FTP=GLOB(0x182e348)<<< 250 Directory successfully changed.
Net::FTP=GLOB(0x182e348)>>> PWD
Net::FTP=GLOB(0x182e348)<<< 257 "/web/example.com/public_html/cgi-bin/links/admin/IMPORT" is the current directory
Currently in: /web/example.com/public_html/cgi-bin/links/admin/IMPORT
Net::FTP=GLOB(0x182e348)>>> PORT 139,162,208,252,155,199
Net::FTP=GLOB(0x182e348)<<< 200 PORT command successful. Consider using PASV.
Net::FTP=GLOB(0x182e348)>>> FEAT
Net::FTP=GLOB(0x182e348)<<< 211-Features:
Net::FTP=GLOB(0x182e348)<<< EPRT
Net::FTP=GLOB(0x182e348)<<< EPSV
Net::FTP=GLOB(0x182e348)<<< MDTM
Net::FTP=GLOB(0x182e348)<<< PASV
Net::FTP=GLOB(0x182e348)<<< REST STREAM
Net::FTP=GLOB(0x182e348)<<< SIZE
Net::FTP=GLOB(0x182e348)<<< TVFS
Net::FTP=GLOB(0x182e348)<<< 211 End
Net::FTP=GLOB(0x182e348)>>> HELP ALLO
Net::FTP=GLOB(0x182e348)<<< 214-The following commands are recognized.
Net::FTP=GLOB(0x182e348)<<< ABOR ACCT ALLO APPE CDUP CWD DELE EPRT EPSV FEAT HELP LIST MDTM MKD
Net::FTP=GLOB(0x182e348)<<< MODE NLST NOOP OPTS PASS PASV PORT PWD QUIT REIN REST RETR RMD RNFR
Net::FTP=GLOB(0x182e348)<<< RNTO SITE SIZE SMNT STAT STOR STOU STRU SYST TYPE USER XCUP XCWD XMKD
Net::FTP=GLOB(0x182e348)<<< XPWD XRMD
Net::FTP=GLOB(0x182e348)<<< 214 Help OK.
Net::FTP=GLOB(0x182e348)>>> ALLO 37954326
Net::FTP=GLOB(0x182e348)<<< 202 ALLO command ignored.
Net::FTP=GLOB(0x182e348)>>> STOR org.csv
Net::FTP=GLOB(0x182e348)<<< 425 Failed to establish connection.
<h1>Software error:</h1>
<pre>Cannot upload Failed to establish connection.
</pre>
<p>
For help, please send mail to this site's webmaster, giving this error message
and the time and date of the error.
</p>
[Fri Nov 10 10:57:33 2017] export-csv-other-sites.cgi: Cannot upload Failed to establish connection.
It seems to work up until the put() command. Any ideas as to what is going
on?
Secondly, it is not connecting. It tells you that twice
Net::FTP=GLOB(0x182e348)<<< 425 Failed to establish connection. and then
again <pre>Cannot upload Failed to establish connection – Gerhard Barnard
Nov 10 at 11:38
@GerhardBarnard - I know that :) The weird part, is that it says it IS connected:
Currently in: /web/example.com/public_html/cgi-bin/links/admin/IMPORT . –
Andrew
Newby
Nov 10 at 11:41
I suspect it is not keeping the connection open. can you also fix the code? it seems
incomplete. $ftp->put("/home/chambres/web/example.com/public_html/cgi-
– Gerhard Barnard
Nov 10 at 11:43
Net::FTP=GLOB(0x182e348)>>> PORT 139,162,208,252,155,199
Net::FTP=GLOB(0x182e348)<<< 200 PORT command successful. Consider using PASV.
FTP uses a control connection for the command and data connections for each data transfer.
With the PORT command your local system is instructing the server to connect to the given IP
address (139.162.208.252) and port (39879=155*256+199). Connecting from outside to some
arbitrary port on your system will not work if you are behind a firewall or some NAT or if
there is a firewall configured on your system. In these cases it might work to use the
passive mode where the client opens a connection to the server and not the server a
connection to the client.
Net::FTP=GLOB(0x182e348)>>> STOR org.csv
Net::FTP=GLOB(0x182e348)<<< 425 Failed to establish connection.
It looks like the server could not connect to your system in order to create a connection
to transfer the data. Probably a firewall or NAT involved. Try passive mode.
It looks like that you tried to use passive mode already:
Only you did it the wrong way. The command above just sends the PASV/EPSV command to the
server but does not change which mode gets used for the next data transfer. To cite from
the documentation :
If for some reason you want to have complete control over the data connection, this
includes generating it and calling the response method when required, then the user can use
these methods to do so.
However calling these methods only affects the use of the methods above that can return a
data connection. They have no effect on methods get, put, put_unique and those that do not
require data connections.
To instead enable passive mode in connection with put , get etc
use passive not pasv :
Strings with arbitrary delimiters after tr, m, s, etc are a special, additional
type of literals. Each with its own rules. And those rules are different from rules that exist
for single quoted strings, or double quoted strings or regex (three most popular types of
literals in Perl).
For example, the treatment of backslash in "tr literal" is different from single quoted
strings:
"A single-quoted, literal string. A backslash represents a backslash unless followed by the
delimiter or another backslash, in which case the delimiter or backslash is interpolated."
This means that in Perl there is a dozen or so of different types of literals, each with its
own idiosyncratic rules. Which create confusion even for long type Perl users as they tend to
forget detail of constructs they use rarely and extrapolate them from more often used
constructs.
For example, in my case, I was burned by the fact that "m literals" allows interpolation of
variables, but "tr literals" do not. And even created a test case to study this behavior
:-)
In other words, the nature of those "context-dependent-literals" (on the level of lexical
scanner they are all literals) is completely defined not by delimiters they are using (which
are arbitrary), but by the operator used before it. If there none, m is assumed.
This "design decision" (in retrospect this is a design decision, although in reality it was
"absence of design decition" situation ;-) adds unnecessary complexity to the language and
several new (and completely unnecessary) types of bugs.
This "design decision" is also poorly documented and for typical "possible blunders" (for tr
that would be usage of "[","$","@" without preceding backslash) there is no warnings.
This trick of putting tr description into http://perldoc.perl.org/perlop.html that I
mentioned before now can be viewed as an attempt to hide this additional complexity. It might
be beneficial to revise the docs along the lines I proposed.
In reality in Perl q, qq, qr, m, s, tr are functions each of which accepts (and interpret) a
specific, unique type of "context-dependent-literal" as the argument. That's the reality of
this, pretty unique, situation with the language, as I see it.
Quote-Like-Operators shows 2 interesting examples with tr: tr[aeiouy][yuoiea] or
tr(+\-*/)/ABCD/.[download]
The second variant look like a perversion for me. I never thought that this is
possible. I thought that the "arbitrary delimiter" is "catched" after the operator and after
that they should be uniform within the operator ;-).
And the first is not without problems either: if you "extrapolate" your skills with regex
into tr you can write instead of tr[aeiouy][yuoiea] obviously incorrect<
code>tr/ aeiouy /]
yuoiea / that will work fine as
long as strings are of equal length.
I don't think this is the code you are using. This code doesn't compile. You are missing a
semicolon at the end of the use File::Find line. And once I fix that, I get
another problem as you are not loading sample.pm in your main program. Please
don't waste our time by posting sample code where we have to fix simple errors like that.
– Dave
Cross
Nov 11 at 6:54
Two more errors. sample.pm does not return a true value. And the filename is
different between this sample code and the error message that you quote. – Dave Cross
Nov 11 at 6:55
Basically i wanted to know whether we can open a file under main.pl and i need this file
handle to be accessible in different Perl modules. – Rotch Miller
Nov 11 at 7:01
This is unrelated to the problem that you are asking about, but what do you think will happen
when you open your logfile in '>' mode, and then discover you're unable to
obtain an exclusive lock because someone else has it locked? – DavidO
Nov 11 at 7:11
Hint: Clobber-output mode will clobber the output file before you've obtained a lock. This
means if someone else already had the file opened with a lock, you just clobbered them.
– DavidO
Nov 11 at 7:13
The reason why you're seeing this error is that $main::LOGFILE refers to the
scalar variable $LOGFILE which contains the filename, sample . The
filehandle, LOGFILE , is a completely different variable. And here we see the
dangers of having two variables of different types (scalar vs filehandle) with the same name.
Bareword filehandles (the ones in capital letters with no sigil attached, the type you are
using) are slightly strange variables. They don't need a sigil, so you shouldn't use one. So
the simplest fix is to just remove the $ .
sub func()
{
print main::LOGFILE ("Printing in subroutine\n");
}
But using global variables like this is a terrible idea. It will quickly lead to your code
turning into an unmaintainable mess.
Far better to use a lexical filehandle and to pass that into your subroutine.
our $LOGFILE="sample";
open( my $log_fh, ">$LOGFILE" ) or die "__ERROR: can't open file\n'",$LOGFILE,"'!\n";
flock( $log_fh, LOCK_EX );
print $log_fh ("Tool Start\n");
&sample::func($log_fh);
flock( $log_fh, LOCK_UN );
close( $log_fh );
And in sample.pm :
sub func
{
my ($fh) = @_;
print $fh ("Printing in subroutine\n");
}
Note that as I'm now passing a parameter to func() . I've removed the
prototype saying that it takes no parameters (although the fact that you were calling it with
& turns off parameter checking!)
A few other points.
You don't need both -w and use warnings . Remove the
-w .
You don't need both use strict and use strict 'refs' . Remove
the latter.
Modules with all lower-case names are reserved for special Perl features called pragmas
. Don't name your modules like that.
There's no need for $LOGFILE to be a package variable (defined with
our ). Just make it a lexical (defined with my ).
There is no reason to call subroutines with & (and, in fact, it has a
couple of downsides that will confused you).
Don't define subroutines with prototypes unless you know what they are for.
No need for a shebang line in modules.
Use strict and warnings in modules.
I'd write your code like this:
# main.pl
use warnings;
use strict;
use File::Basename; # Not used. Remove?
use Fcntl ':flock'; # Not user. Remove?
use feature qw/say switch/;
use File::Spec::Functions; # Not user. Remove?
use File::Find; # Not user. Remove?
use Sample;
my $LOGFILE = 'sample';
# Lexical filehandle. Three-arg version of open()
open( my $log_fh, '>', $LOGFILE )
or die "__ERROR: can't open file\n'$LOGFILE'!\n";
flock( $log_fh, LOCK_EX );
print $log_fh ("Tool Start\n");
sample::func($log_fh);
flock( $log_fh, LOCK_UN );
close( $log_fh );
And...
package Sample;
use strict;
use warnings;
sub func {
my ($fh) = @_;
print $fh ("Printing in subroutine\n");
}
1;
Is there any method where we can avoid passing the file handler to a subroutine ? I need to
directly access the file handler in the perl module which is present in main,pl. Reason for
this requirement is because i may have different Perl modules and different subroutines
inside each modules, every time i need to pass the file handlers to each of these subroutines
in Perl module. Another difficulty will be always subroutine need not be called from main.pl
file, subroutine defined in a *.pm file may call other subroutine which is defined in another
*.pm module. – Rotch Miller
Nov 11 at 7:29
Main underlying problem is the way how the file handler's can be made visible in the
subroutine of different Perl modules. Like how we have to export a scalar variables from one
*.pm module to any perl modules using the EXPORTER, similar concept for file handlers would
be good. – Rotch Miller
Nov 11 at 7:54
@RotchMiller: Exporter works fine for filehandles. Obviously not if they're lexical
variables. But for package variables and bareword filehandles, there's no problem. –
Dave Cross
Nov 11 at 7:59
@RotchMiller The traditional way to make a variable visible within subroutines in many
different modules is to pass it in as a parameter. But if you want to ignore seventy years of
good software engineering practice - feel free :-) – Dave Cross
Nov 11 at 8:03
Here I'd like to offer a way to cleanly provide a log file for all modules to write
to.
Introduce a module that performs the writes to a log file in a sub; load it by all modules
that need that. In that sub open the log file to append, using state filehandle which thus stays
open across the calls. Then the modules write by invoking this sub, and this can be
initiated by a call from main .
The logger module
package LogAll;
use warnings;
use strict;
use feature qw(say state);
use Carp qw(croak);
use Exporter qw(import);
our @EXPORT_OK = qw(write_log);
sub write_log {
state $fh = do { # initialize; stays open across calls
my $log = 'LOG_FILE.txt';
open my $afh, '>>', $log or croak "Can't open $log: $!";
$afh;
};
say $fh $_ for @_;
}
1;
Two other modules, that need to log, are virtually the same for this example; here is
one
package Mod1;
use warnings;
use strict;
use Exporter qw(import);
use LogAll qw(write_log);
our @EXPORT_OK = qw(f1);
sub f1 {
write_log(__PACKAGE__ . ": @_");
}
1;
The main
use warnings;
use strict;
use LogAll qw(write_log);
use Mod1 qw(f1);
use Mod2 qw(f2);
write_log('START');
f1("hi from " . __PACKAGE__);
f2("another " . __PACKAGE__);
A run results in the file LOG_FILE.txt
START
Mod1: hi from main
Mod2: another main
I print START for a demo but the file need not be opened from
main .
Please develop the printer module further as suitable. For example, and a way for the file
name to be passed optionally so that main can name the log (by varying type and
number of arguments), and add a way to close the log controllably,
"... The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern /^ a-zA-Z * 0-9 *\z/ , the increment is done as a string, preserving each character within its range, with carry: ..."
"... print ++($foo = "99"); # prints "100" print ++($foo = "a0"); # prints "a1" print ++($foo = "Az"); # prints "Ba" print ++($foo = "zz"); # prints "aaa" [download] ..."
The thing to consider here is that the .. range operator leverages the semantics
provided by ++ (auto-increment). The documentation for auto-increment says this:
The auto-increment operator has a little extra builtin magic to it. If you increment
a variable that is numeric, or that has ever been used in a numeric context, you get a
normal increment. If, however, the variable has been used in only string contexts since it
was set, and has a value that is not the empty string and matches the pattern /^ a-zA-Z * 0-9 *\z/ , the increment is done as a string,
preserving each character within its range, with carry:
print ++($foo =
"99"); # prints "100" print ++($foo = "a0"); # prints "a1" print ++($foo = "Az"); # prints
"Ba" print ++($foo = "zz"); # prints "aaa"[download]
The components of the range you are trying to construct do not meet the criteria for
Perl's built-in autoincrement behavior.
However, if you're using Perl 5.26 or newer, and enable unicode_strings you can
use the following, as documented in perlop Range Operators .
use
charnames "greek"; my @greek_small = map { chr } (ord("\N{alpha}") .. ord("\N{omega}"));[download]
Or forgo the \N{charname} lookups and just use the actual ordinal
values:
my @chars = map {chr} $ord_first .. $ord_last;[download]
Dave
Your Mother
(Chancellor) on Nov 16, 2017 at 06:13 UTC
I have an HTML file containing a 2-column table which I want to parse in order to extract
pairs of strings representing the columns. The page layout of the HTML (white space, new
lines) is arbitrary, hence I can't parse the file line by line.
I recall that you can parse such a thing by slurping the whole file into a string and
operating on the entire string, which I'm finding a bit more challenging. I'm trying things
like the following:
#!/usr/bin/perl
open(FILE, "Glossary") || die "Couldn't open file\n";
@lines = <FILE>;
close(FILE);
$data = join(' ', @lines);
while ($data =~ /<tr>.*(<td>.*<\/td>).*(<td>.*<\/td>).*<\/tr>/g) {
print $1, ":", $2, "\n";
}
which gives a null output. Here's a section of the input file:
To correct my early comment (removed), while I recommend HTML::TreeBuilder
for general parsing of HTML (and there are others), here you indeed want
HTML::TableExtract . And you do not want to use regex – zdim
Nov 12 at 21:46
There is a HTML::TableExtract module in CPAN, which
simplifies the problem you are trying to solve:
use strict;
use warnings;
use HTML::TableExtract qw(tree);
my $te = HTML::TableExtract->new( headers => qw(Term Meaning) );
my $html_file = "Glossary";
$te->parse_file($html_file);
my $table = $te->first_table_found;
# ...
Thank you and I'm sure TableExtract is the better way of doing it, but the object of my
question was to improve my understanding of how to use regular expressions since they're so
central to Perl. Adding gs to the regexpr as someone suggested (since deleted) was the leg-up
I needed. – pleriche
yesterday
I see your point, and it's really important build a solid knowlegment on regexpr. But, like
other people have said, it's not a goot idea apply regexpr to parsing html documents –
Miguel Prz
yesterday
,
You already have answers explaining why you shouldn't parse HTML with regexes. And you really
shouldn't. But you've asked for an explanation of why your code doesn't work. So here goes...
You have two problems in your code. One stops it working and the other stops it working as
you expect.
Firstly, you are using . in your regex to match any character. But
. doesn't match any character. It matches any character except a newline. And
you have newlines in your string. You fix that by adding the /s option to your
match operator (so it has /gs instead of /s ).
With that fix in place, you get a result from your code. Using your test data, I see:
<td><b>Term</b>
</td>:<td><b>Meaning</b>
</td>
Which is correct. But looking at your test data, I wondered why I wasn't getting two
results - because of the /g . I soon realised it was because your test data is
missing the closing </td> . When I added that, I got this result:
Ok. It's now finding the second result. But what has happened to the first one? That's the
second error in your code.
You have .* a few times in your regex. That means "zero or more of any
character". But it's the "or more" that is a problem here. By default, Perl regex qualifiers
( * or + ) are greedy. That means they will use up as much of the
string as possible. And the first .* in your regex is eating up a lot of your
string. All of it up to the second <tr> in fact.
The solution to that is to make the .* non-greedy. And you do that by adding
? to the end. So you can replace all of the .* with
.*? . Having done that, I get this output:
The pack function puts one or more things together in a single string. It
represents things as octets (bytes) in a way that it can unpack reliably in some other
program. That program might be far away (like, the distance to Mars far away). It doesn't
matter if it starts as something human readable or not. That's not the point.
Consider some task where you have a numeric ID that's up to about 65,000 and a string that
might be up to six characters.
print pack 'S A6', 137, $ARGV[0];
It's easier to see what this is doing if you run it through a hex dumper as you run
it:
The first column counts the position in the output so ignore that. Then the first two
octets represent the S (short, 'word', whatever, but two octets) format. I gave
it the number 137 and it stored that as 0x8900. Then it stored 'Snoopy' in the next six
octets.
Now it truncates the string to fit the six available spaces.
Consider the case where you immediately send this through a socket or some other way of
communicating with something else. The thing on the other side knows it's going to get eight
octets. It also knows that the first two will be the short and the next six will be the name.
Suppose the other side stored that it $tidy_little_package . It gets the
separate values by unpacking them:
That's the idea. You can represent many values of different types in a binary format
that's completely reversible. You send that packed string wherever it needs to be used.
The thing to consider here is that the .. range operator leverages the semantics
provided by ++ (auto-increment). The documentation for auto-increment says this:
The auto-increment operator has a little extra builtin magic to it. If you increment
a variable that is numeric, or that has ever been used in a numeric context, you get a
normal increment. If, however, the variable has been used in only string contexts since it
was set, and has a value that is not the empty string and matches the pattern /^ a-zA-Z * 0-9 *\z/ , the increment is done as a string,
preserving each character within its range, with carry:
print ++($foo =
"99"); # prints "100" print ++($foo = "a0"); # prints "a1" print ++($foo = "Az"); # prints
"Ba" print ++($foo = "zz"); # prints "aaa"[download]
The components of the range you are trying to construct do not meet the criteria for
Perl's built-in autoincrement behavior.
However, if you're using Perl 5.26 or newer, and enable unicode_strings you can
use the following, as documented in perlop Range Operators .
use
charnames "greek"; my @greek_small = map { chr } (ord("\N{alpha}") .. ord("\N{omega}"));[download]
Or forgo the \N{charname} lookups and just use the actual ordinal
values:
my @chars = map {chr} $ord_first .. $ord_last;[download]
Dave
Your Mother
(Chancellor) on Nov 16, 2017 at 06:13 UTC
I have an HTML file containing a 2-column table which I want to parse in order to extract
pairs of strings representing the columns. The page layout of the HTML (white space, new
lines) is arbitrary, hence I can't parse the file line by line.
I recall that you can parse such a thing by slurping the whole file into a string and
operating on the entire string, which I'm finding a bit more challenging. I'm trying things
like the following:
#!/usr/bin/perl
open(FILE, "Glossary") || die "Couldn't open file\n";
@lines = <FILE>;
close(FILE);
$data = join(' ', @lines);
while ($data =~ /<tr>.*(<td>.*<\/td>).*(<td>.*<\/td>).*<\/tr>/g) {
print $1, ":", $2, "\n";
}
which gives a null output. Here's a section of the input file:
To correct my early comment (removed), while I recommend HTML::TreeBuilder
for general parsing of HTML (and there are others), here you indeed want
HTML::TableExtract . And you do not want to use regex – zdim
Nov 12 at 21:46
There is a HTML::TableExtract module in CPAN, which
simplifies the problem you are trying to solve:
use strict;
use warnings;
use HTML::TableExtract qw(tree);
my $te = HTML::TableExtract->new( headers => qw(Term Meaning) );
my $html_file = "Glossary";
$te->parse_file($html_file);
my $table = $te->first_table_found;
# ...
Thank you and I'm sure TableExtract is the better way of doing it, but the object of my
question was to improve my understanding of how to use regular expressions since they're so
central to Perl. Adding gs to the regexpr as someone suggested (since deleted) was the leg-up
I needed. – pleriche
yesterday
I see your point, and it's really important build a solid knowlegment on regexpr. But, like
other people have said, it's not a goot idea apply regexpr to parsing html documents –
Miguel Prz
yesterday
,
You already have answers explaining why you shouldn't parse HTML with regexes. And you really
shouldn't. But you've asked for an explanation of why your code doesn't work. So here goes...
You have two problems in your code. One stops it working and the other stops it working as
you expect.
Firstly, you are using . in your regex to match any character. But
. doesn't match any character. It matches any character except a newline. And
you have newlines in your string. You fix that by adding the /s option to your
match operator (so it has /gs instead of /s ).
With that fix in place, you get a result from your code. Using your test data, I see:
<td><b>Term</b>
</td>:<td><b>Meaning</b>
</td>
Which is correct. But looking at your test data, I wondered why I wasn't getting two
results - because of the /g . I soon realised it was because your test data is
missing the closing </td> . When I added that, I got this result:
Ok. It's now finding the second result. But what has happened to the first one? That's the
second error in your code.
You have .* a few times in your regex. That means "zero or more of any
character". But it's the "or more" that is a problem here. By default, Perl regex qualifiers
( * or + ) are greedy. That means they will use up as much of the
string as possible. And the first .* in your regex is eating up a lot of your
string. All of it up to the second <tr> in fact.
The solution to that is to make the .* non-greedy. And you do that by adding
? to the end. So you can replace all of the .* with
.*? . Having done that, I get this output:
The pack function puts one or more things together in a single string. It
represents things as octets (bytes) in a way that it can unpack reliably in some other
program. That program might be far away (like, the distance to Mars far away). It doesn't
matter if it starts as something human readable or not. That's not the point.
Consider some task where you have a numeric ID that's up to about 65,000 and a string that
might be up to six characters.
print pack 'S A6', 137, $ARGV[0];
It's easier to see what this is doing if you run it through a hex dumper as you run
it:
The first column counts the position in the output so ignore that. Then the first two
octets represent the S (short, 'word', whatever, but two octets) format. I gave
it the number 137 and it stored that as 0x8900. Then it stored 'Snoopy' in the next six
octets.
Now it truncates the string to fit the six available spaces.
Consider the case where you immediately send this through a socket or some other way of
communicating with something else. The thing on the other side knows it's going to get eight
octets. It also knows that the first two will be the short and the next six will be the name.
Suppose the other side stored that it $tidy_little_package . It gets the
separate values by unpacking them:
That's the idea. You can represent many values of different types in a binary format
that's completely reversible. You send that packed string wherever it needs to be used.
There are no arrays in your code. And there are no method calls in your code.
Your hash is defined incorrectly. You cannot embed hashes inside other hashes. You need to
use hash references. Like this:
my %data = (
'a' => {
x => 'Hello',
y => 'World'
},
'b' => {
x => 'Foo',
y => 'Bar'
}
);
Note, I'm using { ... } to define your inner hashes, not ( ... )
.
That still gives us an error though.
Type of arg 1 to main::p must be hash (not hash element) at passhash line 20, near
"})"
If that's unclear, we can always try adding use diagnostics to get more
details of the error:
(F) This function requires the argument in that position to be of a certain type. Arrays
must be @NAME or @{EXPR}. Hashes must be %NAME or %{EXPR}. No implicit dereferencing is
allowed--use the {EXPR} forms as an explicit dereference. See perlref.
Parameter type definitions come from prototypes. Your prototype is \% .
People often think that means a hash reference. It doesn't. It means, "give me a real hash in
this position and I'll take a reference to it and pass that reference to the subroutine".
(See, this is why people say that prototypes shouldn't be used in Perl - they often don't
do what you think they do.)
You're not passing a hash. You're passing a hash reference. You can fix it by
dereferencing the hash in the subroutine call.
p(%{$data{a}});
But that's a really silly idea. Take a hash reference and turn it into a hash, so that
Perl can take its reference to pass it into a subroutine.
What you really want to do is to change the prototype to just $ so the
subroutine accepts a hash reference. You can then check that you have a hash reference using
ref .
But that's still overkill. People advise against using Perl prototypes for very good
reasons. Just remove it
> ,
Your definition of the structure is wrong. Inner hashes need to use {} , not
() .
my %data = (
a => {
x => 'Hello',
y => 'World'
},
b => {
x => 'Foo',
y => 'Bar'
}
);
Also, to get a single hash element, use $data{'a'} (or even
$data{a} ), not %data{'a'} .
Moreover, see Why are
Perl 5's function prototypes bad? on why not to use prototypes. After correcting the
syntax as above, the code works even without the prototype. If you really need the prototype,
use % , not \% . But you clearly don't know exactly what purpose
prototypes serve, so don't use them.
I have a function in perl that returns a list. It is my understanding that when foo() is
assigned to list a copy is made:
sub foo() { return `ping 127.0.0.1` }
my @list = foo();
That @list then needs to be transferred to another list like @oldlist =
@list; and another copy is made. So I was thinking can I just make a reference from
the returned list like my $listref = \foo(); and then I can assign that
reference, but that doesn't work.
The function I'm working with runs a command that returns a pretty big list (the ping
command is just for example purposes) and I have call it often so I want to minimize the
copies if possible. what is a good way to deal with that?
Make an anonymous array reference of the list that is returned
my $listref = [ foo() ];
But, can you not return an arrayref to start with? That is better in general, too.
What you attempted "takes a reference of a list" ... what one cannot do in the literal
sense; lists are "elusive" things , while a
reference
can be taken
By using the backslash operator on a variable, subroutine, or value.
and a "list" isn't either (with a subroutine we need syntax \&sub_name
)
However, with the \ operator a reference is taken, either to all
elements of the list if in list context
or to a scalar if in scalar context, which is what happens in your attempt. Since your sub
returns a list of values, they are evaluated by the comma operator and discarded, one
by one, until the last one. The reference is then taken of that scalar
my $ref_of_LIST = \( 1,2,3 ); #--> $ref_of_LIST: \3
As it happens, all this applies without parens as well, with \foo() .
I don't know how to return an array ref from a command that returns a list. Would it be
acceptable to do it as return [`ping 1.2.3.4`]; – newguy
2 days ago
@newguy Yes, that would be a fine way to do it. Another is to store the command's return in
an array variable (say, @ary ) -- if you need it elsewhere in the sub -- and
then return \@ary; – zdim
2 days ago
@newguy For one, those elements must be stored somewhere, either anonymously by [ ..
] or associated with a named variable by @ary = .. . I don't know whether
yet an extra copy is made in order to construct an array, but I'd expect that it isn't When
you return \@ary no new copies are made. I would expect that they are about the
same. – zdim
2 days ago
"... Characters may be literals or any of the escape sequences accepted in double-quoted strings. But there is no interpolation, so "$" and "@" are treated as literals. ..."
Obviously only the second result in both tests is correct. Looks like only explicitly given first set is correctly compiled. Is
this a feature or a bug ?
Athanasius (Chancellor) on Nov 16, 2017 at 03:08 UTC
The transliteration operator tr/SEARCHLIST/REPLACEMENTLIST/ does not interpolate its SEARCHLIST ,
so in your first example the search list is simply the literal characters , , , , . See
Quote and Quote like Operators
.
Characters may be literals or any of the escape sequences accepted in double-quoted strings. But there is no interpolation,
so "$" and "@" are treated as literals.
A hyphen at the beginning or end, or preceded by a backslash is considered a literal. Escape sequence details are in the
table near the beginning of this section.
So if you want to use a string to specify the values in a tr statement, you'll probably have to do it via a string
eval:
$ cat foo.pl use strict; use warnings;
my $str1 = 'abcde';
my $str2 = 'eda';
my $diff1 = 0;
eval "\$diff1=\$str1=~tr/$str2//";
print "diff1: $diff1\n";
perl foo.pl diff1: 3
Looks like in tr function a scalar variable is accepted as the fist argument, but is not compiled properly into set of characters
:)
you're guessing how tr /// works, you're guessing it
works like s/// or m///, but you can't guess , it doesn't work like that, it doesn't interpolate variables, read perldoc -f
tr for the details
you're guessing how tr/// works, you're guessing it works like s/// or m///, but you can't guess , it doesn't work like
that, it doesn't interpolate variables, read perldoc -f tr for the details
Houston, we have a problem ;-)
First of all that limits tr area of applicability.
The second, it's not that I am guessing, I just (wrongly) extrapolated regex behavior on tr, as people more often use regex
then tr. Funny, but searching my old code and comments in it is clear that I remembered (probably discovered the hard way, not
by reading the documentation ;-) this nuance several years ago. Not now. Completely forgotten. Erased from memory. And that tells
you something about Perl complexity (actually tr is not that frequently used by most programmers, especially for counting characters).
And that's a real situation, that we face with Perl in other areas too (and not only with Perl): Perl exceeds typical human
memory capacity to hold the information about the language. That's why we need "crutches" like strict.
You simply can't remember all the nuances of more then a dozen of string-related built-in functions, can you? You probably
can (and should) for index/rindex and substr , but that's about it.
So here are two problems here:
1. Are / / strings uniformly interpreted in the language, or there is a "gotcha" because they are differently interpreted by
tr (essentially as a single quoted strings) and regex (as double quoted strings) ?
2. If so, what is the quality of warnings about this gotcha? There is no warning issued, if you use strict and warnings. BTW,
it looks like $ can be escaped:
Right now there is zero warnings issued with use strict and use warnings enabled. Looks like this idea of using =~ for tr was
not so good, after all. Regular syntax like tr(set1, set2) would be much better. But it's to late to change and now we need warnings
to be implemented.
"... The Perl Monks website has 83 data tables, two main type hierarchies (nodetypes and perl classes), a core engine of about 12K and about 600 additional code units spread throughout the database. Documentation is scattered and mostly out of date. ..."
"... The initial architecture seems solid but its features have been used inconsistently over time. ..."
Re^2: Swallowing an elephant in 10 easy steps
by ELISHEVA (Prior) on Aug
13, 2009 at 18:27 UTC
The time drivers are the overall quality of the design, ease of access to code and
database schemas, and the size of the system: the number of database tables, the complexity
of the type/class system(s), the amount of code, and the number of features in whatever
subsystem you explore in step 10. Rather than an average, I'll take the most recent example,
Perl Monks.
The Perl Monks website has 83 data tables, two main type hierarchies (nodetypes and
perl classes), a core engine of about 12K and about 600 additional code units spread
throughout the database. Documentation is scattered and mostly out of date.
The initial architecture seems solid but its features have been used inconsistently
over time. Accessing the schema and code samples is slow because there is no tarball to
download - it has to be done through the web interface or manually cut and pasted into files
off line. The database/class assessment (1-4) took about 16 hours. Steps 5-7 took about 30
hours. Steps 8-10 took about 24 hours. All told that is 70 hours, including writing up
documentation and formatting it with HTML.
However, I always like to leave myself some breathing space. If I were contracting to
learn a system that size, I'd want 90 hours and an opportunity to reassess time schedules
after the initial code walk through was complete. If a system is very poorly designed this
process takes somewhat longer.
A crucial element in controlling time is controlling the amount of detail needed to gain
understanding. It is easy to lose sight of the forest for the trees. That is why I advise
stopping and moving onto the next phase once your categories give a place to most design
elements and the categories work together to tell story. That is also why I recommend
backtracking as needed. Sometimes we make mistakes about which details really matter and
which can be temporarily blackboxed. Knowing I can backtrack lets me err on the side of black
boxing.
The other element affecting time is, of course, the skill of the analyst or developer. I
have the advantage that I have worked both at the coding and the architecture level of
software. I doubt I could work that fast if I didn't know how to read code fluently and trace
the flow of data through code. Having been exposed to many different system designs over the
years also helps - architectural strategies leave telltale footprints and experience helps me
pick up on those quickly.
However one can also learn these skills by doing. The more you practice scanning,
categorizing and tracing through code and data the better you get at it. It will take longer,
but the steps are designed to build on themselves and are, in a way, self-teaching. That is
why you can't just do the 10 steps in parallel as jdporter jokingly suggests below.
However some theoretical context and a naturally open mind definitely helps: if you think
that database tables should always have a one-to-one relationship with classes you will be
very very confused by a system where that isn't true. If I had to delegate this work to
someone else I probably would work up a set of reading materials on different design
strategies that have been used in the past 30 years. Alternatively or in addition, I might
pair an analyst with a programmer so that they could learn from each other (with neither
having priority!)
Best, beth
Update: expanded description of the PerlMonks system so that it addresses all of
the time drivers mentioned in the first paragaph.
Having recently done this on a fairly large codebase that grew organically (no design, no
refactoring) over the course of four years, I feel your pain.
Writing a testsuite, on any level, is nearly essential for this. If you're rewriting an
existing module, you'll need to ensure it's compatible with the old one, and the only sane
way to do that is to test. If the old code is monolithic, it might be difficult to test
individual units, but don't let that stop you from testing at a higher level.
B::Xref helped me make sense of the interactions in the old codebase. I didn't bother with
any visualization tools or graph-creation, though. I just took the output of perl
-MO=Xref filename for each file, removed some of the cruft with a text editor, ran it
through mpage -4 to print, and spent a day with coffee and pencil, figuring out how
things worked.
Pretty much the same tactic was used on the actual code. Print it out, annotate it away
from the computer, and then sit down with the notes to implement the refactoring. If your
codebase is huge (mine was about 4-5k lines in several .pl and .pm files, and was still
manageable) you might not want to do this, though.
The Doxygen perl extension creates docs that are great for seeing what classes
re-implement what methods etc. Also the UML::Sequence
sounds intriguing - it pupports to generate a sequence diagram by monitoring code
execution.
With regard to the To Do list, I scatter them throughout my code if there is a place I
need to do further work. However, I have a make rule for todo that searches for all
of the lines with TODO in them and prints them out. So a usage of a TODO:
if ($whatever) {
# TODO - Finish code to take over the world
}
Kinda ugly, but it lets me put the TODO statements where I actually need to do the
work.
So I can proof out a block of code by writing narrative comments with TODO at the start of
the line (behind comment characters of course).
Then fill in the code later and not worry about missing a piece. Also since the TODOs are
where the stuff needs to be filled in, I have lots of context around the issue and don't need
to write as much as I would if they were at the top of the file. Plus anyone without
something to do in the group can just type make todo and add some code. Finally, it
is easier to add a TODO right where you need it, than bop up to the top of the file and then
have to find where you were back in the code.
Debugging is just an extreme case of dynamic analysis. Third-party code can be extremely
convoluted (so can your own code, of course, but you don't usually think of it that way because
you're familiar with it; you knew it when it was just a subroutine); sometimes you just can't
tell how part of the code fits in, or whether it's called at all. The code is laid out in some
arrangement that makes no sense; if only you could see where the program would actually go when
it was run.
Well, you can, using Perl's built-in debugger. Even though you're not actually trying to
find a bug, the code-tracing ability of the debugger is perfect for the job.
This isn't the place for a full treatment of the debugger (you can see more detail in [
SCOTT01 ]), but fortunately
you don't need a full treatment; a subset of the commands is enough for what you need to do.
(Using the debugger is like getting in a fight; it's usually over very quickly without using
many of the fancy moves you trained for.)
-d command-line flag; either edit the program to add -d to the
shebang line, or run the program by invoking Perl explicitly:
% perl -d program argument argument...
Make sure that the perl in your path is the same one in the shebang line of program
or you'll go crazy if there are differences between the two perls.
Basic Debugger Commands
h h h Brief verbose help (verbose brief help prior to 5.8.0)
b subroutine Set breakpoint at first executable statement of subroutine
b line Set breakpoint for line line
b place condition Set breakpoint for place (either line or subroutine) but trigger it
only when the Perl expression condition is true
с Continue until end of program or breakpoint
с line Continue until line line, end of program, or earlier breakpoint
x expression Examine the value of a variable or expression
n Execute current statement, skipping over any subroutines called from it
s Execute next Perl statement, going into a subroutine called from the current statement
if necessary
1 List source code from current line
r Execute statements until return from current subroutine, end of program, or earlier
breakpoint
T Display stack trace
q Quit
Armed with these commands, we can go code spelunking. Suppose you are debugging a program
containing the following code fragment:
77 for my $url (@url_queue)
78 {
79 my $res = $ua->request($url);
80 summarize($res->content);
81 }
and you know that whenever the program gets to the URL http://www.perlmedic.com/fnord.html something
strange happens in the summarize() subroutine. You'd like to check the
HTTP::Response object to see if there were any redirects you didn't know about. You start the
program under the debugger and type:
DB<1> b 80 $url =~ /fnord/
DB<2>
The program will run until it has fetched the URL you're interested in, at which point you
can examine the response object -- here's an example of what it might look like:
Perl 5.8.0 and later will give you a stack trace anyway if you run a program under the
debugger and some code triggers a warning. But suppose you are either running under an earlier
perl, or you'd really like to have a debugger prompt at the point the warning was about to
happen.
You can combine two advanced features of Perl to do this: pseudo-signal handlers,
and programmatic debugger control .
A signal handler is a subroutine you can tell Perl to execute whenever your program receives
a signal. For instance, when the user interrupts your program by pressing Control-C, that works
by sending an INT signal to your program, which interprets it by default as an instruction to
stop executing.
There are two pseudo-signals, called __WARN__ and __DIE__ . They
aren't real signals, but Perl "generates" them whenever it's told to issue a warning or to die,
respectively. You can supply code to be run in those events by inserting a subroutine reference
in the %SIG hash (see perlvar ) as follows:
$SIG{__WARN__} = sub { print "Ouch, I'm bad" };
(Try it on some code that generates a warning.)
The next piece of the solution is that the debugger can be controlled from within your
program; the variable $single in the special package DB determines what Perl does
at each statement: 0 means keep going, and 1 or 2 mean give a user prompt. 1
So setting $DB::single to 1 in a pseudo-signal handler will give us a debugger
prompt at just the point we wanted.
1 . The difference
between the two values is that a 1 causes the debugger to act as though the last n
or s command the user typed was s , whereas a 2 is equivalent to an
n . When you type an empty command in the debugger (just hit Return), it repeats
whatever the last n or s command was.
Putting the pieces together, you can start running the program under the debugger and give
the commands:
Now the program will breakpoint where it was about to issue a warning, and you can issue a
T command to see a stack trace, examine data, or do anything else you want.
2 The warning is
still printed first.
2 . Under some
circumstances, the breakpoint might not occur at the actual place of warning: The current
routine might return if the statement triggering the warning is the last one being executed in
that routine.
Unfortunately, no __DIE__ pseudo-signal handler will return control to the
debugger (evidently death is considered too pressing an engagement to be interrupted). However,
you can get a stack trace by calling the confess() function in the Carp
module:
DB<1> use Carp
DB<2> $SIG{__DIE__} = sub { confess (@_) }
The output will look something like this:
DB<3>
Insufficient privilege to launch preemptive strike at wargames line
109.
main::__ANON__[(eval 17)[/usr/lib/perl5/5.6.1/
perl5db.pl:1521]:2]('Insufficient privilege to launch preemptive
strike at wargames line 109.^J') called at wargames line 121
main::preemptive('Strike=HASH(0x82069d4)') called at wargames
line 109
main::make_strike('ICBM=HASH(0x820692c)') called at wargames
line 74
main::icbm('Silo_ND') called at wargames line 32
main::wmd('ICBM') called at wargames line 22
main::strike() called at wargames line 11
main::menu() called at wargames line 5
Debugged program terminated. Use q to quit or R to restart,
use O inhibit_exit to avoid stopping after program termination,
h q, h R or h O to get additional info.
I've often found it amusing that the debugger refers to the program at this point as
"debugged."
likbez has asked for the wisdom of the Perl Monks
concerning the following question:
This is kind of topic that previously was reserved to Cobol and PL/1 forums ;-) but now Perl is almost 30 years old and it looks
like the space for Perl archeology is gradually opening ;-).
I got a dozen of fairly large scripts (several thousand lines each) written in a (very) early version of Perl 5 (below Perl
5.6), I now need:
1. Convert them to use strict pragma. The problem is that all of them share (some heavily, some not) information from
main program to subroutines (and sometimes among subroutines too) via global variables in addition to (or sometimes instead of)
parameters. Those scripts mostly do not use my declarations either.
So I need to map variables into local and global namespaces for each subroutine (around 40 per script; each pretty small --
less then hundred lines) to declare them properly.
As initial step I just plan use global variable with namespace qualification or our lists for each subroutine. Currently I
plan to postprocess output of perl -MO=Xref old_perl_script.pl
and generate such statement. Is there a better way ?
2. If possible, I want to split the main namespace into at least two chunks putting all subroutines into another namespace,
or module. I actually do not know how to export subroutines names into other namespace (for example main::) when just package
statements is used in Perl as in example below. Modules do some magic via exporter that I just use but do not fully understand.
For example if we have
#main_script ... ... ... x:a(1,2,3); ... ... ... package x; sub a {...) sub b {...} sub c {...} package y; ... ... ...[download] How can
I access subs a,b,c without qualifying them with namespace x from the main:: namespace?
3. Generally this task looks like a case of refactoring. I wonder, if any Perl IDE has some of required capabilities, or
are there tools that can helpful.
My time to make the conversion is limited and using some off the shelf tools that speed up the process would be a great help.
Any advice will be greatly appreciated.
AnomalousMonk (Chancellor) on Nov 14, 2017 at 07:20 UTC
Step 0: Write a test suite that the current code passes for all normal modes of operation and for all failure
modes.
With this test suite, you can be reasonably certain that refactored code isn't just going to be spreading the devastation.
Given that you seem to be describing a spaghetti-coded application with communication from function to function via all kinds
of secret tunnels and spooky-action-at-a-distance global variables, I'd say you have a job on your hands just with Step 0. But
you've already taken a test suite into consideration... Right?
by Monk::Thomas (Friar) on Nov 14, 2017 at 12:14 UTC
This is what I would do after 'Step 0':
identify a function using a global variable.
verify the global variable does not change during execution of this function, e.g. some other function called by this function
modifies it. (insert some code to do this for you)
convert global variable into an argument and update all callers.
If the variable does change during the run then pick a different function first. When you got the global state disentangled
a bit it's a lot easier to reason about what this code is doing. Everything that's still using a global needs to be treated with
very careful attention.
In addition to AnomalousMonk s advice of a test suite,
I would suggest at the very least to invest the time up front to run automatic regression tests between whatever development version
of the program you have and the current "good" (but ugly) version. That way you can easily verify whether your change affected
the output and operation of the program. Ideally, the output of your new program and the old program should remain identical while
you are cleaning things up.
Note that you can enable strict locally in blocks, so you
don't need to make the main program compliant but can start out with subroutines or files and slowly convert them.
For your second question, have a look at Exporter . Basically
it allows you to im/export subroutine names between packages:
package x;
use Exporter 'import';
our @EXPORT_OK = ('a', 'b', 'c'); [download]#main_script use x 'a', 'b'; # makes a() and b() available in the main namespace[download]
To find and collect the global variables, maybe it helps you to dump the global namespace before and after your program has
run. All these names are good candidates for being at least declared via our to make them visible, and then ideally removed
to pass the parameters explicitly instead of implicitly:
#!perl -w
use strict;
our $already_fixed = 1; # this won't show up
# Put this right before the "uncleaned" part of the script starts
my %initial_variables;
BEGIN {
%initial_variables = %main::; # make a copy at the start of the program
}
END {
#use Data::Dumper;
#warn Dumper \%initial_variables;
#warn Dumper \%main::;
# At the end, look what names came newly into being, and tell us about them:
for my $key (sort keys %main::) {
if( ! exists $initial_variables{ $key } ) {
print "Undeclared global variable '$key' found\n";
my $glob = $main::{ $key };
if( defined *{ $glob }{GLOB}) {
print "used as filehandle *'$key', replace by a lexical filehandle\n";
};
if( defined *{ $glob }{CODE}) {
print "used as subroutine '$key'\n"; # so maybe a false alarm unless you dynamically load code?!
};
if( defined *{ $glob }{SCALAR}) {
print "used as scalar \$'$key', declare as 'our'\n";
};
if( defined *{ $glob }{ARRAY}) {
print "used as array \@'$key', declare as 'our'\n";
};
if( defined *{ $glob }{HASH}) {
print "used as hash \%'$key', declare as 'our'\n";
};
};
};
}
no strict;
$foo = 1;
@bar = (qw(baz bat man));
open LOG, '<', *STDIN;
sub foo_2 {}
The above code is a rough cut and for some reason it claims all global names as scalars in addition to their real use, but
it should give you a start at generating a list of undeclared names.
That pretty much means convert one at a time by hand after you have learned the understanding of importance of knowing
:) Speed kills
2. If possible ... I do not understand ...
That is a hint you shouldn't be refactoring anything programmatically. There are a million nodes on perlmonks, and a readers
digest version might be Modern Perla loose description
of how experienced and effective Perl 5 programmers work....You can learn this too.
Hurry up and bone up
3. Generally this task looks like a case of refactoring. I wonder, if any Perl IDE has some of required capabilities, or
are there tools that can helpful.
"... temporarily changes the value of the variable ..."
"... within the scope ..."
"... Unlike dynamic variables created by the local operator, lexical variables declared with my are totally hidden from the outside world, including any called subroutines. ..."
Dynamic Scoping. It is a neat concept. Many people don't use it, or understand it.
Basically think of my as creating and anchoring a variable to one block of
{}, A.K.A. scope.
my $foo if (true); # $foo lives and dies within the if statement.
So a my variable is what you are used to. whereas with dynamic scoping $var
can be declared anywhere and used anywhere. So with local you basically suspend
the use of that global variable, and use a "local value" to work with it. So
local creates a temporary scope for a temporary variable.
The short answer is that my marks a variable as private in a lexical scope, and
local marks a variable as private in a dynamic scope.
It's easier to understand my , since that creates a local variable in the
usual sense. There is a new variable created and it's accessible only within the enclosing
lexical block, which is usually marked by curly braces. There are some exceptions to the
curly-brace rule, such as:
foreach my $x (@foo) { print "$x\n"; }
But that's just Perl doing what you mean. Normally you have something like this:
sub Foo {
my $x = shift;
print "$x\n";
}
In that case, $x is private to the subroutine and it's scope is enclosed by
the curly braces. The thing to note, and this is the contrast to local , is that
the scope of a my variable is defined with respect to your code as it is written
in the file. It's a compile-time phenomenon.
To understand local , you need to think in terms of the calling stack of your
program as it is running. When a variable is local , it is redefined from the
point at which the local statement executes for everything below that on the
stack, until you return back up the stack to the caller of the block containing the
local .
This can be confusing at first, so consider the following example.
sub foo { print "$x\n"; }
sub bar { local $x; $x = 2; foo(); }
$x = 1;
foo(); # prints '1'
bar(); # prints '2' because $x was localed in bar
foo(); # prints '1' again because local from foo is no longer in effect
When foo is called the first time, it sees the global value of
$x which is 1. When bar is called and local $x runs,
that redefines the global $x on the stack. Now when foo is called
from bar , it sees the new value of 2 for $x . So far that isn't
very special, because the same thing would have happened without the call to
local . The magic is that when bar returns we exit the dynamic
scope created by local $x and the previous global $x comes back
into scope. So for the final call of foo , $x is 1.
You will almost always want to use my , since that gives you the local
variable you're looking for. Once in a blue moon, local is really handy to do
cool things.
But local is misnamed, or at least misleadingly named. Our friend Chip Salzenberg says
that if he ever gets a chance to go back in a time machine to 1986 and give Larry one piece
of advice, he'd tell Larry to call local by the name "save" instead.[14] That's because
local actually will save the given global variable's value away, so it will later
automatically be restored to the global variable. (That's right: these so-called "local"
variables are actually globals!) This save-and-restore mechanism is the same one we've
already seen twice now, in the control variable of a foreach loop, and in the @_ array of
subroutine parameters.
So, local saves a global variable's current value and then set it to some
form of empty value. You'll often see it used to slurp an entire file, rather than leading
just a line:
my $file_content;
{
local $/;
open IN, "foo.txt";
$file_content = <IN>;
}
Calling local $/ sets the input record separator (the value that Perl stops
reading a "line" at) to an empty value, causing the spaceship operator to read the entire
file, so it never hits the input record separator.
Word of warning: both of these articles are quite old, and the second one (by the author's
own warning) is obsolete. It demonstrates techniques for localization of file handles that
have been superseded by lexical file handles in modern versions of Perl. – dan1111
Jan 28 '13 at 11:21
Unlike dynamic variables created by the local operator, lexical variables declared with
my are totally hidden from the outside world, including any called subroutines. This is
true if it's the same subroutine called from itself or elsewhere--every call gets its own
copy.
A local modifies its listed variables to be "local" to the enclosing block, eval, or do
FILE --and to any subroutine called from within that block. A local just gives temporary
values to global (meaning package) variables. It does not create a local variable. This is
known as dynamic scoping. Lexical scoping is done with my, which works more like C's auto
declarations.
I don't think this is at all unclear, other than to say that by "local to the enclosing
block", what it means is that the original value is restored when the block is exited.
While this may be true, it's basically a side effect of the fact that "local"s are intended
to be visible down the callstack, while "my"s are not. And while overriding the value of a
global may be the main reason for using "local", there's no reason you can't use "local" to
define a new variable. – Kevin Crumley
Sep 24 '08 at 20:27
local does not actually define a new variable. For example, try using local to define a
variable when option explicit is enabled. You need to use "our" or "my" to define a new
global or local variable. "local" is correctly used to give a variable a new value –
1800
INFORMATION
Jan 21 '09 at 10:02
Jesus did I really say option explicit to refer to the Perl feature. I meant obviously "use
strict". I've obviously not coded in Perl in a while – 1800 INFORMATION
Jan 29 '09 at 10:45
Unlike dynamic variables created by the local operator, lexical variables declared
with my are totally hidden from the outside world, including any called subroutines.
So, oversimplifying, my makes your variable visible only where it's declared.
local makes it visible down the call stack too. You will usually want to use
my instead of local .
Your confusion is understandable. Lexical scoping is fairly easy to understand but dynamic
scoping is an unusual concept. The situation is made worse by the names my and
local being somewhat inaccurate (or at least unintuitive) for historical
reasons.
my declares a lexical variable -- one that is visible from the point of
declaration until the end of the enclosing block (or file). It is completely independent from
any other variables with the same name in the rest of the program. It is private to that
block.
local , on the other hand, declares a temporary change to the value of a
global variable. The change ends at the end of the enclosing scope, but the variable -- being
global -- is visible anywhere in the program.
As a rule of thumb, use my to declare your own variables and
local to control the impact of changes to Perl's built-in variables.
For a more thorough description see Mark Jason Dominus' article Coping with Scoping .
local is an older method of localization, from the times when Perl had only dynamic scoping.
Lexical scoping is much more natural for the programmer and much safer in many situations. my
variables belong to the scope (block, package, or file) in which they are declared.
local variables instead actually belong to a global namespace. If you refer to a variable
$x with local, you are actually referring to $main::x, which is a global variable. Contrary
to what it's name implies, all local does is push a new value onto a stack of values for
$main::x until the end of this block, at which time the old value will be restored. That's a
useful feature in and of itself, but it's not a good way to have local variables for a host
of reasons (think what happens when you have threads! and think what happens when you call a
routine that genuinely wants to use a global that you have localized!). However, it was the
only way to have variables that looked like local variables back in the bad old days before
Perl 5. We're still stuck with it.
"my" variables are visible in the current code block only. "local" variables are also visible
where ever they were visible before. For example, if you say "my $x;" and call a
sub-function, it cannot see that variable $x. But if you say "local $/;" (to null out the
value of the record separator) then you change the way reading from files works in any
functions you call.
In practice, you almost always want "my", not "local".
dinomite's example of using local to redefine the record delimiter is the only time I have
ran across in a lot of perl programming. I live in a niche perl environment [security
programming], but it really is a rarely used scope in my experience.
I have a variable $x which currently has a local scope in A.pm and I want to use the output
of $x (which is usually PASSED/FAILED) in an if else statement in B.pm
Something like below
A.pm:
if (condition1) { $x = 'PASSED'; }
if (condition2) { $x = 'FAILED'; }
B.pm:
if ($x=='PASSED') { $y=1; } else { $y=0; }
I tried using require ("A.pm"); in B.pm but it gives me an error global
symbol requires an explicit package name which means it is not able to read the
variable from require. Any inputs would help
This sounds like a very strange configuration. Your A.pm has executable code as
well as values that you want to access externally. Is that code in subroutines? Are you
aware that any code outside a subroutine will be executed the first time the external
code requires the file? You need to show us the contents of A.pm or
we can't help you much. – Borodin
Apr 3 '14 at 17:27
Normally, you'd return $x from a function defined in A and called in B; this is
a much cleaner, less pathological way of getting at the information. – Jonathan Leffler
Apr 3 '14 at 17:29
Yes the above if conditions in A.pm are in a subroutine. Is there a way I could read that
subroutine outside to extract the value of $x? – Rancho
Apr 3 '14 at 17:41
I have a variable $x which currently has a local scope in A.pm and I want to use the
output of $x (which is usually PASSED/FAILED) in an if else statement in B.pm
We could show you how to do this, but this is a really bad, awful idea.
There's a reason why variables are scoped, and even global variables declared
with our and not my are still scoped to a particular package.
Imagine someone modifying one of your packages, and not realizing there's a direct
connection to a variable name $x . They could end up making a big mess without
even knowing why.
What I would HIGHLY recommend is that you use functions (subroutines) to pass
around the value you need:
Local/A.pm
package Local::A;
use strict;
use warnings;
use lib qw($ENV{HOME});
use Exporter qw(import);
our @EXPORT_OK = qw(set_condition);
sub set_condition {
if ( condition1 ) {
return "PASSED";
elsif ( condition2 ) {
return "FALSED";
else {
return "Huh?";
}
1;
Here's what I did:
I can't use B as a module name because that's an actual module. Therefore,
I used Local::B and Local::A instead. The Local
module namespace is undefined in CPAN and never used. You can always declare your own
modules under this module namespace.
The use lib allows me to specify where to find my modules.
The package command gives this module a completely separate namespace.
This way, variables in A.pm don't affect B.pm .
use Exporter allows me to export subroutines from one module to
another. @EXPORT_OK are the names of the subroutines I want to export.
Finally, there's a subroutine that runs my test for me. Instead of setting a variable
in A.pm , I return the value from this subroutine.
Check your logic. Your logic is set that $x isn't set if neither condition
is true. You probably don't want that.
Your module can't return a zero as the last value. Thus, it's common to always put
1; as the last line of a module.
Local/B.pm
package Local::B;
use lib qw($ENV{HOME});
use Local::A qw(set_condition);
my $condition = set_contition();
my $y;
if ( $condition eq 'PASSED' ) { # Note: Use `eq` and not `==` because THIS IS A STRING!
$y = 1;
else {
$y = 0;
}
1;
Again, I define a separate module namespace with package .
I use Local::A qw(set_condition); to export my set_condition
subroutine into B.pm . Now, I can call this subroutine without prefixing it
with Local::A all of the time.
I set a locally scoped variable called $condition to the status of my
condition.
Now, I can set $y from the results of the subroutine
set_condition . No messy need to export variables from one package to
another.
If all of this looks like mysterious magic, you need to read about Perl modules . This isn't light summer
reading. It can be a bit impenetrable, but it's definitely worth the struggle. Or, get
Learning Perl
and read up on Chapter 11.
print "$Robert has canned $name's sorry butt\n"; I tried running this in PERL and it yelled
at me saying that it didn't like $name::s. I changed this line of code to: print "$Robert has
canned $name sorry butt\n"; And it worked fine 0_o An error in the tutorial perhaps?
Aristotle (Chancellor)
on Dec 24, 2004 at 01:50 UTC
by Aristotle (Chancellor) on
Dec 24, 2004 at 01:50 UTC
Try
print "$Robert has canned ${name}'s sorry butt\n";[download]
The apostrophe is the old-style package separator, still supported, so $name's
is indeed equivalent to $name::s . By putting the curlies in there, you tell Perl
exactly which part of the string to consider part of the variable name, and which part to
consider a literal value.
I'd put more emphasis on the fact that the first argument to split is always, always, always a regular expression (except
for the one special case where it isn't :-). Too often do I see people write code like this:
@stuff = split "|", $string; # or worse ... $delim = "|"; @stuff = split $delim, $string;[download] And expect
it to split on the pipe symbol because they have fooled themselves into thinking that the first argument is somehow interpreted as
a string rather than a regular expression. duff
There are cases where it is equally easy to use a regexp in list context to split a string as it is to use the split function.
Consider the following examples:my @list = split /\s+/, $string; my @list = $string =~ /(\S+)/g;[download]In
the first example you're defining what to throw away. In the second, you're defining what to keep. But you're getting the same
results. That is a case where it's equally easy to use either syntax.
In your regexp example you don't need the parentheses, it will work the same without them.
If $string contains leading whitespace then you will NOT get the same results. To demonstrate examples
that produce the same results:
my @list = split ' ', $string; my @list = $string =~ /\S+/g;[download]
chromatic (Archbishop) on Dec 29, 2006 at 00:52 UTC
What happens if the delimiter is indicated to be a null string (a string of zero characters)?
perl behaves inconsistently with regard to the "empty" regex:
chromatic has pointed out that split treats an empty pattern
normally, not as a directive to reuse the last successfully matching pattern, as m// and s/// do.
A pattern that split treats specially but m// and s/// treat normally is /^/. Normally, ^ only matches at the beginning of
a string. Given the /m flag, it also matches after newlines in the interior of the string. It's common to want to break a string
up into lines without removing the newlines as splitting on /\n/ would do. One way to do this is @lines = /^(.*\n?)/mg
. Another, perhaps more straightforward, is @lines = split /^/m . Without the /m, the ^ should match only at the beginning
of the string, so the split should return only one element, containing the entire original string. Since this is useless, and
splitting on /^/m instead is common, /^/ silently becomes /^/m.
This only applies to a pattern consisting of just ^; even the apparently equivalent /^(?#)/ or /^ /x are treated normally and
don't split the string at all.
I am trying to extract the Pod documentation from a Perl file. I do not want to convert the
documentation to text as is done by Pod::Simple::Text . I just want
the Pod text as Pod text, such that I can feed it into Pod::Template later. For example:
use warnings;
use strict;
use Pod::Simple::Text;
my $ps=Pod::Simple::Text->new();
my $str;
$ps->output_string( \$str );
$ps->parse_file($0);
print $str;
__END__
=head1 SYNOPSIS
prog [OPTIONS]
This will print the Pod as text. Is there a CPAN module that can give me the Pod text,
that is:
=head1 SYNOPSIS
prog [OPTIONS]
instead?
Update
The solution should be able to handle Pod docs in strings, like
use strict;
use warnings;
use PPI;
# Slurp source code
my $src = do { local ( @ARGV, $/ ) = $0; <> };
# Load a document
my $doc = PPI::Document->new( \$src );
# Find all the pod within the doc
my $pod = $doc->find('PPI::Token::Pod');
for (@$pod) {
print $_->content, "\n";
}
=comment
Hi Pod
=cut
1;
__END__
=head1 SYNOPSIS
prog [OPTIONS]
Outputs:
=comment
Hi Pod
=cut
=head1 SYNOPSIS
prog [OPTIONS]
Thanks for this great solution. It even works with Pod docs embedded in strings, like
my $str='__END__ =head1 SYNOPSIS'; – Håkon
Hægland
Nov 3 '14 at 12:51
Use the -u option for perldoc . This strips out the POD and
displays it raw.
If you want to extract the POD from within a Perl program, you could do something like
this:
my $rawpod;
if (open my $fh, '-|', 'perldoc', '-u', $filename) {
local $/;
my $output = <$fh>;
if (close $fh) {
$rawpod = $output;
}
}
If you really don't want to run perldoc as an executable, you might
be interested that the perldoc executable is a very simple wrapper around
Pod::Perldoc which you might want to consider using yourself.
Well, if you change the question, it's not that surprising that a given answer no longer
works. I'm pleased you've found a solution to your new question. – Tim
Nov 3 '14 at 18:54
"... "Biological data are typically huge. For reasons of efficiency, when dealing with this type of data, you should choose a fast solution over a slower one. Perl's string handling functions ... are measurably faster than regexes ..." ..."
I have a subroutine for a basic one frame translation that is giving me an error for "Use
of uninitialized value $codon in hash element" and "substr outside of string". I think my
problem is I need to modify the subroutine's for loop to account for nucleotide sequences
with odd numbers of acids (i.e. not in multiples of 3).
Does anyone have suggestions for how to modify the code properly?
Here is the subroutine I'm using in a simple example:
my @seqarray = split(//,$seq); ## Explodes the string
for (my $i=0; $i
Re: Translation Substring
Error (updated)
by haukex
(Monsignor) on Nov 09, 2017 at 15:47 UTC
@seqarray and $seqarray are two
different variables, and you never assign anything to $seqarray,
so using substr on it does not
make much sense, I suspect you just want to look directly at $seq
instead of splitting it (BTW, to
get multiple elements out of an array, use
Slices or
splice). Also, note that you
overwrite $amino_acid on every loop iteration. The following
minimal changes make your code work for me:
my $seq = shift; my $amino_acid; for (my $i=0; $i<=length($seq)-3; $i=$i+3) { my
$codon = substr($seq,$i,3); $amino_acid .= $genetic_code{$codon}; } return $amino_acid;[download]
<update2> Fixed an off-by-one error in the above code; I initially
incorrectly translated your $#seqarray-2 into length($seq)-2 (
$#seqarray returns the last index of the array, not its length like
scalar(@seqarray) does, or length does for strings). That's a good
argument against the classic for(;;) and for the two solutions below instead :-)
</update2>
If you output the return value from OneFrameTranslation (your current code is
ignoring the return value), this gives you:
By the way, you can probably move your %genetic_code to the top of your code
(outside of the sub ), so that it only gets initialized once instead of on every
call to the sub , and making its name uppercase is the usual convention to indicate
it is a constant that should not be changed.
Another way to break up a string is using regular expressions, the following also works -
it matches three characters, and then matches again at the position that the previous match
finished, and so on:
my $amino_acid; while ($seq=~/\G(...)/sg) { $amino_acid .=
$genetic_code{$1}; } return $amino_acid;[download]
Or, possibly going a little overboard, here's a technique I describe in Building Regex Alternations Dynamically to
make the replacements using a single regex. I have left out the quotemeta and sort steps only because I know for certain
that all keys are three-character strings without any special characters, if you have any
doubts about the input data, put those steps back in!
# build the regex, this only
needs to be done once my ($genetic_regex) = map qr/$_/, join '|', keys %genetic_code; # apply
the regex (my $amino_acid = $seq) =~ s/($genetic_regex)/$genetic_code{$1}/g; return
$amino_acid;[download]
However, note this produces slightly different output for the first input: " MPVC
" (the leftover C remains unchanged). Whether or not you want this behavior or not
is up to you; it can also be accomplished in the first two solutions (although slightly less
elegantly than with a regex). Update: Also, in the first two solutions you haven't
defined what would happen if a code happens to not be available in the table; the third regex
solution would simply leave it unchanged. Also minor edits for clarification.
Good point. If a nucleotide triplet with an unknown nucleotide appears (ex. ANC instead
of ATC), I'd want to either skip those, or mark them with a letter like 'X'.
I do like the regex solution though, it's quite elegant.
If a nucleotide triplet with an unknown nucleotide appears (ex. ANC instead of ATC),
I'd want to either skip those, or mark them with a letter like 'X'.
In the first two solutions, you can use exists , e.g.:
if ( exists
$genetic_code{$codon} ) { $amino_acid .= $genetic_code{$codon}; } else { $amino_acid .=
$codon; # - OR - $amino_acid .= 'X'; # or something else... }[download]
Update: Or, written more tersely, either $amino_acid .= exists
$genetic_code{$codon} ? $genetic_code{$codon} : 'X'; or $amino_acid .=
$genetic_code{$codon} // 'X'; (the former uses the Conditional Operator , and
the latter uses Logical Defined Or instead of
exists ,
assuming you don't have any undef values in your hash).
I do like the regex solution though, it's quite elegant.
You can combine my second and third suggestions (for nonexistent codes, this uses the
defined-or solution I showed here , the exists solution would work as
well):
The reason for the "substr outside of string" warning is that you assign the $seqarray
variable to the empty string and you never assign it any other value. You are likely getting
confused because you use the same name for two variables (an array and a scalar): $seqarray
is a different variable from @seqarray. If you can specify what you want for output, you will
get more specific help.
use strict; use warnings; my $s1 = 'ATGCCCGTAC'; ## Sequence 1 my $s2 =
'GCTTCCCAGCGC'; ## Sequence 2 print "Sequence 1 Translation:"; my $amino_acid =
OneFrameTranslation ($s1); ## Calls subroutine print "$amino_acid\n"; print "Sequence 2
Translation:"; $amino_acid = OneFrameTranslation ($s2); ## Calls subroutine print
"$amino_acid\n"; ### Subroutine ### sub OneFrameTranslation { my ($seq) = shift; my
$amino_acid=''; my $seqarray=''; my %genetic_code = ( 'TTT' => 'F', 'TTC' => 'F', 'TTA'
=> 'L', 'TTG' => 'L', 'CTT' => 'L', 'CTC' => 'L', 'CTA' => 'L', 'CTG' =>
'L', 'ATT' => 'I', 'ATC' => 'I', 'ATA' => 'I', 'ATG' => 'M', 'GTT' => 'V',
'GTC' => 'V', 'GTA' => 'V', 'GTG' => 'V', 'TCT' => 'S', 'TCC' => 'S', 'TCA'
=> 'S', 'TCG' => 'S', 'CCT' => 'P', 'CCC' => 'P', 'CCA' => 'P', 'CCG' =>
'P', 'ACT' => 'T', 'ACC' => 'T', 'ACA' => 'T', 'ACG' => 'T', 'GCT' => 'A',
'GCC' => 'A', 'GCA' => 'A', 'GCG' => 'A', 'TAT' => 'Y', 'TAC' => 'Y', 'TAA'
=> '*', 'TAG' => '*', 'CAT' => 'H', 'CAC' => 'H', 'CAA' => 'Q', 'CAG' =>
'Q', 'AAT' => 'N', 'AAC' => 'N', 'AAA' => 'K', 'AAG' => 'K', 'GAT' => 'D',
'GAC' => 'D', 'GAA' => 'E', 'GAG' => 'E', 'TGT' => 'C', 'TGC' => 'C', 'TGA'
=> '*', 'TGG' => 'W', 'CGT' => 'R', 'CGC' => 'R', 'CGA' => 'R', 'CGG' =>
'R', 'AGT' => 'S', 'AGC' => 'S', 'AGA' => 'R', 'AGG' => 'R', 'GGT' => 'G',
'GGC' => 'G', 'GGA' => 'G', 'GGG' => 'G' ); ## '---' = 3 character codon in hash
above ## '-' = one letter amino acid abbreviation in hash above my @seqarray =
split(//,$seq); ## Explodes the string for (my $i=0; $i<=$#seqarray-2; $i=$i+3) { my
$codon = substr($seq,$i,3); $amino_acid .= $genetic_code{$codon}; } return ($amino_acid);
}[download]
The main errors in your code is that the $seqarray is never initialized to anything
(note that this is different from @seqarray ) and that you don't use the return
values from your subroutines.
Update:haukex and
toolic were faster than me.
Also note I only made the minimal changes, you don't really need to create @seqarray
, since you're not really using it (except in the $i<=$#seqarray-2 for loop
termination clause where you could simply use the length of the sequence).
This is not addressing the problem you were having, rather it is a suggestion for a
simpler way of initialising your %genetic_code hash that would save some typing. The
glob function can
be used to generate combinations of letters. Your hash contains 64 keys which are all
possible 3-character combinations of A, C, G and T. These can be generated using glob like this
...
Arranging the corresponding amino acid letters in an array allows us to map keys (genetic codes) and values
(amino acids) shift 'ed from the array together to
create the hash lookup.
my %genetic_code = do { my @amino_acids = qw{ K N K N T T T T
R S R S I I M I Q H Q H P P P P R R R R L L L L E D E D A A A A G G G G V V V V * Y * Y S S S
S * C W C L F L F }; map { $_ => shift @amino_acids } glob q{{A,C,G,T}} x 3; };[download]
"Biological data are typically huge. For reasons of efficiency, when dealing with this
type of data, you should choose a fast solution over a slower one. Perl's string handling
functions ... are measurably faster than regexes ..."
Here's a solution that uses the string handling functions length and substr (no regexes are used at
all):
#!/usr/bin/env perl -l use strict; use warnings; my @dna_seqs = qw{ATGCCCGTAC
GCTTCCCAGCGC}; print "$_ => ", dna_prot_map($_) for @dna_seqs; { my %code; BEGIN { %code =
qw{ATG M CCC P GTA V GCT A TCC S CAG Q CGC R} } sub dna_prot_map { join '', map $code{substr
$_[0], $_*3, 3}, 0..length($_[0])/3- 1 } }[download]
My %code is just a subset of your %genetic_code : it only has the data
required for your example sequences. You will still need all the data; you can save yourself
some typing by omitting the 128 single quotes around all the keys.
You can use state within your subroutine (if you're
using Perl version 5.10 or higher); although, be aware that limits the scope. I
often find that when I write code like:
sub f { state $static_var = ... ... do
something with $static_var here ... }[download]
instead of like:
{ my $static_var; BEGIN { $static_var = ... } sub f { ... do
something with $static_var here ... } }[download]
I subsequently find I need to share $static_var with another routine. This
requires a major rewrite which ends up looking very much like the version with BEGIN
:
{ my $static_var; BEGIN { $static_var = ... } sub f { ... do something with
$static_var here ... } sub g { ... do something with $static_var here ... } }[download]
Just having to add ' sub g { ... } ' to existing code is a lot less work and a
lot less error-prone.
How you choose to do it is up to you: I'm only providing advice of possible pitfalls based
on my experience.
I am trying to extract the Pod documentation from a Perl file. I do not want to convert the
documentation to text as is done by Pod::Simple::Text . I just want
the Pod text as Pod text, such that I can feed it into Pod::Template later. For example:
use warnings;
use strict;
use Pod::Simple::Text;
my $ps=Pod::Simple::Text->new();
my $str;
$ps->output_string( \$str );
$ps->parse_file($0);
print $str;
__END__
=head1 SYNOPSIS
prog [OPTIONS]
This will print the Pod as text. Is there a CPAN module that can give me the Pod text,
that is:
=head1 SYNOPSIS
prog [OPTIONS]
instead?
Update
The solution should be able to handle Pod docs in strings, like
use strict;
use warnings;
use PPI;
# Slurp source code
my $src = do { local ( @ARGV, $/ ) = $0; <> };
# Load a document
my $doc = PPI::Document->new( \$src );
# Find all the pod within the doc
my $pod = $doc->find('PPI::Token::Pod');
for (@$pod) {
print $_->content, "\n";
}
=comment
Hi Pod
=cut
1;
__END__
=head1 SYNOPSIS
prog [OPTIONS]
Outputs:
=comment
Hi Pod
=cut
=head1 SYNOPSIS
prog [OPTIONS]
Thanks for this great solution. It even works with Pod docs embedded in strings, like
my $str='__END__ =head1 SYNOPSIS'; – Håkon
Hægland
Nov 3 '14 at 12:51
Use the -u option for perldoc . This strips out the POD and
displays it raw.
If you want to extract the POD from within a Perl program, you could do something like
this:
my $rawpod;
if (open my $fh, '-|', 'perldoc', '-u', $filename) {
local $/;
my $output = <$fh>;
if (close $fh) {
$rawpod = $output;
}
}
If you really don't want to run perldoc as an executable, you might
be interested that the perldoc executable is a very simple wrapper around
Pod::Perldoc which you might want to consider using yourself.
Well, if you change the question, it's not that surprising that a given answer no longer
works. I'm pleased you've found a solution to your new question. – Tim
Nov 3 '14 at 18:54
I have a subroutine for a basic one frame translation that is giving me an error for "Use
of uninitialized value $codon in hash element" and "substr outside of string". I think my
problem is I need to modify the subroutine's for loop to account for nucleotide sequences
with odd numbers of acids (i.e. not in multiples of 3).
Does anyone have suggestions for how to modify the code properly?
Here is the subroutine I'm using in a simple example:
my @seqarray = split(//,$seq); ## Explodes the string
for (my $i=0; $i
Re: Translation Substring
Error (updated)
by haukex
(Monsignor) on Nov 09, 2017 at 15:47 UTC
@seqarray and $seqarray are two
different variables, and you never assign anything to $seqarray,
so using substr on it does not
make much sense, I suspect you just want to look directly at $seq
instead of splitting it (BTW, to
get multiple elements out of an array, use
Slices or
splice). Also, note that you
overwrite $amino_acid on every loop iteration. The following
minimal changes make your code work for me:
my $seq = shift; my $amino_acid; for (my $i=0; $i<=length($seq)-3; $i=$i+3) { my
$codon = substr($seq,$i,3); $amino_acid .= $genetic_code{$codon}; } return $amino_acid;[download]
<update2> Fixed an off-by-one error in the above code; I initially
incorrectly translated your $#seqarray-2 into length($seq)-2 (
$#seqarray returns the last index of the array, not its length like
scalar(@seqarray) does, or length does for strings). That's a good
argument against the classic for(;;) and for the two solutions below instead :-)
</update2>
If you output the return value from OneFrameTranslation (your current code is
ignoring the return value), this gives you:
By the way, you can probably move your %genetic_code to the top of your code
(outside of the sub ), so that it only gets initialized once instead of on every
call to the sub , and making its name uppercase is the usual convention to indicate
it is a constant that should not be changed.
Another way to break up a string is using regular expressions, the following also works -
it matches three characters, and then matches again at the position that the previous match
finished, and so on:
my $amino_acid; while ($seq=~/\G(...)/sg) { $amino_acid .=
$genetic_code{$1}; } return $amino_acid;[download]
Or, possibly going a little overboard, here's a technique I describe in Building Regex Alternations Dynamically to
make the replacements using a single regex. I have left out the quotemeta and sort steps only because I know for certain
that all keys are three-character strings without any special characters, if you have any
doubts about the input data, put those steps back in!
# build the regex, this only
needs to be done once my ($genetic_regex) = map qr/$_/, join '|', keys %genetic_code; # apply
the regex (my $amino_acid = $seq) =~ s/($genetic_regex)/$genetic_code{$1}/g; return
$amino_acid;[download]
However, note this produces slightly different output for the first input: " MPVC
" (the leftover C remains unchanged). Whether or not you want this behavior or not
is up to you; it can also be accomplished in the first two solutions (although slightly less
elegantly than with a regex). Update: Also, in the first two solutions you haven't
defined what would happen if a code happens to not be available in the table; the third regex
solution would simply leave it unchanged. Also minor edits for clarification.
Good point. If a nucleotide triplet with an unknown nucleotide appears (ex. ANC instead
of ATC), I'd want to either skip those, or mark them with a letter like 'X'.
I do like the regex solution though, it's quite elegant.
If a nucleotide triplet with an unknown nucleotide appears (ex. ANC instead of ATC),
I'd want to either skip those, or mark them with a letter like 'X'.
In the first two solutions, you can use exists , e.g.:
if ( exists
$genetic_code{$codon} ) { $amino_acid .= $genetic_code{$codon}; } else { $amino_acid .=
$codon; # - OR - $amino_acid .= 'X'; # or something else... }[download]
Update: Or, written more tersely, either $amino_acid .= exists
$genetic_code{$codon} ? $genetic_code{$codon} : 'X'; or $amino_acid .=
$genetic_code{$codon} // 'X'; (the former uses the Conditional Operator , and
the latter uses Logical Defined Or instead of
exists ,
assuming you don't have any undef values in your hash).
I do like the regex solution though, it's quite elegant.
You can combine my second and third suggestions (for nonexistent codes, this uses the
defined-or solution I showed here , the exists solution would work as
well):
The reason for the "substr outside of string" warning is that you assign the $seqarray
variable to the empty string and you never assign it any other value. You are likely getting
confused because you use the same name for two variables (an array and a scalar): $seqarray
is a different variable from @seqarray. If you can specify what you want for output, you will
get more specific help.
This is what I'm getting with your program modified as in my earlier post below: $ perl
dna.pl Sequence 1 Translation:MPV Sequence 2 Translation:ASQR[download]
use strict; use warnings; my $s1 = 'ATGCCCGTAC'; ## Sequence 1 my $s2 =
'GCTTCCCAGCGC'; ## Sequence 2 print "Sequence 1 Translation:"; my $amino_acid =
OneFrameTranslation ($s1); ## Calls subroutine print "$amino_acid\n"; print "Sequence 2
Translation:"; $amino_acid = OneFrameTranslation ($s2); ## Calls subroutine print
"$amino_acid\n"; ### Subroutine ### sub OneFrameTranslation { my ($seq) = shift; my
$amino_acid=''; my $seqarray=''; my %genetic_code = ( 'TTT' => 'F', 'TTC' => 'F', 'TTA'
=> 'L', 'TTG' => 'L', 'CTT' => 'L', 'CTC' => 'L', 'CTA' => 'L', 'CTG' =>
'L', 'ATT' => 'I', 'ATC' => 'I', 'ATA' => 'I', 'ATG' => 'M', 'GTT' => 'V',
'GTC' => 'V', 'GTA' => 'V', 'GTG' => 'V', 'TCT' => 'S', 'TCC' => 'S', 'TCA'
=> 'S', 'TCG' => 'S', 'CCT' => 'P', 'CCC' => 'P', 'CCA' => 'P', 'CCG' =>
'P', 'ACT' => 'T', 'ACC' => 'T', 'ACA' => 'T', 'ACG' => 'T', 'GCT' => 'A',
'GCC' => 'A', 'GCA' => 'A', 'GCG' => 'A', 'TAT' => 'Y', 'TAC' => 'Y', 'TAA'
=> '*', 'TAG' => '*', 'CAT' => 'H', 'CAC' => 'H', 'CAA' => 'Q', 'CAG' =>
'Q', 'AAT' => 'N', 'AAC' => 'N', 'AAA' => 'K', 'AAG' => 'K', 'GAT' => 'D',
'GAC' => 'D', 'GAA' => 'E', 'GAG' => 'E', 'TGT' => 'C', 'TGC' => 'C', 'TGA'
=> '*', 'TGG' => 'W', 'CGT' => 'R', 'CGC' => 'R', 'CGA' => 'R', 'CGG' =>
'R', 'AGT' => 'S', 'AGC' => 'S', 'AGA' => 'R', 'AGG' => 'R', 'GGT' => 'G',
'GGC' => 'G', 'GGA' => 'G', 'GGG' => 'G' ); ## '---' = 3 character codon in hash
above ## '-' = one letter amino acid abbreviation in hash above my @seqarray =
split(//,$seq); ## Explodes the string for (my $i=0; $i<=$#seqarray-2; $i=$i+3) { my
$codon = substr($seq,$i,3); $amino_acid .= $genetic_code{$codon}; } return ($amino_acid);
}[download]
The main errors in your code is that the $seqarray is never initialized to anything
(note that this is different from @seqarray ) and that you don't use the return
values from your subroutines.
Update:haukex and
toolic were faster than me.
Also note I only made the minimal changes, you don't really need to create @seqarray
, since you're not really using it (except in the $i<=$#seqarray-2 for loop
termination clause where you could simply use the length of the sequence).
This is not addressing the problem you were having, rather it is a suggestion for a
simpler way of initialising your %genetic_code hash that would save some typing. The
glob function can
be used to generate combinations of letters. Your hash contains 64 keys which are all
possible 3-character combinations of A, C, G and T. These can be generated using glob like this
...
Arranging the corresponding amino acid letters in an array allows us to map keys (genetic codes) and values
(amino acids) shift 'ed from the array together to
create the hash lookup.
my %genetic_code = do { my @amino_acids = qw{ K N K N T T T T
R S R S I I M I Q H Q H P P P P R R R R L L L L E D E D A A A A G G G G V V V V * Y * Y S S S
S * C W C L F L F }; map { $_ => shift @amino_acids } glob q{{A,C,G,T}} x 3; };[download]
"Biological data are typically huge. For reasons of efficiency, when dealing with this
type of data, you should choose a fast solution over a slower one. Perl's string handling
functions ... are measurably faster than regexes ..."
Here's a solution that uses the string handling functions length and substr (no regexes are used at
all):
#!/usr/bin/env perl -l use strict; use warnings; my @dna_seqs = qw{ATGCCCGTAC
GCTTCCCAGCGC}; print "$_ => ", dna_prot_map($_) for @dna_seqs; { my %code; BEGIN { %code =
qw{ATG M CCC P GTA V GCT A TCC S CAG Q CGC R} } sub dna_prot_map { join '', map $code{substr
$_[0], $_*3, 3}, 0..length($_[0])/3- 1 } }[download]
My %code is just a subset of your %genetic_code : it only has the data
required for your example sequences. You will still need all the data; you can save yourself
some typing by omitting the 128 single quotes around all the keys.
You can use state within your subroutine (if you're
using Perl version 5.10 or higher); although, be aware that limits the scope. I
often find that when I write code like:
sub f { state $static_var = ... ... do
something with $static_var here ... }[download]
instead of like:
{ my $static_var; BEGIN { $static_var = ... } sub f { ... do
something with $static_var here ... } }[download]
I subsequently find I need to share $static_var with another routine. This
requires a major rewrite which ends up looking very much like the version with BEGIN
:
{ my $static_var; BEGIN { $static_var = ... } sub f { ... do something with
$static_var here ... } sub g { ... do something with $static_var here ... } }[download]
Just having to add ' sub g { ... } ' to existing code is a lot less work and a
lot less error-prone.
How you choose to do it is up to you: I'm only providing advice of possible pitfalls based
on my experience.
#!/usr/local/bin/perl use strict;
foreach my $name ('A', 'B') { my $res = 'Init' if (0); if (defined ($res)) { print "$name: res
= $res\n"; } else { print "$name: res is undef\n" } $res = 'Post'; }[download]
Result:
A: res is undef
B: res = Post
As $res is under lexical variable scope, shouldn't it disappear at the bottom of the
block
and be recreated by the second pass, producing an identical result?
Bug? Feature? Saving CPU?
perl -v
This is perl, v5.10.1 (*) built for x86_64-linux-thread-multi
NOTE: The behaviour of a my , state , or our modified
with a statement modifier conditional or loop construct (for example, my $x if ...
) is undefined . The value of the my variable may be undef , any
previously assigned value, or possibly anything else. Don't rely on it. Future versions of
perl might do something different from the version of perl you try it out on. Here be
dragons.
Update: Heh, Eily and
I posted within 4 seconds of another ;-)
BEGIN { my $static_val = 0; sub gimme_another { return
++$static_val; } } # - OR - in Perl >=5.10: use feature 'state'; sub gimme_another { state
$static_val = 0; return ++$static_val; }[download]
$ perl -e 'my $x if
0' Deprecated use of my() in false conditional. This will be a fatal erro r in Perl 5.30 at
-e line 1.[download]
Update 3: Apparently, the warning " Deprecated use of my() in false
conditional " first showed up in Perl 5.10 and became a default warning in 5.12. Note
that your Perl 5.10.1 is now more than eight years old, and you should upgrade. Also, you
should generally use warnings; ( Use strict and warnings ).
NOTE: The behaviour of a my, state, or our modified with a statement modifier conditional
or loop construct (for example, my $x if ... ) is undefined. The value of the my variable
may be undef, any previously assigned value, or possibly anything else. Don't rely on it.
Future versions of perl might do something different from the version of perl you try it
out on. Here be dragons.
So neither bug nor feature, third option.
AnomalousMonk
(Chancellor) on Nov 10, 2017 at 17:07 UTC
I'm new to Perl programming. I've noticed that every time I want to declare a new variable, I
should use the my keyword before that variable if strict and
warnings are on (which I was told to do, for reasons also I do not know.)
So how to declare a variable in perl without using my and without getting
warnings?
My question is: Is it possible to declare a variable without using my and
without omitting the use strict; and use warnings; and without
getting warnings at all?
It seems to me that many of the questions in the Perl tag could be solved if people would
use:
use strict;
use warnings;
I think some people consider these to be akin to training wheels, or unnecessary
complications, which is clearly not true, since even very skilled Perl programmers use
them.
It seems as though most people who are proficient in Perl always use these two pragmas,
whereas those who would benefit most from using them seldom do. So, I thought it would be a
good idea to have a question to link to when encouraging people to use strict
and warnings .
So, why should a Perl developer use strict and warnings ?
I always wonder for stuff like this why they don't just make it the default and have the dev
actually have to actively loosen stuff, where is the use loose; –
Paul Tyng
Nov 5 '11 at 23:08
Like many cool and useful things Perl started as a hack, as a tool for the guy who invents
it. Later it became more popular and an increasing number of unskilled people started using
it. This is when you start thinking something like use strict was a good idea
but backwards compatibility has already become a real problem to you:-( – Daniel
Böhmer
Nov 5 '11 at 23:15
@JB Nizet, @Paul T., Actually, use strict; is on by default when you request the
Perl 5.12 (or higher) language. Try perl -e"use v5.012; $x=123;" . no
strict; actually turns it off. – ikegami
Nov 6 '11 at 0:04
Though in the end your point is true, the more times we say it, maybe the more people will
hear. There has been some rumbling lately of trying to make more/better/modern Perl tutorials
available and certainly strict/warnings will be on the top of each of these. For mine I plan
to have s/w on the top of every snippet, just so that all newbies see it every time –
Joel Berger
Nov 6 '11 at 3:05
@JoelBerger No, actually it is nothing like it. Just like I said, it only has similar words
in the title. It's for backwards compatibility. is the first sentence in the accepted
answer, how do you propose that applies to my question? – TLP
Nov 6 '11 at 5:04
For starters, it helps find typos in variable names. Even experienced programmers make such
errors. A common case is forgetting to rename an instance of a variable when cleaning up or
refactoring code.
The pragmas catch many errors sooner than they would be caught otherwise, which makes it
easier to find the root causes of the errors. The root cause might be the need for an error
or validation check, and that can happen regardless or programmer skill.
What's good about Perl warnings is that they are rarely spurious, so there's next to no
cost to using them.
@TLP, I'm not about to make a study to quantify how much it helps. It should suffice to say
that they help unconditionally. – ikegami
Nov 6 '11 at 19:42
Why is it made optional then if it has so many benefits ? Why not enable it by default (like
someone commented above) ? Is it for compatibility reasons ? – Jean
Sep 26 '13 at 16:11
@Jean, backwards compatibility. Note that use strict; is enabled by default if
you use version 5.12 or newer of the language ( use 5.012; ). – ikegami
Sep 26 '13 at 16:34
@Jean if you are writing a simple script you really don't want to get alerted by warnings
about file handler names or for not declaring the variable before using them :-) –
user2676847
Aug 17 '14 at 8:51
Apparently use strict should(must) be used when you want to force perl to code
properly which could be forcing declaration, being explicit on strings and subs i.e.
barewords or using refs with caution. Note: if there are errors use strict will abort the
execution if used.
While use warnings; will help you find typing mistakes in program like you
missed a semicolon, you used 'elseif' and not 'elsif', you are using deprecated syntax or
function, whatever like that. Note: use warnings will only provide warnings and continue
execution i.e. wont abort the execution..
Anyway, It would be better if we go into details, which I am specifiying below
which means that you must always declare variables before you use them.
If you don't declare you will probably get error message for the undeclared variable
Global symbol "$variablename" requires explicit package name at scriptname.pl line 3
This warning mean Perl is not exactly clear about what the scope of variable is. So you
need to be explicit about your variables, which means either declaring them with
my so they are restricted to the current block, or referring to them with their
fully qualified name (for ex: $MAIN::variablename).
So, a compile-time error is triggered if you attempt to access a variable that hasn't met
at least one of the following criteria:
Predefined by Perl itself, such as @ARGV, %ENV, and all the global punctuation
variables such as $. or $_.
Declared with our (for a global) or my (for a lexical).
Imported from another package. (The use vars pragma fakes up an import, but use our
instead.)
Fully qualified using its package name and the double-colon package separator.
use strict 'subs';
Consider two programs
# prog 1
$a = test_value;
print "First program: ", $a, "\n";
sub test_value { return "test passed"; }
Output: First program's result: test_value
# prog 2
sub test_value { return "test passed"; }
$a = test_value;
print "Second program: ", $a, "\n";
Output: Second program's result: test passed
In both cases we have a test_value() sub and we want to put its result into $a. And yet,
when we run the two programs, we get two different results:
In the first program, at the point we get to $a = test_value; , Perl doesn't
know of any test_value() sub, and test_value is interpreted as string 'test_value'. In the
second program, the definition of test_value() comes before the $a = test_value;
line. Perl thinks test_value as sub call.
The technical term for isolated words like test_value that might be subs and might be
strings depending on context, by the way, is bareword . Perl's handling of
barewords can be
confusing, and it can cause bug in program.
The bug is what we encountered in our first program, Remember that Perl won't look forward
to find test_value() , so since it hasn't already seen test_value(), it assumes
that you want a string. So if you use strict subs; , it will cause this program
to die with an error:
Bareword "test_value" not allowed while "strict subs" in use at ./a6-strictsubs.pl line
3.
Solution to this error would be
1. Use parentheses to make it clear you're calling a sub. If Perl sees $a =
test_value();,
2. Declare your sub before you first use it
use strict;
sub test_value; # Declares that there's a test_value() coming later ...
my $a = test_value; # ...so Perl will know this line is okay.
.......
sub test_value { return "test_passed"; }
3. And If you mean to use it as a string, quote it.
So, This stricture makes Perl treat all barewords as syntax errors. *A bareword
is any bare name or identifier that has no other interpretation forced by context. (Context
is often forced by a nearby keyword or token, or by predeclaration of the word in question.)*
So If you mean to use it as a string, quote it and If you mean to use it as a function call,
predeclare it or use parentheses.
Barewords are dangerous because of this unpredictable behavior. use strict; (or use
strict 'subs';) makes them predictable, because barewords that might cause strange
behavior in the future will make your program die before they can wreak havoc
There's one place where it's OK to use barewords even when you've turned on strict subs:
when you are assigning hash keys.
$hash{sample} = 6; # Same as $hash{'sample'} = 6
%other_hash = ( pie => 'apple' );
Barewords in hash keys are always interpreted as strings, so there is no ambiguity.
use strict 'refs';
This generates a run-time error if you use symbolic references, intentionally or
otherwise. A value that is not a hard reference is then treated as a symbolic
reference . That is, the reference is interpreted as a string representing the name of a
global variable.
use strict 'refs';
$ref = \$foo; # Store "real" (hard) reference.
print $$ref; # Dereferencing is ok.
$ref = "foo"; # Store name of global (package) variable.
print $$ref; # WRONG, run-time error under strict refs.
use warnings;
This lexically scoped pragma permits flexible control over Perl's built-in warnings, both
those emitted by the compiler as well as those from the run-time system.
So The majority of warning messages from the classifications below i.e. W, D & S can
be controlled using the warnings pragma.
(W) A warning (optional)
(D) A deprecation (enabled by default)
(S) A severe warning (enabled by default)
I have listed some of warnings messages those occurs often below by classifications. For
detailed info on them and others messages refer perldiag
(W) A warning (optional):
Missing argument in %s
Missing argument to -%c
(Did you mean &%s instead?)
(Did you mean "local" instead of "our"?)
(Did you mean $ or @ instead of %?)
'%s' is not a code reference
length() used on %s
Misplaced _ in number
(D) A deprecation (enabled by default):
defined(@array) is deprecated
defined(%hash) is deprecated
Deprecated use of my() in false conditional
$# is no longer supported
(S) A severe warning (enabled by default)
elseif should be elsif
%s found where operator expected
(Missing operator before %s?)
(Missing semicolon on previous line?)
%s never introduced
Operator or semicolon missing before %s
Precedence problem: open %s should be open(%s)
Prototype mismatch: %s vs %s
Warning: Use of "%s" without parentheses is ambiguous
Can't open %s: %s
Actually, you have to delay the FATAL => "all" till runtime, by assigning to
$SIG{__WARN__} = sub { croak "fatalized warning @_" }; or else you screw up the
compiler trying to tell you what it needs to. – tchrist
Nov 5 '11 at 23:52
@tchrist: This has always worked for me as-is and as documented. If you have found a case
where it doesn't work as documented, please patch the documentation using
perlbug . – toolic
Nov 6 '11 at 0:06
Use will export functions and variable names to the main namespace by calling modules
import() function.
A pragma is a module which influences some aspect of the compile time or run time
behavior of perl.Pragmas give hints to the compiler.
Use warnings - perl complaints about variables used only once,improper conversions of
strings into numbers,.Trying to write to files that are not opened .it happens at compile
time.It is used to control warnings.
Use strict - declare variables scope. It is used to set some kind of discipline in the
script.If barewords are used in the code they are interpreted.All the variables should be
given scope ,like my,our or local.
The "use strict" directive tells Perl to do extra checking during the compilation of your
code. Using this directive will save you time debugging your Perl code because it finds
common coding bugs that you might overlook otherwise.
strict and warnings are the mode for the perl program,and it is allowing the user to enter
the code more liberally and more than that,that perl code will be look formal and its coding
standard will be effective.
warnings means same like "-w" in the perl shabang line,so it will provide you the warnings
generated by the perl program,it will display inthe terminal
checklist
of tips and techniques to get you started.
This list is meant for debugging some of the most common Perl programming problems; it
assumes no prior working experience with the Perl debugger ( perldebtut ). Think of it as a First Aid kit,
rather than a fully-staffed state-of-the-art operating room.
These tips are meant to act as a guide to help you answer the following questions:
Are you sure your data is what you think it is?
Are you sure your code is what you think it is?
Are you inadvertently ignoring error and warning messages?
Display the contents of variables using print or warnwarn "$var\n"; print "@things\n";
# array with spaces between elements[download]
Check for unexpected whitespace
chomp , then print with
delimiters of your choice, such as colons or balanced brackets, for visibility chomp
$var; print ">>>$var<<<\n";[download]
Check for unprintable characters by converting them into their ASCII hex codes using
ordmy $copy = $str; $copy =~ s/([^\x20-\x7E])/sprintf '\x{%02x}', ord $1/eg; print
":$copy:\n";[download]
Dump arrays,
hashes and arbitrarily complex data structures. You can get started using the core module
Data::Dumper . Should
the output prove to be unsuitable to you, other alternatives can be downloaded from CPAN,
such as Data::Dump , YAML , or JSON . See also How can I visualize my complex data structure?use Data::Dumper; print Dumper(\%hash); print Dumper($ref);[download]
If you were expecting a ref erence, make sure it is the
right kind (ARRAY, HASH, etc.) print ref $ref, "\n";[download]
Check to see if your code is what you thought it was: B::Deparse$ perl -MO=Deparse -p
program.pl[download]
Check the return ( error ) status of your commands
open with $!open my $fh,
'<', 'foo.txt' or die "can not open foo.txt: $!";[download]
system and backticks (
qx )
with $?if (system $cmd) {
print "Error: $? for command $cmd" } else { print "Command $cmd is OK" } $out = `$cmd`;
print $? if $?;[download]
Demystify regular expressions by installing and using the CPAN module YAPE::Regex::Explain# what the heck does /^\s+$/ mean? use YAPE::Regex::Explain; print
YAPE::Regex::Explain->new('/^\s+$/')->explain();[download]
Neaten up your code by installing and using the CPAN script perltidy . Poor indentation can often obscure
problems.
Checklist for debugging when using CPAN modules:
Check the Bug List by following the module's "View Bugs" link.
Is your installed version the latest version? If not, check the change log by
following the "Changes" link. Also follow the "Other Tools" link to "Diff" and "Grep" the
release.
If a module provides status methods, check them in your code as you would check
return status of built-in functions: use WWW::Mechanize; if ($mech->success()) {
... }[download]
What's next? If you are not already doing so, use an editor that understands Perl syntax
(such as vim or emacs), a GUI debugger (such as Devel::ptkdb ) or use a full-blown IDE.
Lastly, use a version control system so that you can fearlessly make these temporary hacks to
your code without trashing the real thing.
Damned decent posting :D ... just a couple of suggestions tho'...
Step 5 - Use a stringified ref. to provide straightforward visual comparison of 2, or
more, ref.s - I've recently been using this to verify that a ref. in 2 different places is
actually the same object.
Step 7 - add use autodie; to provide default exception throwing on
failure
Step 7 & 8 - add use CGI::Carp; for CGI/WWW scripts
Your final observation WRT IDEs etc. could, IMHO, suggest that the use of Eclipse, for
perl dev't, isn't for the fainthearted...
When debugging warnings from the perl core like Use of uninitialized value ... let
the debugger pause right there. Then have a good look at the context that led to this
situation and investigate variables and the callstack.
To let the debugger do this automatically I use a debugger customization script:
sub afterinit
{
$::SIG{'__WARN__'} = sub {
my $warning = shift;
if ( $warning =~ m{\s at \s \S+ \s line \s \d+ \. $}xms ) {
$DB::single = 1; # debugger stops here automatically
}
warn $warning;
};
print "sigwarn handler installed!\n";
return;
}
Save the content to file .perldb (or perldb.ini on Windows) and place it in
the current or in your HOME directory.
The subroutine will be called initially by the debugger and installs a signal handler for
all warnings. If the format matches one from the perl core, execution in the debugger is
paused by setting $DB::single = 1 .
If you don't quite understand what you're looking at (output of deparse, perl syntax),
then ppi_dumper can help
you look at the right part of the manual, an example
Regular expressions are used to match delimiters with the split function, to break up strings into a
list of substrings. The join function is in some ways the inverse of
split. It takes a list of strings and joins them together again, optionally, with a delimiter.
We'll discuss split first, and then move on to join.
A simple example...
Let's first consider a simple use of split: split a string on whitespace.
$line =
"Bart Lisa Maggie Marge Homer"; @simpsons = split ( /\s/, $line ); # Splits line and uses
single whitespaces # as the delimiter.[download]
@simpsons now contains "Bart", "", "Lisa", "Maggie", "Marge", and "Homer".
There is an empty element in the list that split placed in @simpsons . That is
because \s matched exactly one whitespace character. But in our string, $line
, there were two spaces between Bart and Lisa. Split, using single whitespaces as delimiters,
created an empty string at the point where two whitespaces were found next to each other. That
also includes preceding whitespace. In fact, empty delimiters found anywhere in the string will
result in empty strings being returned as part of the list of strings.
We can specify a more
flexible delimiter that eliminates the creation of an empty string in the list. @simpsons =
split ( /\s+/, $line ); #Now splits on one-or-more whitespaces.[download]
@simpsons now contains "Bart", "Lisa", "Maggie", "Marge", and "Homer", because the
delimiter match is seen as one or more whitespaces, multiple whitespaces next to each other are
consumed as one delimiter.
Where do delimiters go?
"What does split do with the delimiters?" Usually it discards them, returning only what is
found to either side of the delimiters (including empty strings if two delimiters are next to
each other, as seen in our first example). Let's examine that point in the following
example:
The delimiter is something visible: 'humility'. And after this code executes, @japh
contains four strings, "Just ", "another ", "Perl ", and "hacker.". 'humility' bit the
bit-bucket, and was tossed aside.
Preserving delimiters
If you want to keep the delimiters you can. Here's an example of how. Hint, you use
capturing parenthesis.
@list now contains "alpha","-", "bravo","-", "charlie", and so on. The parenthesis
caused the delimiters to be captured into the list passed to @list right alongside the stuff
between the delimiters.
The null delimiter
What happens if the delimiter is indicated to be a null string (a string of zero
characters)? Let's find out.
Now @letters contains a list of four letters, "M", "o", "n", and "k". If split is
given a null string as a delimiter, it splits on each null position in the string, or in other
words, every character boundary. The effect is that the split returns a list broken into
individual characters of $string .
Split's return value
Earlier I mentioned that split returns a list. That list, of course, can be stored in an
array, and often is. But another use of split is to store its return values in a list of
scalars. Take the following code:
@mydata = ( "Simpson:Homer:1-800-000-0000:40:M",
"Simpson:Marge:1-800-111-1111:38:F", "Simpson:Bart:1-800-222-2222:11:M",
"Simpson:Lisa:1-800-333-3333:9:F", "Simpson:Maggie:1-800-444-4444:2:F" ); foreach ( @mydata ) {
( $last, $first, $phone, $age ) = split ( /:/ ); print "You may call $age year old $first $last
at $phone.\n"; }[download]
What happened to the person's sex? It's just discarded because we're only accepting four of
the five fields into our list of scalars. And how does split know what string to split up? When
split isn't explicitly given a string to split up, it assumes you want to split the contents of
$_ . That's handy, because foreach aliases $_ to each element (one at a time)
of @mydata .
Words about Context
Put to its normal use, split is used in list context. It may also be used in scalar context,
though its use in scalar context is deprecated. In scalar context, split returns the number of
fields found, and splits into the @_ array. It's easy to see why that might not be desirable,
and thus, why using split in scalar context is frowned upon.
The limit argument
Split can optionally take a third argument. If you specify a third argument to split, as in
@list = split ( /\s+/, $string, 3 ); split returns no more than the number of fields
you specify in the third argument. So if you combine that with our previous
example.....
Now, $everything_else contains Bart's phone number, his age, and his sex, delimited
by ":", because we told split to stop early. If you specify a negative limit value, split
understands that as being the same as an arbitrarily large limit.
Unspecified split
pattern
As mentioned before, limit is an optional parameter. If you leave limit off, you
may also, optionally, choose to not specify the split string. Leaving out the split string
causes split to attempt to split the string contained in $_. And if you leave off the split
string (and limit), you may also choose to not specify a delimiter pattern.
If you leave off the pattern, split assumes you want to split on /\s+/ . Not
specifying a pattern also causes split to skip leading whitespace. It then splits on any
whitespace field (of one or more whitespaces), and skips past any trailing whitespace. One
special case is when you specify the string literal, " " (a quoted space), which does the same
thing as specifying no delimiter at all (no argument).
The star quantifier (zero or
more)
Finally, consider what happens if we specify a split delimiter of /\s*/ . The
quantifier "*" means zero or more of the item it is quantifying. So this split can split on
nothing (character boundaries), any amount of whitespace. And remember, delimiters get thrown
away. See this in action:
@letters now contains "H", "e", "l", "l", "o", "w", "o", "r", "l", "d", and
"!".
Notice that the whitespace is gone. You just split $string , character by character
(because null matches boundaries), and on whitespace (which gets discarded because
it's a delimiter).
Using split versus Regular Expressions
There are cases where it is equally easy to use a regexp in list context to split a string
as it is to use the split function. Consider the following examples:
my @list = split
/\s+/, $string; my @list = $string =~ /(\S+)/g;[download]
In the first example you're defining what to throw away. In the second, you're defining what
to keep. But you're getting the same results. That is a case where it's equally easy to use
either syntax.
But what if you need to be more specific as to what you keep, and perhaps are a little less
concerned with what comes between what you're keeping? That's a situation where a regexp is
probably a better choice. See the following example:
my @bignumbers = $string =~
/(\d{4,})/g;[download]
That type of a match would be difficult to accomplish with split. Try not to fall into the
pitfall of using one where the other would be handier. In general, if you know what you want to
keep, use a regexp. If you know what you want to get rid of, use split. That's an
oversimplification, but start there and if you start tearing your hair out over the code,
consider taking another approach. There is always more than one way to do it
.
Regular expressions are used to match delimiters with the split function, to break up strings into a
list of substrings. The join function is in some ways the inverse of
split. It takes a list of strings and joins them together again, optionally, with a delimiter.
We'll discuss split first, and then move on to join.
A simple example...
Let's first consider a simple use of split: split a string on whitespace.
$line =
"Bart Lisa Maggie Marge Homer"; @simpsons = split ( /\s/, $line ); # Splits line and uses
single whitespaces # as the delimiter.[download]
@simpsons now contains "Bart", "", "Lisa", "Maggie", "Marge", and "Homer".
There is an empty element in the list that split placed in @simpsons . That is
because \s matched exactly one whitespace character. But in our string, $line
, there were two spaces between Bart and Lisa. Split, using single whitespaces as delimiters,
created an empty string at the point where two whitespaces were found next to each other. That
also includes preceding whitespace. In fact, empty delimiters found anywhere in the string will
result in empty strings being returned as part of the list of strings.
We can specify a more
flexible delimiter that eliminates the creation of an empty string in the list. @simpsons =
split ( /\s+/, $line ); #Now splits on one-or-more whitespaces.[download]
@simpsons now contains "Bart", "Lisa", "Maggie", "Marge", and "Homer", because the
delimiter match is seen as one or more whitespaces, multiple whitespaces next to each other are
consumed as one delimiter.
Where do delimiters go?
"What does split do with the delimiters?" Usually it discards them, returning only what is
found to either side of the delimiters (including empty strings if two delimiters are next to
each other, as seen in our first example). Let's examine that point in the following
example:
The delimiter is something visible: 'humility'. And after this code executes, @japh
contains four strings, "Just ", "another ", "Perl ", and "hacker.". 'humility' bit the
bit-bucket, and was tossed aside.
Preserving delimiters
If you want to keep the delimiters you can. Here's an example of how. Hint, you use
capturing parenthesis.
@list now contains "alpha","-", "bravo","-", "charlie", and so on. The parenthesis
caused the delimiters to be captured into the list passed to @list right alongside the stuff
between the delimiters.
The null delimiter
What happens if the delimiter is indicated to be a null string (a string of zero
characters)? Let's find out.
Now @letters contains a list of four letters, "M", "o", "n", and "k". If split is
given a null string as a delimiter, it splits on each null position in the string, or in other
words, every character boundary. The effect is that the split returns a list broken into
individual characters of $string .
Split's return value
Earlier I mentioned that split returns a list. That list, of course, can be stored in an
array, and often is. But another use of split is to store its return values in a list of
scalars. Take the following code:
@mydata = ( "Simpson:Homer:1-800-000-0000:40:M",
"Simpson:Marge:1-800-111-1111:38:F", "Simpson:Bart:1-800-222-2222:11:M",
"Simpson:Lisa:1-800-333-3333:9:F", "Simpson:Maggie:1-800-444-4444:2:F" ); foreach ( @mydata ) {
( $last, $first, $phone, $age ) = split ( /:/ ); print "You may call $age year old $first $last
at $phone.\n"; }[download]
What happened to the person's sex? It's just discarded because we're only accepting four of
the five fields into our list of scalars. And how does split know what string to split up? When
split isn't explicitly given a string to split up, it assumes you want to split the contents of
$_ . That's handy, because foreach aliases $_ to each element (one at a time)
of @mydata .
Words about Context
Put to its normal use, split is used in list context. It may also be used in scalar context,
though its use in scalar context is deprecated. In scalar context, split returns the number of
fields found, and splits into the @_ array. It's easy to see why that might not be desirable,
and thus, why using split in scalar context is frowned upon.
The limit argument
Split can optionally take a third argument. If you specify a third argument to split, as in
@list = split ( /\s+/, $string, 3 ); split returns no more than the number of fields
you specify in the third argument. So if you combine that with our previous
example.....
Now, $everything_else contains Bart's phone number, his age, and his sex, delimited
by ":", because we told split to stop early. If you specify a negative limit value, split
understands that as being the same as an arbitrarily large limit.
Unspecified split
pattern
As mentioned before, limit is an optional parameter. If you leave limit off, you
may also, optionally, choose to not specify the split string. Leaving out the split string
causes split to attempt to split the string contained in $_. And if you leave off the split
string (and limit), you may also choose to not specify a delimiter pattern.
If you leave off the pattern, split assumes you want to split on /\s+/ . Not
specifying a pattern also causes split to skip leading whitespace. It then splits on any
whitespace field (of one or more whitespaces), and skips past any trailing whitespace. One
special case is when you specify the string literal, " " (a quoted space), which does the same
thing as specifying no delimiter at all (no argument).
The star quantifier (zero or
more)
Finally, consider what happens if we specify a split delimiter of /\s*/ . The
quantifier "*" means zero or more of the item it is quantifying. So this split can split on
nothing (character boundaries), any amount of whitespace. And remember, delimiters get thrown
away. See this in action:
@letters now contains "H", "e", "l", "l", "o", "w", "o", "r", "l", "d", and
"!".
Notice that the whitespace is gone. You just split $string , character by character
(because null matches boundaries), and on whitespace (which gets discarded because
it's a delimiter).
Using split versus Regular Expressions
There are cases where it is equally easy to use a regexp in list context to split a string
as it is to use the split function. Consider the following examples:
my @list = split
/\s+/, $string; my @list = $string =~ /(\S+)/g;[download]
In the first example you're defining what to throw away. In the second, you're defining what
to keep. But you're getting the same results. That is a case where it's equally easy to use
either syntax.
But what if you need to be more specific as to what you keep, and perhaps are a little less
concerned with what comes between what you're keeping? That's a situation where a regexp is
probably a better choice. See the following example:
my @bignumbers = $string =~
/(\d{4,})/g;[download]
That type of a match would be difficult to accomplish with split. Try not to fall into the
pitfall of using one where the other would be handier. In general, if you know what you want to
keep, use a regexp. If you know what you want to get rid of, use split. That's an
oversimplification, but start there and if you start tearing your hair out over the code,
consider taking another approach. There is always more than one way to do it
.
I currently use the following Perl to check if a variable is defined and contains text. I
have to check defined first to avoid an 'uninitialized value' warning:
if (defined $name && length $name > 0) {
# do something with $name
}
Is there a better (presumably more concise) way to write this?
You often see the check for definedness so you don't have to deal with the warning for using
an undef value (and in Perl 5.10 it tells you the offending variable):
Use of uninitialized value $name in ...
So, to get around this warning, people come up with all sorts of code, and that code
starts to look like an important part of the solution rather than the bubble gum and duct
tape that it is. Sometimes, it's better to show what you are doing by explicitly turning off
the warning that you are trying to avoid:
In other cases, use some sort of null value instead of the data. With
Perl 5.10's defined-or operator , you can give length an explicit empty
string (defined, and give back zero length) instead of the variable that will trigger the
warning:
use 5.010;
if( length( $name // '' ) ) {
...
}
In Perl 5.12, it's a bit easier because
length on an undefined value also returns undefined . That might seem like a bit
of silliness, but that pleases the mathematician I might have wanted to be. That doesn't
issue a warning, which is the reason this question exists.
use 5.012;
use warnings;
my $name;
if( length $name ) { # no warning
...
}
Also, in v5.12 and later, length undef returns undef, instead of warning and
returning 0. In boolean context, undef is just as false as 0, so if you're targeting v5.12 or
later, you can just write if (length $name) { ... } – rjbs
Jul 9 '14 at 17:14
As mobrule indicates, you could use the following instead for a small savings:
if (defined $name && $name ne '') {
# do something with $name
}
You could ditch the defined check and get something even shorter, e.g.:
if ($name ne '') {
# do something with $name
}
But in the case where $name is not defined, although the logic flow will work
just as intended, if you are using warnings (and you should be), then you'll get
the following admonishment:
Use of uninitialized value in string ne
So, if there's a chance that $name might not be defined, you really do need
to check for definedness first and foremost in order to avoid that warning. As Sinan
Ünür points out, you can use Scalar::MoreUtils to get code that does
exactly that (checks for definedness, then checks for zero length) out of the box, via the
empty() method:
use Scalar::MoreUtils qw(empty);
if(not empty($name)) {
# do something with $name
}
First, since length always returns a non-negative number,
if ( length $name )
and
if ( length $name > 0 )
are equivalent.
If you are OK with replacing an undefined value with an empty string, you can use Perl
5.10's //= operator which assigns the RHS to the LHS unless the LHS is
defined:
#!/usr/bin/perl
use feature qw( say );
use strict; use warnings;
my $name;
say 'nonempty' if length($name //= '');
say "'$name'";
Note the absence of warnings about an uninitialized variable as $name is
assigned the empty string if it is undefined.
However, if you do not want to depend on 5.10 being installed, use the functions provided
by Scalar::MoreUtils . For example, the
above can be written as:
#!/usr/bin/perl
use strict; use warnings;
use Scalar::MoreUtils qw( define );
my $name;
print "nonempty\n" if length($name = define $name);
print "'$name'\n";
If you don't want to clobber $name , use default .
I wouldn't use //= in this case since it changes the data as a side effect. Instead, use the
slightly shorter length( $name // '' ) . – brian d foy
Sep 26 '09 at 19:16
As @rjbs pointed out in my answer, with v5.12 and later length can now return
something that is not a number (but not NaN ;) – brian d foy
Aug 29 '15 at 1:44
@RET: you can't use the || operator here since it replaces the string '0' with ''. You have
to check if it is defined, not true. – brian d foy
Sep 29 '09 at 4:25
Chris, RET: Yup, I know. I was specifically trying to suggest that if Jessica was not
concerned with the difference between undef and "" , she should
just change one to the other and use a single test. This won't work in the general case, for
which the other solutions posted are way better, but in this specific case leads to neat
code. Should I rephrase my answer to make this clearer? – Gaurav
Sep 29 '09 at 4:27
This will still give you a warning. The reason people check definedness first is to avoid the
'uninitialized value' warning. – brian d foy
Sep 26 '09 at 19:20
It isn't always possible to do repetitive things in a simple and elegant way.
Just do what you always do when you have common code that gets replicated across many
projects:
Search CPAN, someone may have already the code for you. For this issue I found Scalar::MoreUtils .
If you don't fined something you like on CPAN, make a module and put the code in a
subroutine:
package My::String::Util;
use strict;
use warnings;
our @ISA = qw( Exporter );
our @EXPORT = ();
our @EXPORT_OK = qw( is_nonempty);
use Carp qw(croak);
sub is_nonempty ($) {
croak "is_nonempty() requires an argument"
unless @_ == 1;
no warnings 'uninitialized';
return( defined $_[0] and length $_[0] != 0 );
}
1;
=head1 BOILERPLATE POD
blah blah blah
=head3 is_nonempty
Returns true if the argument is defined and has non-zero length.
More boilerplate POD.
=cut
Then in your code call it:
use My::String::Util qw( is_nonempty );
if ( is_nonempty $name ) {
# do something with $name
}
Or if you object to prototypes and don't object to the extra parens, skip the prototype in
the module, and call it like: is_nonempty($name) .
@Zoran No. Factoring code out like this beats having a complicated condition replicated in
many different places. That would be like using pinpricks to kill an elephant. @daotoad: I
think you should shorten your answer to emphasize the use of Scalar::MoreUtils .
– Sinan Ünür
Sep 26 '09 at 7:05
@Zoran: Scalar::MoreUtils is a very lightweight module with no dependencies. Its semantics
are also well known. Unless you are allergic to CPAN, there's not much reason to avoid using
it. – Adam Bellaire
Sep 26 '09 at 11:45
@Chris Lutz, yeah, I shouldn't. But prototypes are semi-broken--there are easy ways to break
prototype enforcement. For example, crappy and/or outdated tutorials continue to encourage
the use of the & sigil when calling functions. So I tend not to rely on
prototypes to do all the work. I suppose I could add "and quit using the & sigil on sub
calls unless you really mean it" to the error message. – daotoad
Sep 28 '09 at 7:35
It's easier to think about prototypes as hints to the perl compiler so it knows how to parse
something. They aren't there to validate arguments. They may be broken in terms of people's
expectations, but so many things are. :) – brian d foy
Sep 28 '09 at 18:52
if (length ($name || '')) {
# do something with $name
}
This isn't quite equivalent to your original version, as it will also return false if
$name is the numeric value 0 or the string '0' , but will behave
the same in all other cases.
In perl 5.10 (or later), the appropriate approach would be to use the defined-or operator
instead:
use feature ':5.10';
if (length ($name // '')) {
# do something with $name
}
This will decide what to get the length of based on whether $name is defined,
rather than whether it's true, so 0/ '0' will handle those cases correctly, but
it requires a more recent version of perl than many people have available.
Because, as I also mentioned, 5.10 is "a more recent version of perl than many people have
available." YMMV, but "this is a 99% solution that I know you can use, but there's a better
one that maybe you can use, maybe you can't" seems better to me than "here's the perfect
solution, but you probably can't use it, so here's an alternative you can probably get by
with as a fallback." – Dave Sherohman
Sep 26 '09 at 22:20
if ($name )
{
#since undef and '' both evaluate to false
#this should work only when string is defined and non-empty...
#unless you're expecting someting like $name="0" which is false.
#notice though that $name="00" is not false
}
; , Sep 29, 2009 at 15:20
Unfortunately this will be false when $name = 0; – user180804
Sep 29 '09 at 15:20
I am attempting to parse a CSV, but am not allowed to install the CSV parsing module
because of "security reasons" (what a joke), so I'm attempting to use 'split' to break up a
comma-delimited file.
My issue is that as soon as an "empty" field comes up (two commas in a row), split seems
to think the line is done and goes to the next one.
Everything I've read online says that split will return a null field, but I don't know how
to get it to go to the next element and not just skip to the next line.
Expand|Select|Wrap|Line Numbers
while (<INFILE>) {
# use 'split' to avoid module-dependent functionality
# split line on commas, OS info in [3] (4th group, but
# counting starts first element at 0)
# line = <textonly>,<text+num>,<ip>,<whatIwant>,
chomp($_);
@a_splitLine = split (/,/, $_);
# move OS info out of string to avoid accidentally
# parsing over stuff
$s_info = $a_splitLine[3];
Could anyone see either a better way to accomplish what I'm trying to do, or help get
split to capture all the elements?
I was thinking I could run a simple substitution before parsing of a known string
(something ridiculous that'll never show up in my data - like &^%$#), then split, and
then when printing, if that matches the current item, just print some sort of whitespace, but
that doesn't sound like the best method to me - like I'm overcomplicating it.
Interesting, so then how would I access the b or the 6?
#!/bin/perl
use strict;
use warnings;
use Data::Dumper;
my $str = 'a,,,b,,,,6,,';
my @fields = split /,/, $str;
my $n = 0;
print Dumper @fields;
while ($fields[$n]) {
print "$n: $fields[$n]\n";
$n++;
}
print "done!\n";
Expand|Select|Wrap|Line Numbers
$ ./splitTest.pl
$VAR1 = 'a';
$VAR2 = '';
$VAR3 = '';
$VAR4 = 'b';
$VAR5 = '';
$VAR6 = '';
$VAR7 = '';
$VAR8 = '6';
0: a
done!
In the above, my attempt to print with a while loop stops as soon as the first
empty set is reached. I'm guessing I'd have to check each one to see which are
valid and which are not, but what am I looking for - null?
I have to agree with Ron. Since this is a csv file, you should already know
which field is what. All you would have to do is reference it by its index.
Otherwise, you can use the code above to iterate through each one and pull out the
variables with values other than null.
Cool, thanks. I am really only interested in one of those fields, but then have
to make sure once I edit that field, I re-append all the others back on, so I will
play around with that.
There's an alternative way of option parsing using the core module Getopt::Long:
use Getopt::Long;
use constant WEB => 1;
use constant SQL => 2;
use constant REG => 4;
my %dbg_flag = (
'WEB' => WEB,
'SQL' => SQL,
'REG' => REG);
my %dbg;
GetOptions(\%dbg, "D=s@");
my $DEBUG = WEB | REG unless $dbg{D};
$DEBUG |= $dbg_flag{$_}
|| die "Unknown debug flag $_\n"
foreach @{$dbg{D}};
(We've shown only the option-parsing part.) This has a slightly different interface;
instead of separating multiple values with commas, we must repeat the -D flag:
% whizzbang.pl -D WEB -D REG
(We could also say "-D=WEB -D=REG".)
Getopt::Long permits many more choices than this, of course. If you have
a complex program (particularly one worked on by multiple programmers), look to it for support for
the kind of debug flag setting interface you'll find useful. Brad Appleton says
Sometimes I will have a debug flag per each important subsystem of a large
Perl program encompassing many modules. Each one will have an integer value. Sometimes I will
have a debug argument along the lines of
-debug Pkg1=f1,f2,... -debug Pkg2=f3,f4,...
If the value for Pkg1 is an integer, it sets a package-wide debug
level/flag. Otherwise, it says which specific functions to debug in the package.
The -d command line option makes your script run under the debugger. You can either
add it to the options in the #! line at the beginning of your script, or you can
override the options by explicitly running the script through perl. So for example, if
wombat . pl currently has the -w option set, you can either change
its first line to
#!/usr/bin/perl -wd
or you can type
% perl -wd wombat.pl
to debug it without having to change the script. Unlike some debuggers, with this one you
supply arguments to the program on the command line, not as part of a debugger command; for
example:
% perl -wd wombat.pl kangaroo platypus wallaby
The debugger will announce itself and provide a prompt:
Loading DB routines from perl5db.pl version 1.07
Emacs support available.
Enter h or `h h' for help.
main::(wombat.pl:1): my $marsupial = shift;
DB<1>
From now on we will elide everything before the first prompt (and the code on which it is
stopped) when reproducing debugger sessions.
To begin, let's look at some simple commands. The very first of interest, of course, is
h for help. The output from this is several screens long, which gives us an
opportunity to mention an option we can apply to all commands: put a vertical bar ( |
) before any command and it will run the output through your pager (the program that prints
things one screen at a time, waiting for you to tell it when to continue -- more or less
more or less ).
7.2.1. Watch the Code Execute: s, n, r
Enter this simple program into debug . pl:
#!/usr/local/bin/perl -w
use strict;
my @parole = qw(Salutations Hello Hey);
print_line(@parole);
print "Done\n";
# Our subroutine accepts an array, then prints the
# value of each element appended to "Perl World."
sub print_line
{
my @parole = @_;
foreach (@parole)
{
print "$_ Perl World\n";
}
}
Now run it under the debugger and step through the program one statement at a time using the
n (next) command:
% perl -dw debug.pl
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> n
main::(debug.pl:6): &print_line(@parole);
DB<1> n
Salutations Perl World
Hello Perl World
Hey Perl World
main::(debug.pl:7): print "Done\n";
DB<1> n
Done
Debugged program terminated. Use q to quit or R to restart, use
O inhibit_exit to avoid stopping after program termination, h
q, h R or h O to get additional info.
DB<1> q
Before the prompt, the debugger prints the source line(s) containing the statement to be
executed in the next step. (If you have more than one executable statement in a line, it prints
the line each time you type n until it's done executing all the statements on the
line.) Notice the output of our program going to the terminal is intermingled with the debugger
text. Notice also when we called print_line(@parole) , we executed all the statements
in the subroutine before we got another prompt.
(From now on, we won't reproduce the optimistic Debugged program terminated blurb
printed by the debugger.)
Suppose we wanted to step through the code inside subroutines like print_line .
That's the reason for s (single step). Let's see how it's used, along with another
handy stepping command, r (return):
% perl -d debug.pl
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> n
main::(debug.pl:6): print_line(@parole);
DB<1> s
main::print_line(debug.pl:13): my @parole = @_;
DB<1> n
main::print_line(debug.pl:14): foreach (@parole)
main::print_line(debug.pl:15): {
DB<1>
main::print_line(debug.pl:16): print "$_ Perl World\n";
DB<1> r
Salutations Perl World
Hello Perl World
Hey Perl World
void context return from main::print_line
main::(debug.pl:7): print "Done\n";
DB<1> s
Done
Debugged program terminated.
The effect of r is to execute all the code up to the end of the current subroutine.
(All these command letters are copied from existing popular Unix command line debuggers and are
mnemonic -- n ext, s tep, r eturn). In addition, note that just
hitting carriage return as a command repeats the last n or s command (and if
there hasn't been one yet, it does nothing).
7.2.2. Examining Variables: p, x, V
Stepping through code is dandy, but how do we check our variables' values? Use either
pexpression to print the result of the expression (which is equivalent to
print ing to the filehandle $DB::OUT , so expression is put in list
context) or xvariable , which prints a variable in a pleasantly formatted
form, following references. Once again, with the simple program:
% perl -wd debug.pl
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> p
@parole
DB<2> n
main::(debug.pl:6): print_line(@parole);
DB<2> p
@parole
SalutationsHelloHey
DB<3> x
@parole
0 'Salutations'
1 'Hello'
2 'Hey'
In the first command, we instruct the debugger to print the value of @parole .
However, the @parole assignment has yet to execute, so nothing comes out. Step past
the assignment and then print the value with p ; we see the current state of the array
in a list format. Print the array value with x , and we see the individual elements
formatted with array indices (a pretty print).
This might look familiar if you've been playing with the Data::Dumper module we
referenced in Chapter 5 . In fact, the output of
x is intentionally very similar.
You can see all of the dynamic variables in a given package (default: main:: ) with
the V command. This isn't as useful as it sounds because, unlike the x or
p commands, it won't show you any lexical variables (which you declared with
my ). Yet you want to make as many of your variables as possible lexical ones (see
Perl of Wisdom #8). Unfortunately there is no (easy) way to dump out all the lexical variables
in a package, so you're reduced to printing the ones you know about.
A common problem is running off the end of the program and getting the Debugged program
terminated message. At that point, all your variables have been destroyed. If you want to
inspect the state of variables after the last line of your program has executed, add a dummy
line (a 1 by itself will work) so that you can set a breakpoint on it.
Tip
when e x amining a hash, examine a reference to it instead. This lets the
x command see the datatype you're inspecting instead of being handed the list that it
evaluates to, and it can format it more appealingly:
Examine references to hashes instead of the hashes themselves in the debugger to get
well-formatted output.
7.2.3. Examining Source: l, -, w,
Sometimes you want more of the context of your program than just the current line. The
following commands show you parts of your source code:
l
List successive windows of source code starting from the
current line about to be executed.
lx+y
List + 1 lines of source starting from line x.
lx-y
List source lines through y.
-
List successive windows of source code before the current
line.
w
List a window of lines around the current line.
wline
List a window of lines around line.
.
Reset pointer for window listings to current line.
Source lines that are breakable (i.e. can have a breakpoint inserted before them -- see the
following section) have a colon after the line number.
7.2.4. Playing in the Sandbox
Since the debugger is a full-fledged Perl environment, you can type in Perl code on the fly
to examine its effects under the debugger; [1] some people do this as a way of testing code
quickly without having to enter it in a script or type in everything perfectly before hitting
end-of-file. (You just saw us do this at the end of section 7.2.2 .)
[1] So, you might wonder, how would you
enter Perl code which happened to look like a debugger command (because you'd defined a
subroutine l , perhaps)? In versions of Perl prior to 5.6.0, if you enter leading
white space before text, the debugger assumes it must be Perl code and not a debugger
command. So be careful not to hit the space bar by accident before typing a debugger command.
This was no longer true as of version 5.6.0.
Type perl-de0 to enter this environment. [2] Let's use this as a sandbox for testing Perl
constructs:
[2] There are many expressions other than
0 that would work equally well, of course. Perl just needs something innocuous to
run.
You can even use this feature to change the values of variables in a program you are
debugging, which can be a legitimate strategy for seeing how your program behaves under
different circumstances. If the way your program constructs the value of some internal variable
is complex and it would require numerous changes in the input to have it form the variable
differently, then a good way of playing "What if?" is to stop the program at the right place in
the debugger and change the value by hand. How would we stop it? Let's see
7.2.5.
Breakpointing: c, b, L
An important feature of a debugger is the ability to allow your program to continue
executing until some condition is met. The most common such condition is the arrival of the
debugger at a particular line in your source. You can tell the Perl debugger to run until a
particular line number with the c (for continue) command:
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> c
16
main::print_line(debug.pl:16): print "$_ Perl World\n";
DB<2>
What the debugger actually did was set a one-time breakpoint at line 16 and then executed
your code until it got there. If it had hit another breakpoint earlier, it would have stopped
there first.
So what's a breakpoint? It's a marker set by you immediately before a line of code,
invisible to anyone but the perl debugger, which causes it to halt when it gets there and
return control to you with a debugger prompt. If you have a breakpoint set at a line of code
that gets printed out with one of the source examination commands listed earlier, you'll see a
b next to it. It's analogous to putting a horse pill in the trail of bread crumbs the
mouse follows so the mouse gets indigestion and stops to take a breather (we really have to
give up this metaphor soon).
You set a breakpoint with the b command; the most useful forms are bline
or bsubroutine to set a breakpoint either at a given line number or
immediately upon entering a subroutine. To run until the next breakpoint, type c . To
delete a breakpoint, use dline to delete the breakpoint at line number
line or D to delete all breakpoints.
In certain situations you won't want to break the next time you hit a particular breakpoint,
but only when some condition is true, like every hundredth time through a loop. You can add a
third argument to b specifying a condition that must be true before the debugger will
stop at the breakpoint. For example,
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> l
4==> my @parole = qw(Salutations Hello Hey);
5
6: print_line(@parole);
7: print "Done\n";
8
9 # Our subroutine which accepts an array, then prints
10 # the value of each element appended to "Perl World."
11 sub print_line
12 {
13: my @parole = @_;
DB<1> l
14: foreach (@parole)
15 {
16: print "$_ Perl World\n";
17 }
18 }
DB<1> b
16 /Hey/
DB<2> c
Salutations Perl World
Hello Perl World
main::print_line(debug.pl:16): print "$_ Perl World\n";
DB<2> p
Hey
Notice that we've demonstrated several things here: the source listing command l ,
the conditional breakpoint with the criterion that $_ must match /Hey/ , and
that $_ is the default variable for the p command (because p just
calls print ).
The capability of the debugger to insert code that gets executed in the context of the
program being debugged does not exist in compiled languages and is a significant example of the
kind of thing that is possible in a language as well designed as Perl.
The command L lists all breakpoints.
7.2.6. Taking Action: a, A
An even more advanced use of the facility to execute arbitrary code in the debugger is the
action capability. With the a command (syntax: aline code ), you
can specify code to be executed just before a line would be executed. (If a breakpoint is set
for that line, the action executes first; then you get the debugger prompt.) The action can be
arbitrarily complicated and, unlike this facility in debuggers for compiled languages, lets you
reach into the program itself:
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> a
16 s/Hey/Greetings/
DB<2> c
Salutations Perl World
Hello Perl World
Greetings Perl World
Done
Debugged program terminated.
L also lists any actions you have created. Delete all the installed actions with
the A command. This process is commonly used to insert tracing code on the fly. For
example, suppose you have a program executing a loop containing way too much code to step
through, but you want to monitor the state of certain variables each time it goes around the
loop. You might want to confirm what's actually being ordered in a shopping cart application
test (looking at just a fragment of an imaginary such application here):
Suppose you want to break on a condition that is dictated not by a particular line of code
but by a change in a particular variable. This is called a watchpoint, and in Perl you set it
with the W command followed by the name of a variable. [3]
[3] In fact, Perl can monitor anything
that evaluates to an lvalue, so you can watch just specific array or hash entries, for
example.
Let's say you're reading a file of telephone numbers and to whom they belong into a hash,
and you want to stop once you've read in the number 555-1212 to inspect the next input line
before going on to check other things:
main::(foo:1): my %phone;
DB<1> l
1-5
1==> my %phone;
2: while (<>) {
3: my ($k, $v) = split;
4: $phone{$k} = $v;
5 }
DB<2> W
$phone{'555-1212'}
DB<3> c
Watchpoint 0: $phone{'555-1212'} changed:
old value: undef
new value: 'Information'
main::(foo:2): while (<>) {
DB<3> n
main::(foo:3): my ($k, $v) = split;
DB<3> p
555-1234 Weather
Delete all watchpoints with a blank W command.
7.2.8. Trace:
The debugger's t command provides a trace mode for those instances that require a
complete trace of program execution. Running the program with an active trace mode:
% perl -wd debug.pl
main::(debug.pl:4): my @parole = qw(Salutations Hello Hey);
DB<1> t
Trace = on
DB<1> n
main::(debug.pl:6): print_line(@parole);
DB<1> n
main::print_line(debug.pl:13): my @parole = @_;
main::print_line(debug.pl:14): foreach (@parole)
main::print_line(debug.pl:15): {
main::print_line(debug.pl:16): print "$_ Perl World\n";
Salutations Perl World
main::print_line(debug.pl:14): foreach (@parole)
main::print_line(debug.pl:15): {
main::print_line(debug.pl:16): print "$_ Perl World\n";
Hello Perl World
main::print_line(debug.pl:14): foreach (@parole)
main::print_line(debug.pl:15): {
main::print_line(debug.pl:16): print "$_ Perl World\n";
Hey Perl World
main::print_line(debug.pl:14): foreach (@parole)
main::print_line(debug.pl:15): {
main::(debug.pl:7): print "Done\n";
Notice that trace mode causes the debugger to output the call tree when execution enters the
print_line subroutine.
7.2.9. Programmatic Interaction with the Debugger
You can put code in your program to force a call to the debugger at a particular point. For
instance, suppose you're processing a long input file line by line and you want to start
tracing when it reaches a particular line. You could set a conditional breakpoint, but you
could also extend the semantics of your input by creating "enable debugger" lines. Consider the
following code:
while (<INPUT>)
{
$DB::trace = 1, next if /debug/;
$DB::trace = 0, next if /nodebug/;
# more code
}
When run under the debugger, this enables tracing when the loop encounters an input line
containing "debug" and ceases tracing upon reading one containing " nodebug ". You can
even force the debugger to breakpoint by setting the variable $DB::single to
1 , which also happens to provide a way you can debug code in BEGIN blocks (which
otherwise are executed before control is given to the debugger).
7.2.10. Optimization
Although the Perl debugger displays lines of code as it runs, it's important to note that
these are not what actually executes. Perl internally executes its compiled opcode tree, which
doesn't always have a contiguous mapping to the lines of code you typed, due to the processes
of compilation and optimization. If you have used interactive debuggers on C code in the past,
you may be familiar with this process.
When debugging C programs on VAX/VMS, it was common for me to want to examine an
important variable only to get the message that the variable was not in memory and had
been "optimized away."
Perl has an optimizer to do as good a job as it can -- in the short amount of time people
will wait for compilation -- of taking shortcuts in the code you've given it. For instance, in
a process called constant folding, it does things like build a single string in places where
you concatenate various constant strings together so that the concatenation operator need not
be called at run-time.
The optimization process also means that perl may execute opcodes in an order different from
the order of statements in your program, and therefore when the debugger displays the current
statement, you may see it jump around oddly. As recently as version 5.004_04 of perl, this
could be observed in a program like the following:
1 my @a = qw(one two three);
2 while ($_ = pop @a)
3 {
4 print "$_\n";
5 }
6 1;
See what happens when we step through this, again using perl 5.004_04 or earlier:
main::(while.pl:1): my @a = qw(one two three);
DB<1> n
main::(while.pl:6): 1;
DB<1>
main::(while.pl:4): print "$_\n";
DB<1>
three
main::(while.pl:2): while ($_ = pop @a)
DB<1>
main::(while.pl:4): print "$_\n";
DB<1>
two
In fact, if we set a breakpoint for line 6 and ran to it, we'd get there before the loop
executed at all. So it's important to realize that under some circumstances, what the debugger
tells you about where you are can be confusing. If this inconveniences you,
upgrade.
7.2.11. Another "Gotcha"
If you set a lexical variable as the last statement of a block, there is no way to see what
it was set to if the block exits to a scope that doesn't include the lexical. Why would code do
that? In a word, closures. For example,
{ # Start a closure-enclosing block
my $spam_type; # This lexical will outlive its block
sub type_spam
{
# ...
$spam_type = $spam_types[complex_func()];
}
}
In this case, either type_spam or some other subroutine in the closure block would
have a good reason for seeing the last value of $spam_type . But if you're stepping
through in the debugger, you won't see the value it gets set to on the last line because, after
the statement executes, the debugger pops out to a scope where $spam_type is not in
scope (unless type_spam() was called from within the enclosing block). Unfortunately,
in this case, if the result of the function is not used by the caller, you're out of luck.
The package directive sets the namespace. As such, the namespace is also called
the package.
Perl doesn't have a formal definition of module. There's a lot of variance, but the
following holds for a huge majority of modules:
A file with a .pm extension.
The file contains a single package declaration that covers the entirety of
the code. (But see below.)
The file is named based on the namespace named by that package .
The file is expected to return a true value when executed.
The file is expected to be executed no more than once per interpreter.
It's not uncommon to encounter .pm files with multiple packages. Whether
that's a single module, multiple modules or both is up for debate.
Namespace is a general computing term meaning a container for a distinct set of
identifiers. The same identifier can appear independently in different namespaces and refer
to different objects, and a fully-qualified identifier which unambiguously identifies an
object consists of the namespace plus the identifier.
Perl implements namespaces using the package keyword.
A Perl module is a different thing altogether. It is a piece of Perl code that
can be incorporated into any program with the use keyword. The filename should
end with .pm - for erl odule - and the code it contains should have a
package statement using a package name that is equivalent to the file's name,
including its path. For instance, a module written in a file called
My/Useful/Module.pm should have a package statement like
package My::Useful::Module .
What you may have been thinking of is a class which, again, is a general
computing term, this time meaning a type of object-oriented data. Perl uses its packages as
class names, and an object-oriented module will have a constructor subroutine -
usually called new - that will return a reference to data that has been
blessed to make it behave in
an object-oriented fashion. By no means all Perl modules are object-oriented ones: some can
be simple libraries of subroutines.
1 does not matter. It can be 2 , it can be "foo" , it
can be ["a", "list"] . What matters is it's not 0 , or anything
else that evaluates as false, or use would fail. – Amadan
Aug 4 '10 at 5:32
.pl is actually a perl library - perl scripts, like C programs or programs written in other
languages, do not have an ending, except on operating systems that need one to functiopn,
such as windows. – Marc Lehmann
Oct 16 '15 at 22:08
At the very core, the file extension you use makes no difference as to how perl
interprets those files.
However, putting modules in .pm files following a certain directory structure
that follows the package name provides a convenience. So, if you have a module
Example::Plot::FourD and you put it in a directory
Example/Plot/FourD.pm in a path in your @INC , then use and require will do the
right thing when given the package name as in use Example::Plot::FourD .
The file must return true as the last statement to indicate successful execution of any
initialization code, so it's customary to end such a file with 1; unless
you're sure it'll return true otherwise. But it's better just to put the 1; ,
in case you add more statements.
If EXPR is a bareword, the require assumes a ".pm" extension
and replaces "::" with "/" in the filename for you, to make it easy to load standard
modules. This form of loading of modules does not risk altering your namespace.
All use does is to figure out the filename from the package name provided,
require it in a BEGIN block and invoke import on the
package. There is nothing preventing you from not using use but taking those
steps manually.
For example, below I put the Example::Plot::FourD package in a file called
t.pl , loaded it in a script in file s.pl .
C:\Temp> cat t.pl
package Example::Plot::FourD;
use strict; use warnings;
sub new { bless {} => shift }
sub something { print "something\n" }
"Example::Plot::FourD"
C:\Temp> cat s.pl
#!/usr/bin/perl
use strict; use warnings;
BEGIN {
require 't.pl';
}
my $p = Example::Plot::FourD->new;
$p->something;
C:\Temp> s
something
This example shows that module files do not have to end in 1 , any true value
will do.
In .pm ( Perl Module ) you have
functions that you can use from other Perl scripts:
A Perl module is a self-contained piece of Perl code that can be used by a Perl program
or by other Perl modules. It is conceptually similar to a C link library, or a C++
class.
"A .pl is a single script." Not true. It's only on broken operating systems that you need to
identify Perl programs with a .pl extension. And originally .pl indicated a "Perl library" -
external subroutines that you loaded with a "require" or "do" command. – Dave Cross
Sep 17 '10 at 9:37
You may be creating more and more scripts for your systems, which need to use the same functions.
You already mastered the ancient art of copy-paste, but you are not satisfied with the result.
You probably know lots of Perl modules that allow you to use their functions and you also want to create one.
However, you don't know how to create such a module.
The module
package My :: Math
use strict
use warnings
use Exporter qw import );
our @EXPORT_OK = qw add multiply );
sub add
my $x $y = @_
return $x $y
sub multiply
my $x $y = @_
return $x $y
Save this in somedir/lib/My/Math.pm (or somedir\lib\My\Math.pm on Windows).
The script
#!/usr/bin/perl
use strict
use warnings
use My :: Math qw add );
print add 19 23 );
Save this in somedir/bin/app.pl (or somedir\bin\app.pl on Windows).
Now run perl somedir/bin/app.pl . (or perl somedir\bin\app.pl on Windows).
It is going to print an error like this:
Can't locate My/Math.pm in @INC (@INC contains:
...
...
...
BEGIN failed--compilation aborted at somedir/bin/app.pl line 9.
What is the problem?
In the script we loaded the module with the use keyword. Specifically with the use My::Math qw(add); line. This searches the directories
listed in the built-in @INC variable looking for a subdirectory called My and in that subdirectory for a file called Math.pm
.
The problem is that your .pm file is not in any of the standard directories of perl: it is not in any of the directories listed in
@INC.
You could either move your module, or you could change @INC.
The former can be problematic, especially on systems where there is a strong separation between the system administrator and the
user. For example on Unix and Linux system only the user "root" (the administrator) has write access to these directories. So in
general it is easier and more correct to change @INC.
Change @INC from the command line
Before we try to load the module, we have to make sure the directory of the module is in the @INC array.
Try this:
perl -Isomedir/lib/ somedir/bin/app.pl .
This will print the answer: 42.
In this case, the -I flag of perl helped us add a directory path to @INC.
Change @INC from inside the script
Because we know that the "My" directory that holds our module is in a fixed place relative to the script, we have another
possibility for changing the script:
#!/usr/bin/perl
use strict
use warnings
use File :: Basename qw dirname );
use Cwd qw abs_path );
use lib dirname dirname abs_path $0 '/lib'
use My :: Math qw add );
print add 19 23 );
and run it again with this command:
perl somedir/bin/app.pl .
Now it works.
Let's explain the change:
How to change @INC to point to a relative directory
This line: use lib dirname(dirname abs_path $0) . '/lib'; adds the relative lib directory to the beginning of @INC
$0 holds the name of the current script. abs_path() of Cwd returns the absolute path to the script.
Given a path to a file or to a directory the call to dirname() of File::Basename returns the directory part, except of the last
part.
In our case $0 contains app.pl
abs_path($0) returns .../somedir/bin/app.pl
dirname(abs_path $0) returns .../somedir/bin
dirname( dirname abs_path $0) returns .../somedir
That's the root directory of our project.
dirname( dirname abs_path $0) . '/lib' then points to .../somedir/lib
So what we have there is basically
use lib '.../somedir/lib';
but without hard-coding the actual location of the whole tree.
The whole task of this call is to add the '.../somedir/lib' to be the first element of @INC.
Once that's done, the subsequent call to use My::Math qw(add); will find the 'My' directory in '.../somedir/lib' and the Math.pm
in '.../somedir/lib/My'.
The advantage of this solution is that the user of the script does not have to remember to put the -I... on the command line.
So as I wrote earlier, the use call will look for the My directory and the Math.pm file in it.
The first one it finds will be loaded into memory and the import function of My::Math will be called with the parameters after
the name of the module. In our case import( qw(add) ) which is just the same as calling import( 'add' )
The explanation of the script
There is not much left to explain in the script. After the use statement is done calling the import function, we can just call
the newly imported add function of the My::Math module. Just as if I declared the function in the same script.
What is more interesting is to see the parts of the module.
The explanation of the module
A module in Perl is a namespace in the file corresponding to that namespace. The package keyword creates the namespace. A module
name My::Math maps to the file My/Math.pm. A module name A::B::C maps to the file A/B/C.pm somewhere in the directories listed in
@INC.
As you recall, the use My::Math qw(add); statement in the script will load the module and then call the import function. Most
people don't want to implement their own import function, so they load the Exporter module and import the 'import' function.
Yes, it is a bit confusing. The important thing to remember is that Exporter gives you the import.
That import function will look at the @EXPORT_OK array in your module and arrange for on-demand importing of the functions listed
in this array.
OK, maybe I need to clarify: The module "exports" functions and the script "imports" them.
The last thing I need to mention is the 1; at the end of the module. Basically the use statement is executing the module and it
needs to see some kind of a true statement there. It could be anything. Some people put there 42; , others, the really funny ones
put "FALSE" there. After all every string with letters in it is
considered to be true in Perl . That confuses about everyone.
There are even people who put quotes from poems there.
"Famous last words."
That's actually nice, but might still confuse some people at first.
There are also two functions in the module. We decided to export both of them, but the user (the author of the script) wanted
to import only one of the subroutines.
Conclusion
Aside from a few lines that I explained above, it is quite simple to create a Perl module. Of course there are other things you
might want to learn about modules that will appear in other articles, but there is nothing stopping you now from moving some common
functions into a module.
Maybe one more advice on how to call your module:
Naming of modules
It is highly recommended to use capital letter as the first letter of every part in the module name and lower case for the rest
of the letters. It is also recommended to use a namespace several levels deep.
If you work in a company called Abc, I'd recommend preceding all the modules with the Abc:: namespace. If within the company the
project is called Xyz, then all its modules should be in Abc::Xyz::.
So if you have a module dealing with configuration you might call the package Abc::Xyz::Config which indicates the file .../projectdir/lib/Abc/Xyz/Config.pm
Please avoid calling it just Config.pm. That will confuse both Perl (that comes with its own Config.pm) and you.
Great question: How does our differ from my and what
does our do?
In Summary:
Available since Perl 5, my is a way to declare:
non-package variables, that are
private,
new ,
non-global variables,
separate from any package. So that the variable cannot be accessed in the form
of $package_name::variable .
On the other hand, our variables are:
package variables, and thus automatically
global variables,
definitely not private ,
nor are they necessarily new; and they
can be accessed outside the package (or lexical scope) with the qualified namespace,
as $package_name::variable .
Declaring a variable with our allows you to predeclare variables in order
to use them under use strict without getting typo warnings or compile-time
errors. Since Perl 5.6, it has replaced the obsolete use vars , which was
only file-scoped, and not lexically scoped as is our
For example, the formal, qualified name for variable $x inside package main
is $main::x . Declaring our $x allows you to use the bare
$x variable without penalty (i.e., without a resulting error), in the scope of the
declaration, when the script uses use strict or use strict "vars"
. The scope might be one, or two, or more packages, or one small block.
@Nathan Fellman, local doesn't create variables. It doesn't relate to my
and our at all. local temporarily backs up the value of variable and
clears its current value. –
ikegami
Sep 21 '11 at 16:57
our variables are not package variables. They aren't globally-scoped, but lexically-scoped
variables just like my variables. You can see that in the following program:
package Foo; our $x = 123; package Bar; say $x; . If you want to "declare" a package
variable, you need to use use vars qw( $x ); . our $x; declares a lexically-scoped
variable that is aliased to the same-named variable in the package in which the our
was compiled. – ikegami
Nov 20 '16 at 1:15
The PerlMonks and PerlDoc links from cartman and Olafur are a great reference - below is my crack
at a summary:
my variables are lexically scoped within a single block defined by {}
or within the same file if not in {} s. They are not accessible from packages/subroutines
defined outside of the same lexical scope / block.
our variables are scoped within a package/file and accessible from any code that
use or require that package/file - name conflicts are resolved between
packages by prepending the appropriate namespace.
Just to round it out, local variables are "dynamically" scoped, differing from
my variables in that they are also accessible from subroutines called within the
same block.
use strict;
for (1 .. 2){
# Both variables are lexically scoped to the block.
our ($o); # Belongs to 'main' package.
my ($m); # Does not belong to a package.
# The variables differ with respect to newness.
$o ++;
$m ++;
print __PACKAGE__, " >> o=$o m=$m\n"; # $m is always 1.
# The package has changed, but we still have direct,
# unqualified access to both variables, because the
# lexical scope has not changed.
package Fubb;
print __PACKAGE__, " >> o=$o m=$m\n";
}
# The our() and my() variables differ with respect to privacy.
# We can still access the variable declared with our(), provided
# that we fully qualify its name, but the variable declared
# with my() is unavailable.
print __PACKAGE__, " >> main::o=$main::o\n"; # 2
print __PACKAGE__, " >> main::m=$main::m\n"; # Undefined.
# Attempts to access the variables directly won't compile.
# print __PACKAGE__, " >> o=$o\n";
# print __PACKAGE__, " >> m=$m\n";
# Variables declared with use vars() are like those declared
# with our(): belong to a package; not private; and not new.
# However, their scoping is package-based rather than lexical.
for (1 .. 9){
use vars qw($uv);
$uv ++;
}
# Even though we are outside the lexical scope where the
# use vars() variable was declared, we have direct access
# because the package has not changed.
print __PACKAGE__, " >> uv=$uv\n";
# And we can access it from another package.
package Bubb;
print __PACKAGE__, " >> main::uv=$main::uv\n";
Coping with Scoping
is a good overview of Perl scoping rules. It's old enough that our is not discussed
in the body of the text. It is addressed in the Notes section at the end.
The article talks about package variables and dynamic scope and how that differs from lexical
variables and lexical scope.
Be careful tossing around the words local and global. The proper terms are lexical and package.
You can't create true global variables in Perl, but some already exist like $_, and local refers
to package variables with localized values (created by local), not to lexical variables (created
with my). – Chas. Owens
May 11 '09 at 0:16
It's an old question, but I ever met some pitfalls about lexical declarations in Perl that messed
me up, which are also related to this question, so I just add my summary here:
1. definition or declaration?
local $var = 42;
print "var: $var\n";
The output is var: 42 . However we couldn't tell if local $var = 42;
is a definition or declaration. But how about this:
use strict;
use warnings;
local $var = 42;
print "var: $var\n";
The second program will throw an error:
Global symbol "$var" requires explicit package name.
$var is not defined, which means local $var; is just a declaration!
Before using local to declare a variable, make sure that it is defined as a global
variable previously.
But why this won't fail?
use strict;
use warnings;
local $a = 42;
print "var: $a\n";
The output is: var: 42 .
That's because $a , as well as $b , is a global variable pre-defined
in Perl. Remember the sort
function?
2. lexical or global?
I was a C programmer before starting using Perl, so the concept of lexical and global variables
seems straightforward to me: just corresponds to auto and external variables in C. But there're
small differences:
In C, an external variable is a variable defined outside any function block. On the other hand,
an automatic variable is a variable defined inside a function block. Like this:
int global;
int main(void) {
int local;
}
While in Perl, things are subtle:
sub main {
$var = 42;
}
&main;
print "var: $var\n";
The output is var: 42 , $var is a global variable even it's defined
in a function block! Actually in Perl, any variable is declared as global by default.
The lesson is to always add use strict; use warnings; at the beginning of a Perl
program, which will force the programmer to declare the lexical variable explicitly, so that we
don't get messed up by some mistakes taken for granted.
Unlike my, which both allocates storage for a variable and associates a simple name with
that storage for use within the current scope, our associates a simple name with a package
variable in the current package, for use within the current scope. In other words, our has
the same scoping rules as my, but does not necessarily create a variable.
This is only somewhat related to the question, but I've just discovered a (to me) obscure bit
of perl syntax that you can use with "our" (package) variables that you can't use with "my" (local)
variables.
Not so. $foo ${foo} ${'foo'} ${"foo"} all work the same for variable assignment or dereferencing.
Swapping the our in the above example for my does work. What you probably experienced
was trying to dereference $foo as a package variable, such as $main::foo or $::foo which will
only work for package globals, such as those defined with our . –
Cosmicnet
Oct 21 '14 at 14:08
My test (on windows): perl -e "my $foo = 'bar'; print $foo; ${foo} = 'baz'; pr int $foo"
output: barbazperl -e "my $foo = 'bar'; print $foo; ${"foo"} = 'baz'; print
$foo" output: barbazperl -e "my $foo = 'bar'; print $foo; ${\"foo\"}
= 'baz'; print $foo" output: barbar So in my testing I'd fallen into the same
trap. ${foo} is the same as $foo, the brackets are useful when interpolating. ${"foo"} is actually
a look up to $main::{} which is the main symbol table, as such only contains package scoped variables.
– Cosmicnet
Nov 22 '14 at 13:44
${"main::foo"}, ${"::foo"}, and $main::foo are the same as ${"foo"}. The shorthand is package
sensitive perl -e "package test; our $foo = 'bar'; print $foo; ${\"foo\"} = 'baz'; print
$foo" works, as in this context ${"foo"} is now equal to ${"test::foo"}.
Of Symbol Tables and Globs
has some information on it, as does the Advanced Perl programming book. Sorry for my previous
mistake. – Cosmicnet
Nov 22 '14 at 13:57
print "package is: " . __PACKAGE__ . "\n";
our $test = 1;
print "trying to print global var from main package: $test\n";
package Changed;
{
my $test = 10;
my $test1 = 11;
print "trying to print local vars from a closed block: $test, $test1\n";
}
&Check_global;
sub Check_global {
print "trying to print global var from a function: $test\n";
}
print "package is: " . __PACKAGE__ . "\n";
print "trying to print global var outside the func and from \"Changed\" package: $test\n";
print "trying to print local var outside the block $test1\n";
Will Output this:
package is: main
trying to print global var from main package: 1
trying to print local vars from a closed block: 10, 11
trying to print global var from a function: 1
package is: Changed
trying to print global var outside the func and from "Changed" package: 1
trying to print local var outside the block
In case using "use strict" will get this failure while attempting to run the script:
Global symbol "$test1" requires explicit package name at ./check_global.pl line 24.
Execution of ./check_global.pl aborted due to compilation errors.
in simple words: Our (as the name sais) is a variable decliration to use that variable from any
place in the script (function, block etc ...), every variable by default (in case not declared)
belong to "main" package, our variable still can be used even after decliration of another package
in the script. "my" variable in case declared in a block or function, can be used in that block/function
only. in case "my" variable was declared not closed in a block, it can be used any where in the
scriot, in a closed block as well or in a function as "our" variable, but can't used in case package
changed – Lavi
Buchnik
Sep 6 '14 at 20:08
My script above shows that by default we are in the "main" package, then the script print an "our"
variable from "main" package (not closed in a block), then we declare two "my" variables in a
function and print them from that function. then we print an "our" variable from another function
to show it can be used in a function. then we changing the package to "changed" (not "main" no
more), and we print again the "our" variable successfully. then trying to print a "my" variable
outside of the function and failed. the script just showing the difference between "our" and "my"
usage. – Lavi Buchnik
Sep 6 '14 at 20:13
#!/usr/local/bin/perl
use feature ':5.10';
#use warnings;
package a;
{
my $b = 100;
our $a = 10;
print "$a \n";
print "$b \n";
}
package b;
#my $b = 200;
#our $a = 20 ;
print "in package b value of my b $a::b \n";
print "in package b value of our a $a::a \n";
This explains the difference between my and our. The my variable goes out of scope outside the
curly braces and is garbage collected but the our variable still lives. –
Yugdev
Nov 5 '15 at 14:03
#!/usr/bin/perl -l
use strict;
# if string below commented out, prints 'lol' , if the string enabled, prints 'eeeeeeeee'
#my $lol = 'eeeeeeeeeee' ;
# no errors or warnings at any case, despite of 'strict'
our $lol = eval {$lol} || 'lol' ;
print $lol;
Can you explain what this code is meant to demonstrate? Why are our and my
different? How does this example show it? –
Nathan Fellman
May 16 '13 at 11:07
Let us think what an interpreter actually is: it's a piece of code that stores values in memory
and lets the instructions in a program that it interprets access those values by their names,
which are specified inside these instructions. So, the big job of an interpreter is to shape the
rules of how we should use the names in those instructions to access the values that the interpreter
stores.
On encountering "my", the interpreter creates a lexical variable: a named value that the interpreter
can access only while it executes a block, and only from within that syntactic block. On encountering
"our", the interpreter makes a lexical alias of a package variable: it binds a name, which the
interpreter is supposed from then on to process as a lexical variable's name, until the block
is finished, to the value of the package variable with the same name.
The effect is that you can then pretend that you're using a lexical variable and bypass the
rules of 'use strict' on full qualification of package variables. Since the interpreter automatically
creates package variables when they are first used, the side effect of using "our" may also be
that the interpreter creates a package variable as well. In this case, two things are created:
a package variable, which the interpreter can access from everywhere, provided it's properly designated
as requested by 'use strict' (prepended with the name of its package and two colons), and
its lexical alias.
Often in programs we would like to have symbols that represent a constant value. Symbols
that we can set to a specific values once, and be sure they never change. As with many other
problems, there are several ways to solve this in Perl, but in most cases enforcement of
"constantness" is not necessary.
In most cases we can just adhere to the established consensus, that variables with all
upper-case names should be treated as constants .
Later we'll see a couple of solutions that actually enforce the "constantness" of the
variables, but for most purposes, having a variable in upper case is enough.
Treat
upper-case variables as constants
We declare and set the values just as we'd do with any other variable in Perl:
use strict
use warnings
use 5.010
my $SPEED_OF_LIGHT 299 _792_458 # m/s
my DATA
Mercury => 0.4 0.055 ],
Venus => 0.7 0.815 ],
Earth => ],
Mars => 1.5 0.107 ],
Ceres => 2.77 0.00015 ],
Jupiter => 5.2 318 ],
Saturn => 9.5 95 ],
Uranus => 19.6 14 ],
Neptune => 30 17 ],
Pluto => 39 0.00218 ],
Charon => 39 0.000254 ],
);
my @PLANETS sort keys DATA
Each planet in the Solar
System has two values. The first is their average distance from the Sun and the second is
their mass, relative to the Earth.
Once the values are initially set, they should NOT be
changed. Nothing enforces it, besides a secret agreement among Perl programmers and
Astronomers.
say join ', ' @PLANETS
say $SPEED_OF_LIGHT
$SPEED_OF_LIGHT 300 _000_000
say "The speed of light is now $SPEED_OF_LIGHT"
We can use these "constants" in the same way as we would use any variable. We could even
change the values, but it is not recommended.
Besides its simplicity, one of the nice things in this solution is that we can actually
compute the values of these constants during run time, as we did with the @PLANETS array.
In many cases this is enough and the cost of creating "real" constants is
unnecessary.
Nevertheless, let's see two other solutions:
The Readonly module
The Readonly module from
CPAN allow us to designate some of our "variables" to be read-only. Effectively turning them
into constants.
use strict
use warnings
use 5.010
use Readonly
Readonly my $SPEED_OF_LIGHT => 299 _792_458 # m/s
Readonly my DATA =>
Mercury => 0.4 0.055 ],
Venus => 0.7 0.815 ],
Earth => ],
Mars => 1.5 0.107 ],
Ceres => 2.77 0.00015 ],
Jupiter => 5.2 318 ],
Saturn => 9.5 95 ],
Uranus => 19.6 14 ],
Neptune => 30 17 ],
Pluto => 39 0.00218 ],
Charon => 39 0.000254 ],
);
Readonly my @PLANETS => sort keys DATA
The declaration of the read-only variables (our constants) is very similar to what happens
with regular variables, except that we precede each declaration with the Readonly keyword, and
instead of assignment , we separate the name of the variable and their values by a fat-arrow:
=>
While the names of the read-only variables can be in any case, it is recommended to only use
UPPER-CASE names, to make it easy for the reader of the code to recognize them, even without
looking at the declaration.
Readonly allows us to create constants during the run-time as we have done above with the
@PLANETS array.
say join ', ' @PLANETS
say "The speed of light is $SPEED_OF_LIGHT"
$SPEED_OF_LIGHT 300 _000_000
say "The speed of light is now $SPEED_OF_LIGHT"
If we run the above code, we'll get an exception that says: Modification of a read-only
value attempted at ... at the line where we tried to assign the new value to the
$SPEED_OF_LIGHT
The same would have happened if we attempted to change one of the internal values such as
either of these:
$DATA Sun 'big'
$DATA Mercury }[
The biggest drawback of Readonly, is its relatively slow performance.
Readonly::XS
There is also the Readonly::XS module that can be installed. One does
not need to make any changes to their code, once the use Readonly; statement notices that
Readonly::XS is also installed, the latter will be used to provide a speed
improvement.
The constant pragma
Perl comes with the constant pragma that can create constants.
The constants themselves can only hold scalars or references to complex data structure
(arrays and hashes). The names of the constants do not have any sigils in front of them. The
names can be any case, but even in the documentation of constant all the examples use upper case, and it is
probably better to stick to that style for clarity.
use strict
use warnings
use 5.010
use constant SPEED_OF_LIGHT => 299 _792_458 # m/s
use constant DATA =>
Mercury => 0.4 0.055 ],
Venus => 0.7 0.815 ],
Earth => ],
Mars => 1.5 0.107 ],
Ceres => 2.77 0.00015 ],
Jupiter => 5.2 318 ],
Saturn => 9.5 95 ],
Uranus => 19.6 14 ],
Neptune => 30 17 ],
Pluto => 39 0.00218 ],
Charon => 39 0.000254 ],
};
use constant PLANETS => sort keys %{ DATA () ];
Creating a constant with a scalar value, such as the SPEED_OF_LIGHT is easy. We just need to
use the constant pragma. We cannot create a constant hash, but we can create a constant
reference to an anonymous hash. The difficulty comes when we would like to use it as a real
hash. We need to dereference it using the %{ } construct, but in order to make it work we have
to put a pair of parentheses after the name DATA .
This might look strange, but the reason is that the constant actually creates functions with
the given names, that return the fixed values. In the above case use constant DATA ... created
a function called DATA()
We don't have to always use the parentheses. For example we can write:
say SPEED_OF_LIGHT
and that will work. On the other hand the following code will print The speed of light is
now SPEED_OF_LIGHT . Because these constants don't have sigils, they cannot interpolate in a
string.
say "The speed of light is now SPEED_OF_LIGHT"
If we try to modify the constant:
SPEED_OF_LIGHT 300 _000_000
we get an exception: Can't modify constant item in scalar assignment at ... . but we can
re-declare them:
use constant SPEED_OF_LIGHT => 300 _000_000 # m/s
say SPEED_OF_LIGHT
that will print 300000000. It will give a warning Constant subroutine main::SPEED_OF_LIGHT
redefined only if we have use warnings; enabled.
So the constant pragma does not fully protect us from changing the "constant".
Note, fetching the values from a constant that holds a reference to an array also requires
the parentheses again, and the de-referencing construct:
One of the first Perl operators to learn is the "dot" concatenation
operator (.) for strings. For example:
my $string = 'foo' . 'bar';
# $string is 'foobar'.
On the other hand, if you have an array of strings @arr, then you can concatenate them by
joining them with an empty string in-between:
my $string = join('', @arr);
But what if you just want 10 "foo"s in a line? You might try the Python approach with 'foo' *
10 but Perl with its type conversion on the fly will try to convert 'foo' into a number and say
something like:
Argument "foo" isn't numeric in multiplication (*) at...
Instead you should use the repetition operator (x) which takes a string on the left and
a number on the right:
my $string = 'foo' x 10;
and $string is then
foofoofoofoofoofoofoofoofoofoo
Note that even if you have integers on both sides, the 'x' repetition operator will cast the
left operand into a string so that:
my $str = 20 x 10;
# $str is "2020202020202020202020"
Now this isn't all the repetition operator is good for - it can also be used for repetition of
lists. For example:
('x','y','z') x 10
evaluates as:
('x','y','z','x','y','z','x','y', ...)
But be
warned : if the left operand is not enclosed in parentheses it is treated as a scalar.
my @arr = ('x', 'y', 'z');
my @bar = @arr x 10;
is equivalent to
my @bar = scalar(@arr) x 10;
# @bar is an array of a single integer (3333333333)
while, turning the array into a list of its elements by enclosing it in parentheses:
my @foo = ( (@arr) x 10 );
# then @foo is ('x','y','z','x','y','z','x','y', ...)
In summary, if you remember that 'x' is different to '*' and lists are treated differently to
scalars, it's less likely your code will give you an unpleasant surprise!
"... Usually when we use split , we provide a regex. Whatever the regex matches is "thrown away" and the pieces between these matches are returned. In this example we get back the two strings even including the spaces. ..."
"... If however, the regex contains capturing parentheses, then whatever they captured will be also included in the list of returned strings. In this case the string of digits '23' is also included. ..."
Usually when we use split ,
we provide a regex. Whatever the regex matches is "thrown away" and the pieces between these
matches are returned. In this example we get back the two strings even including the spaces.
examples/split_str.pl
use strict
use warnings
use Data :: Dumper qw Dumper );
my $str "abc 23 def"
my @pieces split \d +/, $str
print Dumper \@pieces
$VAR1 = [
'abc ',
' def'
];
If however, the regex contains capturing parentheses, then whatever they captured will be
also included in the list of returned strings. In this case the string of digits '23' is also
included.
examples/split_str_retain.pl
use strict
use warnings
use Data :: Dumper qw Dumper );
my $str "abc 23 def"
my @pieces split /( \d +)/, $str
print Dumper \@pieces
$VAR1 = [
'abc ',
'23',
' def'
];
Only what is captured is retained
Remember, not the separator, the substring that was matched, will be retained, but whatever
is in the capturing parentheses.
examples/split_str_multiple.pl
use strict
use warnings
use Data :: Dumper qw Dumper );
my $str "abc 2=3 def "
my @pieces split /( \d +)=( \d +)/, $str
print Dumper \@pieces
$VAR1 = [
'abc ',
'2',
'3',
' def '
];
In this example the sign is not in the resulting list.
Coming to Perl from PHP can be confusing because of the apparently similar and yet quite different
ways the two languages use the $ identifier as a variable prefix. If you're accustomed
to PHP, you'll be used to declaring variables of various types, using the $ prefix each
time:
So when you begin working in Perl, you're lulled into a false sense of security because it looks
like you can do the same thing with any kind of data:
#!/usr/bin/perl
my $string = "string";
my $integer = 6;
my $float = 1.337;
my $object = Object->new();
But then you start dealing with arrays and hashes, and suddenly everything stops working the way
you expect. Consider this snippet:
#!/usr/bin/perl
my $array = (1, 2, 3);
print $array. "\n";
That's perfectly valid syntax. However, when you run it, expecting to see all your elements or
at least an Array like in PHP, you get the output of "3". Not being able to assign a
list to a scalar, Perl just gives you the last item of the list instead. You sit there confused,
wondering how on earth Perl could think that's what you meant.
References in PHP
In PHP, every identifier is a reference, or pointer, towards some underlying data. PHP handles
the memory management for you. When you declare an object, PHP writes the data into memory, and puts
into the variable you define a reference to that data. The variable is not the data itself,
it's just a pointer to it. To oversimplify things a bit, what's actually stored in your $variable
is an address in memory, and not a sequence of data.
When PHP manages all this for you and you're writing basic programs, you don't really notice,
because any time you actually use the value it gets dereferenced implicitly, meaning that
PHP will use the data that the variable points to. So when you write something like:
$string = "string";
print $string;
The output you get is "string", and not "0x00abf0a9", which might be the "real" value of
$string as an address in memory. In this way, PHP is kind of coddling you a bit. In fact,
if you actually want two identifiers to point to the same piece of data rather than making
a copy in memory, you have to use a special reference syntax:
$string2 = &$string1;
Perl and C programmers aren't quite as timid about hiding references, because being able to manipulate
them a bit more directly turns out to be very useful for writing quick, clean code, in particular
for conserving memory and dealing with state intelligently.
References in Perl
In Perl, you have three basic data types; scalars arrays , and hashes . These all use different
identifiers; Scalars use $ , arrays use @ , and hashes use %
.
my $string = "string";
my $integer = 0;
my $float = 0.0;
my @array = (1,2,3);
my %hash = (name => "Tom Ryder",
blog => "Arabesque");
So scalars can refer directly to data in the way you're accustomed to in PHP, but they can also
be references to any other kind of data. For example, you could write:
my $string = "string";
my $copy = $string;
my $reference = \$string;
The value of both $string and $copy , when printed, will be "string",
as you might expect. However, the $reference scalar becomes a reference to
the data stored in $string , and when printed out would give something like SCALAR(0x2160718)
. Similarly, you can define a scalar as a reference to an array or hash:
my @array = (1,2,3);
my $arrayref = \@array;
my %hash = (name => "Tom Ryder",
blog => "Arabesque");
my $hashref = \%hash;
There are even shorthands for doing this, if you want to declare a reference and the data it references
inline. For array references, you use square brackets, and for hash references, curly brackets:
I'll answer seriously. I do not know of any program to translate a shell script into Perl,
and I doubt any interpreter module would provide the performance benefits. So I'll give an
outline of how I would go about it.
Now, you want to reuse your code as much as possible. In that case, I suggest selecting
pieces of that code, write a Perl version of that, and then call the Perl script from the
main script. That will enable you to do the conversion in small steps, assert that the
converted part is working, and improve gradually your Perl knowledge.
As you can call outside programs from a Perl script, you can even replace some bigger
logic with Perl, and call smaller shell scripts (or other commands) from Perl to do something
you don't feel comfortable yet to convert. So you'll have a shell script calling a perl
script calling another shell script. And, in fact, I did exactly that with my own very first
Perl script.
Of course, it's important to select well what to convert. I'll explain, below, how many
patterns common in shell scripts are written in Perl, so that you can identify them inside
your script, and create replacements by as much cut&paste as possible.
First, both Perl scripts and Shell scripts are code+functions. Ie, anything which is not a
function declaration is executed in the order it is encountered. You don't need to declare
functions before use, though. That means the general layout of the script can be preserved,
though the ability to keep things in memory (like a whole file, or a processed form of it)
makes it possible to simplify tasks.
A Perl script, in Unix, starts with something like this:
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
#other libraries
(rest of the code)
The first line, obviously, points to the commands to be used to run the script, just like
normal shells do. The following two "use" lines make then language more strict, which should
decrease the amount of bugs you encounter because you don't know the language well (or plain
did something wrong). The third use line imports the "Dumper" function of the "Data" module.
It's useful for debugging purposes. If you want to know the value of an array or hash table,
just print Dumper(whatever).
Note also that comments are just like shell's, lines starting with "#".
Now, you call external programs and pipe to or pipe from them. For example:
open THIS, "cat $ARGV[0] |";
That will run cat, passing " $ARGV[0] ", which would be $1 on shell -- the
first argument passed to it. The result of that will be piped into your Perl script through
"THIS", which you can use to read that from it, as I'll show later.
You can use "|" at the beginning or end of line, to indicate the mode "pipe to" or "pipe
from", and specify a command to be run, and you can also use ">" or ">>" at the
beginning, to open a file for writing with or without truncation, "<" to explicitly
indicate opening a file for reading (the default), or "+<" and "+>" for read and write.
Notice that the later will truncate the file first.
Another syntax for "open", which will avoid problems with files with such characters in
their names, is having the opening mode as a second argument:
open THIS, "-|", "cat $ARGV[0]";
This will do the same thing. The mode "-|" stands for "pipe from" and "|-" stands for
"pipe to". The rest of the modes can be used as they were ( >, >>, <,
+>, +< ). While there is more than this to open, it should suffice for most
things.
But you should avoid calling external programs as much as possible. You could open the
file directly, by doing open THIS, "$ARGV[0]"; , for example, and have much
better performance.
So, what external programs you could cut out? Well, almost everything. But let's stay with
the basics: cat, grep, cut, head, tail, uniq, wc, sort.
CAT
Well, there isn't much to be said about this one. Just remember that, if possible, read
the file only once and keep it in memory. If the file is huge you won't do that, of course,
but there are almost always ways to avoid reading a file more than once.
Anyway, the basic syntax for cat would be:
my $filename = "whatever";
open FILE, "$filename" or die "Could not open $filename!\n";
while(<FILE>) {
print $_;
}
close FILE;
This opens a file, and prints all it's contents (" while(<FILE>) " will
loop until EOF, assigning each line to " $_ "), and close it again.
If I wanted to direct the output to another file, I could do this:
my $filename = "whatever";
my $anotherfile = "another";
open (FILE, "$filename") || die "Could not open $filename!\n";
open OUT, ">", "$anotherfile" or die "Could not open $anotherfile for writing!\n";
while(<FILE>) {
print OUT $_;
}
close FILE;
This will print the line to the file indicated by " OUT ". You can use
STDIN , STDOUT and STDERR in the appropriate places as
well, without having to open them first. In fact, " print " defaults to
STDOUT , and " die " defaults to " STDERR ".
Notice also the " or die ... " and " || die ... ". The operators
or and || means it will only execute the following command if the
first returns false (which means empty string, null reference, 0, and the like). The die
command stops the script with an error message.
The main difference between " or " and " || " is priority. If "
or " was replaced by " || " in the examples above, it would not
work as expected, because the line would be interpreted as:
open FILE, ("$filename" || die "Could not open $filename!\n");
Which is not at all what is expected. As " or " has a lower priority, it
works. In the line where " || " is used, the parameters to open are
passed between parenthesis, making it possible to use " || ".
Alas, there is something which is pretty much what cat does:
while(<>) {
print $_;
}
That will print all files in the command line, or anything passed through STDIN.
GREP
So, how would our "grep" script work? I'll assume "grep -E", because that's easier in Perl
than simple grep. Anyway:
my $pattern = $ARGV[0];
shift @ARGV;
while(<>) {
print $_ if /$pattern/o;
}
The "o" passed to $patttern instructs Perl to compile that pattern only once, thus gaining
you speed. Not the style "something if cond". It means it will only execute "something" if
the condition is true. Finally, " /$pattern/ ", alone, is the same as " $_
=~ m/$pattern/ ", which means compare $_ with the regex pattern indicated. If you want
standard grep behavior, ie, just substring matching, you could write:
print $_ if $_ =~ "$pattern";
CUT
Usually, you do better using regex groups to get the exact string than cut. What you would
do with "sed", for instance. Anyway, here are two ways of reproducing cut:
That will get you the fourth column of every line, using "," as separator. Note
@array and $array[3] . The @ sigil means "array"
should be treated as an, well, array. It will receive an array composed of each column in the
currently processed line. Next, the $ sigil means array[3] is a
scalar value. It will return the column you are asking for.
This is not a good implementation, though, as "split" will scan the whole string. I once
reduced a process from 30 minutes to 2 seconds just by not using split -- the lines where
rather large, though. Anyway, the following has a superior performance if the lines are
expected to be big, and the columns you want are low:
while(<>) {
my ($column) = /^(?:[^,]*,){3}([^,]*),/;
print $column, "\n";
}
This leverages regular expressions to get the desired information, and only that.
If you want positional columns, you can use:
while(<>) {
print substr($_, 5, 10), "\n";
}
Which will print 10 characters starting from the sixth (again, 0 means the first
character).
HEAD
This one is pretty simple:
my $printlines = abs(shift);
my $lines = 0;
my $current;
while(<>) {
if($ARGV ne $current) {
$lines = 0;
$current = $ARGV;
}
print "$_" if $lines < $printlines;
$lines++;
}
Things to note here. I use "ne" to compare strings. Now, $ARGV will always point to the
current file, being read, so I keep track of them to restart my counting once I'm reading a
new file. Also note the more traditional syntax for "if", right along with the post-fixed
one.
I also use a simplified syntax to get the number of lines to be printed. When you use
"shift" by itself it will assume "shift @ARGV". Also, note that shift, besides modifying
@ARGV, will return the element that was shifted out of it.
As with a shell, there is no distinction between a number and a string -- you just use it.
Even things like "2"+"2" will work. In fact, Perl is even more lenient, cheerfully treating
anything non-number as a 0, so you might want to be careful there.
This script is very inefficient, though, as it reads ALL file, not only the required
lines. Let's improve it, and see a couple of important keywords in the process:
my $printlines = abs(shift);
my @files;
if(scalar(@ARGV) == 0) {
@files = ("-");
} else {
@files = @ARGV;
}
for my $file (@files) {
next unless -f $file && -r $file;
open FILE, "<", $file or next;
my $lines = 0;
while(<FILE>) {
last if $lines == $printlines;
print "$_";
$lines++;
}
close FILE;
}
The keywords "next" and "last" are very useful. First, "next" will tell Perl to go back to
the loop condition, getting the next element if applicable. Here we use it to skip a file
unless it is truly a file (not a directory) and readable. It will also skip if we couldn't
open the file even then.
Then "last" is used to immediately jump out of a loop. We use it to stop reading the file
once we have reached the required number of lines. It's true we read one line too many, but
having "last" in that position shows clearly that the lines after it won't be executed.
There is also "redo", which will go back to the beginning of the loop, but without
reevaluating the condition nor getting the next element.
TAIL
I'll do a little trick here.
my $skiplines = abs(shift);
my @lines;
my $current = "";
while(<>) {
if($ARGV ne $current) {
print @lines;
undef @lines;
$current = $ARGV;
}
push @lines, $_;
shift @lines if $#lines == $skiplines;
}
print @lines;
Ok, I'm combining "push", which appends a value to an array, with "shift", which takes
something from the beginning of an array. If you want a stack, you can use push/pop or
shift/unshift. Mix them, and you have a queue. I keep my queue with at most 10 elements with
$#lines which will give me the index of the last element in the array. You could
also get the number of elements in @lines with scalar(@lines) .
UNIQ
Now, uniq only eliminates repeated consecutive lines, which should be easy with what you
have seen so far. So I'll eliminate all of them:
my $current = "";
my %lines;
while(<>) {
if($ARGV ne $current) {
undef %lines;
$current = $ARGV;
}
print $_ unless defined($lines{$_});
$lines{$_} = "";
}
Now here I'm keeping the whole file in memory, inside %lines . The use of the
% sigil indicates this is a hash table. I'm using the lines as keys, and storing
nothing as value -- as I have no interest in the values. I check where the key exist with
"defined($lines{$_})", which will test if the value associated with that key is defined or
not; the keyword "unless" works just like "if", but with the opposite effect, so it only
prints a line if the line is NOT defined.
Note, too, the syntax $lines{$_} = "" as a way to store something in a hash
table. Note the use of {} for hash table, as opposed to [] for
arrays.
WC
This will actually use a lot of stuff we have seen:
my $current;
my %lines;
my %words;
my %chars;
while(<>) {
$lines{"$ARGV"}++;
$chars{"$ARGV"} += length($_);
$words{"$ARGV"} += scalar(grep {$_ ne ""} split /\s/);
}
for my $file (keys %lines) {
print "$lines{$file} $words{$file} $chars{$file} $file\n";
}
Three new things. Two are the "+=" operator, which should be obvious, and the "for"
expression. Basically, a "for" will assign each element of the array to the variable
indicated. The "my" is there to declare the variable, though it's unneeded if declared
previously. I could have an @array variable inside those parenthesis. The "keys %lines"
expression will return as an array they keys (the filenames) which exist for the hash table
"%lines". The rest should be obvious.
The third thing, which I actually added only revising the answer, is the "grep". The
format here is:
grep { code } array
It will run "code" for each element of the array, passing the element as "$_". Then grep
will return all elements for which the code evaluates to "true" (not 0, not "", etc). This
avoids counting empty strings resulting from consecutive spaces.
Similar to "grep" there is "map", which I won't demonstrate here. Instead of filtering, it
will return an array formed by the results of "code" for each element.
SORT
Finally, sort. This one is easy too:
my @lines;
my $current = "";
while(<>) {
if($ARGV ne $current) {
print sort @lines;
undef @lines;
$current = $ARGV;
}
push @lines, $_;
}
print sort @lines;
Here, "sort" will sort the array. Note that sort can receive a function to define the
sorting criteria. For instance, if I wanted to sort numbers I could do this:
Here " $a " and " $b " receive the elements to be compared. "
<=> " returns -1, 0 or 1 depending on whether the number is less than,
equal to or greater than the other. For strings, "cmp" does the same thing.
HANDLING FILES, DIRECTORIES & OTHER STUFF
As for the rest, basic mathematical expressions should be easy to understand. You can test
certain conditions about files this way:
for my $file (@ARGV) {
print "$file is a file\n" if -f "$file";
print "$file is a directory\n" if -d "$file";
print "I can read $file\n" if -r "$file";
print "I can write to $file\n" if -w "$file";
}
I'm not trying to be exaustive here, there are many other such tests. I can also do "glob"
patterns, like shell's "*" and "?", like this:
for my $file (glob("*")) {
print $file;
print "*" if -x "$file" && ! -d "$file";
print "/" if -d "$file";
print "\t";
}
If you combined that with "chdir", you can emulate "find" as well:
sub list_dir($$) {
my ($dir, $prefix) = @_;
my $newprefix = $prefix;
if ($prefix eq "") {
$newprefix = $dir;
} else {
$newprefix .= "/$dir";
}
chdir $dir;
for my $file (glob("*")) {
print "$prefix/" if $prefix ne "";
print "$dir/$file\n";
list_dir($file, $newprefix) if -d "$file";
}
chdir "..";
}
list_dir(".", "");
Here we see, finally, a function. A function is declared with the syntax:
sub name (params) { code }
Strictly speakings, "(params)" is optional. The declared parameter I used, "
($$) ", means I'm receiving two scalar parameters. I could have " @
" or " % " in there as well. The array " @_ " has all the
parameters passed. The line " my ($dir, $prefix) = @_ " is just a simple way of
assigning the first two elements of that array to the variables $dir and
$prefix .
This function does not return anything (it's a procedure, really), but you can have
functions which return values just by adding " return something; " to it, and
have it return "something".
The rest of it should be pretty obvious.
MIXING EVERYTHING
Now I'll present a more involved example. I'll show some bad code to explain what's wrong
with it, and then show better code.
For this first example, I have two files, the names.txt file, which names and phone
numbers, the systems.txt, with systems and the name of the responsible for them. Here they
are:
names.txt
John Doe, (555) 1234-4321
Jane Doe, (555) 5555-5555
The Boss, (666) 5555-5555
systems.txt
Sales, Jane Doe
Inventory, John Doe
Payment, That Guy
I want, then, to print the first file, with the system appended to the name of the person,
if that person is responsible for that system. The first version might look like this:
#!/usr/bin/perl
use strict;
use warnings;
open FILE, "names.txt";
while(<FILE>) {
my ($name) = /^([^,]*),/;
my $system = get_system($name);
print $_ . ", $system\n";
}
close FILE;
sub get_system($) {
my ($name) = @_;
my $system = "";
open FILE, "systems.txt";
while(<FILE>) {
next unless /$name/o;
($system) = /([^,]*)/;
}
close FILE;
return $system;
}
This code won't work, though. Perl will complain that the function was used too early for
the prototype to be checked, but that's just a warning. It will give an error on line 8 (the
first while loop), complaining about a readline on a closed filehandle. What happened here is
that " FILE " is global, so the function get_system is changing it.
Let's rewrite it, fixing both things:
#!/usr/bin/perl
use strict;
use warnings;
sub get_system($) {
my ($name) = @_;
my $system = "";
open my $filehandle, "systems.txt";
while(<$filehandle>) {
next unless /$name/o;
($system) = /([^,]*)/;
}
close $filehandle;
return $system;
}
open FILE, "names.txt";
while(<FILE>) {
my ($name) = /^([^,]*),/;
my $system = get_system($name);
print $_ . ", $system\n";
}
close FILE;
This won't give any error or warnings, nor will it work. It returns just the sysems, but
not the names and phone numbers! What happened? Well, what happened is that we are making a
reference to " $_ " after calling get_system , but, by reading the
file, get_system is overwriting the value of $_ !
To avoid that, we'll make $_ local inside get_system . This will
give it a local scope, and the original value will then be restored once returned from
get_system :
#!/usr/bin/perl
use strict;
use warnings;
sub get_system($) {
my ($name) = @_;
my $system = "";
local $_;
open my $filehandle, "systems.txt";
while(<$filehandle>) {
next unless /$name/o;
($system) = /([^,]*)/;
}
close $filehandle;
return $system;
}
open FILE, "names.txt";
while(<FILE>) {
my ($name) = /^([^,]*),/;
my $system = get_system($name);
print $_ . ", $system\n";
}
close FILE;
And that still doesn't work! It prints a newline between the name and the system. Well,
Perl reads the line including any newline it might have. There is a neat command which will
remove newlines from strings, " chomp ", which we'll use to fix this problem.
And since not every name has a system, we might, as well, avoid printing the comma when that
happens:
#!/usr/bin/perl
use strict;
use warnings;
sub get_system($) {
my ($name) = @_;
my $system = "";
local $_;
open my $filehandle, "systems.txt";
while(<$filehandle>) {
next unless /$name/o;
($system) = /([^,]*)/;
}
close $filehandle;
return $system;
}
open FILE, "names.txt";
while(<FILE>) {
my ($name) = /^([^,]*),/;
my $system = get_system($name);
chomp;
print $_;
print ", $system" if $system ne "";
print "\n";
}
close FILE;
That works, but it also happens to be horribly inefficient. We read the whole systems file
for every line in the names file. To avoid that, we'll read all data from systems once, and
then use that to process names.
Now, sometimes a file is so big you can't read it into memory. When that happens, you
should try to read into memory any other file needed to process it, so that you can
do everything in a single pass for each file. Anyway, here is the first optimized version of
it:
#!/usr/bin/perl
use strict;
use warnings;
our %systems;
open SYSTEMS, "systems.txt";
while(<SYSTEMS>) {
my ($system, $name) = /([^,]*),(.*)/;
$systems{$name} = $system;
}
close SYSTEMS;
open NAMES, "names.txt";
while(<NAMES>) {
my ($name) = /^([^,]*),/;
chomp;
print $_;
print ", $systems{$name}" if defined $systems{$name};
print "\n";
}
close NAMES;
Unfortunately, it doesn't work. No system ever appears! What has happened? Well, let's
look into what " %systems " contains, by using Data::Dumper :
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
our %systems;
open SYSTEMS, "systems.txt";
while(<SYSTEMS>) {
my ($system, $name) = /([^,]*),(.*)/;
$systems{$name} = $system;
}
close SYSTEMS;
print Dumper(%systems);
open NAMES, "names.txt";
while(<NAMES>) {
my ($name) = /^([^,]*),/;
chomp;
print $_;
print ", $systems{$name}" if defined $systems{$name};
print "\n";
}
close NAMES;
The output will be something like this:
$VAR1 = ' Jane Doe';
$VAR2 = 'Sales';
$VAR3 = ' That Guy';
$VAR4 = 'Payment';
$VAR5 = ' John Doe';
$VAR6 = 'Inventory';
John Doe, (555) 1234-4321
Jane Doe, (555) 5555-5555
The Boss, (666) 5555-5555
Those $VAR1/$VAR2/etc is how Dumper displays a hash table. The
odd numbers are the keys, and the succeeding even numbers are the values. Now we can see that
each name in %systems has a preceeding space! Silly regex mistake, let's fix
it:
#!/usr/bin/perl
use strict;
use warnings;
our %systems;
open SYSTEMS, "systems.txt";
while(<SYSTEMS>) {
my ($system, $name) = /^\s*([^,]*?)\s*,\s*(.*?)\s*$/;
$systems{$name} = $system;
}
close SYSTEMS;
open NAMES, "names.txt";
while(<NAMES>) {
my ($name) = /^\s*([^,]*?)\s*,/;
chomp;
print $_;
print ", $systems{$name}" if defined $systems{$name};
print "\n";
}
close NAMES;
So, here, we are aggressively removing any spaces from the beginning or end of name and
system. There are other ways to form that regex, but that's beside the point. There is still
one problem with this script, which you'll have seen if your "names.txt" and/or "systems.txt"
files have an empty line at the end. The warnings look like this:
Use of uninitialized value in hash element at ./exemplo3e.pl line 10, <SYSTEMS> line 4.
Use of uninitialized value in hash element at ./exemplo3e.pl line 10, <SYSTEMS> line 4.
John Doe, (555) 1234-4321, Inventory
Jane Doe, (555) 5555-5555, Sales
The Boss, (666) 5555-5555
Use of uninitialized value in hash element at ./exemplo3e.pl line 19, <NAMES> line 4.
What happened here is that nothing went into the " $name " variable when the
empty line was processed. There are many ways around that, but I choose the following:
#!/usr/bin/perl
use strict;
use warnings;
our %systems;
open SYSTEMS, "systems.txt" or die "Could not open systems.txt!";
while(<SYSTEMS>) {
my ($system, $name) = /^\s*([^,]+?)\s*,\s*(.+?)\s*$/;
$systems{$name} = $system if defined $name;
}
close SYSTEMS;
open NAMES, "names.txt" or die "Could not open names.txt!";
while(<NAMES>) {
my ($name) = /^\s*([^,]+?)\s*,/;
chomp;
print $_;
print ", $systems{$name}" if defined($name) && defined($systems{$name});
print "\n";
}
close NAMES;
The regular expressions now require at least one character for name and system, and we
test to see if " $name " is defined before we use it.
CONCLUSION
Well, then, these are the basic tools to translate a shell script. You can do MUCH more
with Perl, but that was not your question, and it wouldn't fit here anyway.
Just as a basic overview of some important topics,
A Perl script that might be attacked by hackers need to be run with the -T option, so
that Perl will complain about any vulnerable input which has not been properly
handled.
There are libraries, called modules, for database accesses, XML&cia handling,
Telnet, HTTP & other protocols. In fact, there are miriads of modules which can be
found at CPAN .
As mentioned by someone else, if you make use of AWK or SED, you can translate those
into Perl with A2P and S2P .
Perl can be written in an Object Oriented way.
There are multiple versions of Perl. As of this writing, the stable one is 5.8.8 and
there is a 5.10.0 available. There is also a Perl 6 in development, but experience has
taught everyone not to wait too eagerly for it.
There is a free, good, hands-on, hard & fast book about Perl called Learning Perl The Hard Way . It's style is
similar to this very answer. It might be a good place to go from here.
I hope this helped.
DISCLAIMER
I'm NOT trying to teach Perl, and you will need to have at least some reference
material. There are guidelines to good Perl habits, such as using " use strict;
" and " use warnings; " at the beginning of the script, to make it less lenient
of badly written code, or using STDOUT and STDERR on the print lines, to indicate the correct
output pipe.
This is stuff I agree with, but I decided it would detract from the basic goal of showing
patterns for common shell script utilities.
When using the multiline quoting mechanism called a here document , the text
must be flush against the margin, which looks out of place in the code. You would like to
indent the here document text in the code, but not have the indentation appear in the
final string value. Solution
Use a s///
# all in one
($var = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGET
# or with two steps
$var = <<HERE_TARGET;
your text
goes here
HERE_TARGET
$var =~ s/^\s+//gm;
The substitution is straightforward. It removes leading whitespace from the text of
the here document. The /m modifier lets the ^ character match
at the start of each line in the string, and the /g modifier makes the
pattern matching engine repeat the substitution as often as it can (i.e., for every line
in the here document).
($definition = <<'FINIS') =~ s/^\s+//gm;
The five varieties of camelids
are the familiar camel, his friends
the llama and the alpaca, and the
rather less well-known guanaco
and vicuЯa.
FINIS
Be warned: all the patterns in this recipe use \s\s with
[^\S\n] in the patterns.
The substitution makes use of the property that the result of an assignment can be
used as the left-hand side of =~ . This lets us do it all in one line, but
it only works when you're assigning to a variable. When you're using the here document
directly, it would be considered a constant value and you wouldn't be able to modify it.
In fact, you can't change a here document's value unless you first put it into a
variable.
Not to worry, though, because there's an easy way around this, particularly if you're
going to do this a lot in the program. Just write a subroutine to do it:
sub fix {
my $string = shift;
$string =~ s/^\s+//gm;
return $string;
}
print fix(<<"END");
My stuff goes here
END
# With function predeclaration, you can omit the parens:
print fix <<"END";
My stuff goes here
END
As with all here documents, you have to place this here document's target (the token
that marks its end, END in this case) flush against the left-hand margin. If
you want to have the target indented also, you'll have to put the same amount of
whitespace in the quoted string as you use to indent the token.
($quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you would
deliver us. You are a liar, Saruman, and a corrupter of men's
hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s+--/\n--/; #move attribution to line of its own
If you're doing this to strings that contain code you're building up for an
eval , or just text to print out, you might not want to blindly strip off
all leading whitespace because that would destroy your indentation. Although
eval wouldn't care, your reader might.
Another embellishment is to use a special leading string for code that stands out. For
example, here we'll prepend each line with @@@ , properly indented:
if ($REMEMBER_THE_MAIN) {
$perl_main_C = dequote<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
@@@ while ( op = (*op->op_ppaddr)() ) ;
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
# add more code here if you want
}
Destroying indentation also gets you in trouble with poets.
sub dequote;
$poem = dequote<<EVER_ON_AND_ON;
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
EVER_ON_AND_ON
print "Here's your poem:\n\n$poem\n";
Here is its sample output:
Here's your poem:
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
The following dequote
sub dequote {
local $_ = shift;
my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}
If that pattern makes your eyes glaze over, you could always break it up and add
comments by adding /x :
if (m{
^ # start of line
\s * # 0 or more whitespace chars
(?: # begin first non-remembered grouping
( # begin save buffer $1
[^\w\s] # one byte neither space nor word
+ # 1 or more of such
) # end save buffer $1
( \s* ) # put 0 or more white in buffer $2
.* \n # match through the end of first line
) # end of first grouping
(?: # begin second non-remembered grouping
\s * # 0 or more whitespace chars
\1 # whatever string is destined for $1
\2 ? # what'll be in $2, but optionally
.* \n # match through the end of the line
) + # now repeat that group idea 1 or more
$ # until the end of the line
}x
)
{
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s{
^ # start of each line (due to /m)
\s * # any amount of leading whitespace
? # but minimally matched
$leader # our quoted, saved per-line leader
(?: # begin unremembered grouping
$white # the same amount
) ? # optionalize in case EOL after leader
}{}xgm;
There, isn't that much easier to read? Well, maybe not; sometimes it doesn't help to
pepper your code with insipid comments that mirror the code. This may be one of those
cases. See Also
If you're tempted to write multi-line output with multiple print() statements,
because that's what you're used to in some other language, consider using a HERE-document
instead.
Inspired by the here-documents in the Unix command line shells, Perl HERE-documents provide
a convenient way to handle the quoting of multi-line values.
So you can replace this:
print "Welcome to the MCG Carpark.\n";
print "\n";
print "There are currently 2,506 parking spaces available.\n";
print "Please drive up to a booth and collect a ticket.\n";
with this:
print <<'EOT';
Welcome to the MCG Carpark.
There are currently 2,506 parking spaces available.
Please drive up to a booth and collect a ticket.
EOT
The EOT in this example is an arbitrary string that you provide to indicate the
start and end of the text being quoted. The terminating string must appear on a line by
itself.
The usual Perl quoting conventions apply, so if you want to interpolate variables in a
here-document, use double quotes around your chosen terminating string:
print <<"EOT";
Welcome to the MCG Carpark.
There are currently $available_places parking spaces available.
Please drive up to booth and collect a ticket.
EOT
Note that whilst you can quote your terminator with " or ' , you
cannot use the equivalent qq() and q() operators. So this code is
invalid:
# This example will fail
print <<qq(EOT);
Welcome to the MCG Carpark.
There are currently $available_places parking spaces available.
Please drive up to booth and collect a ticket.
EOT
Naturally, all of the text you supply to a here-document is quoted by the starting and
ending strings. This means that any indentation you provide becomes part of the text that is
used. In this example, each line of the output will contain four leading spaces.
# Let's indent the text to be displayed. The leading spaces will be
# preserved in the output.
print <<"EOT";
Welcome to the MCG Carpark.
CAR PARK FULL.
EOT
The terminating string must appear on a line by itself, and it must have no whitespace
before or after it. In this example, the terminating string EOT is preceded by
four spaces, so Perl will not find it:
# Let's indent the following lines. This introduces an error
print <<"EOT";
Welcome to the MCG Carpark.
CAR PARK FULL.
EOT
Can't find string terminator "EOT" anywhere before EOF at ....
The here-document mechanism is just a generalized means of quoting text, so you can just as
easily use it in an assignment:
my $message = <<"EOT";
Welcome to the MCG Carpark.
CAR PARK FULL.
EOT
print $message;
And don't let the samples you've seen so far stop from considering the full range of
possibilities. The terminating tag doesn't have to appear at the end of a statement.
Here is an example from CPAN.pm that conditionally assigns some text to
$msg .
$msg = <<EOF unless $configpm =~ /MyConfig/;
# This is CPAN.pm's systemwide configuration file. This file provides
# defaults for users, and the values can be changed in a per-user
# configuration file. The user-config file is being looked for as
# ~/.cpan/CPAN/MyConfig.pm.
EOF
And this example from Module::Build::PPMMaker uses a here-document to construct the format
string for sprintf() :
I have two questions, the first one is most important:
How do I take 65 and turn it into A?
\'A converts an ASCII character to its value using printf. Is the syntax
specific to printf or is it used anywhere else in BASH? (Such small
strings are hard to Google for.)
For your second question, it seems the leading-quote syntax ( \'A ) is specific
to printf :
If the leading character is a single-quote or double-quote, the value shall be the
numeric value in the underlying codeset of the character following the single-quote or
double-quote.
You can use tr to convert from DOS to Unix; however, you can only do this safely
if CR appears in your file only as the first byte of a CRLF byte pair. This is usually the
case. You then use:
tr -d '\015' <DOS-file >UNIX-file
Note that the name DOS-file is different from the name UNIX-file
; if you try to use the same name twice, you will end up with no data in the file.
You can't do it the other way round (with standard 'tr').
If you know how to enter carriage return into a script ( control-V ,
control-M to enter control-M), then:
sed 's/^M$//' # DOS to Unix
sed 's/$/^M/' # Unix to DOS
where the '^M' is the control-M character. You can also use the bashANSI-C Quoting
mechanism to specify the carriage return:
sed $'s/\r$//' # DOS to Unix
sed $'s/$/\r/' # Unix to DOS
However, if you're going to have to do this very often (more than once, roughly speaking),
it is far more sensible to install the conversion programs (e.g. dos2unix and unix2dos , or perhaps dtou and
utod ) and use
them.
# IN UNIX ENVIRONMENT: convert DOS newlines (CR/LF) to Unix format.
sed 's/.$//' # assumes that all lines end with CR/LF
sed 's/^M$//' # in bash/tcsh, press Ctrl-V then Ctrl-M
sed 's/\x0D$//' # works on ssed, gsed 3.02.80 or higher
# IN UNIX ENVIRONMENT: convert Unix newlines (LF) to DOS format.
sed "s/$/`echo -e \\\r`/" # command line under ksh
sed 's/$'"/`echo \\\r`/" # command line under bash
sed "s/$/`echo \\\r`/" # command line under zsh
sed 's/$/\r/' # gsed 3.02.80 or higher
Use sed -i
for in-place conversion e.g. sed -i 's/..../' file .
This problem can be solved with standard tools, but there are sufficiently many traps for the
unwary that I recommend you install the flip command, which was written over
20 years ago by Rahul Dhesi, the author of zoo . It does an excellent job
converting file formats while, for example, avoiding the inadvertant destruction of binary
files, which is a little too easy if you just race around altering every CRLF you see...
The solutions posted so far only deal with part of the problem, converting DOS/Windows' CRLF
into Unix's LF; the part they're missing is that DOS use CRLF as a line separator ,
while Unix uses LF as a line terminator . The difference is that a DOS file
(usually) won't have anything after the last line in the file, while Unix will. To do the
conversion properly, you need to add that final LF (unless the file is zero-length, i.e. has
no lines in it at all). My favorite incantation for this (with a little added logic to handle
Mac-style CR-separated files, and not molest files that're already in unix format) is a bit
of perl:
brew install dos2unix
for csv in *.csv; do dos2unix -c mac ${csv}; done;
Make sure you have made copies of the files, as this command will modify the files in
place. The -c mac option makes the switch to be compatible with osx.
You can use awk. Set the record separator ( RS ) to a regexp that matches all
possible newline character, or characters. And set the output record separator (
ORS ) to the unix-style newline character.
Had just to ponder that same question (on Windows-side, but equally applicable to linux.)
Surprisingly nobody mentioned a very much automated way of doing CRLF<->LF conversion
for text-files using good old zip -ll option (Info-ZIP):
zip -ll textfiles-lf.zip files-with-crlf-eol.*
unzip textfiles-lf.zip
NOTE: this would create a zip file preserving the original file names but converting the
line endings to LF. Then unzip would extract the files as zip'ed, that is with
their original names (but with LF-endings), thus prompting to overwrite the local original
files if any.
Relevant excerpt from the zip --help :
zip --help
...
-l convert LF to CR LF (-ll CR LF to LF)
q// is generally the same thing as using single quotes - meaning it doesn't interpolate values
inside the delimiters.
qq// is the same as double quoting a string. It interpolates.
qw// return a list of white space delimited words. @q = qw/this is a test/ is functionally the
same as @q = ('this', 'is', 'a', 'test')
qx// is the same thing as using the backtick operators.
Relational databases started to get to be a big deal in the 1970's, and they're still a big deal
today, which is a little peculiar, because they're a 1960's technology.
A relational database is a bunch of rectangular tables. Each row of a table is a record about
one person or thing; the record contains several pieces of information called fields . Here
is an example table:
LASTNAME FIRSTNAME ID POSTAL_CODE AGE SEX
Gauss Karl 119 19107 30 M
Smith Mark 3 T2V 3V4 53 M
Noether Emmy 118 19107 31 F
Smith Jeff 28 K2G 5J9 19 M
Hamilton William 247 10139 2 M
The names of the fields are LASTNAME , FIRSTNAME , ID ,
POSTAL_CODE , AGE , and SEX . Each line in the table is a
record , or sometimes a row or tuple . For example, the first row of the
table represents a 30-year-old male whose name is Karl Gauss, who lives at postal code 19107, and
whose ID number is 119.
Sometimes this is a very silly way to store information. When the information naturally has a
tabular structure it's fine. When it doesn't, you have to squeeze it into a table, and some of the
techniques for doing that are more successful than others. Nevertheless, tables are simple and are
easy to understand, and most of the high-performance database systems you can buy today operate under
this 1960's model.
SQL stands for Structured Query Language . It was invented at IBM in the 1970's. It's
a language for describing searches and modifications to a relational database.
SQL was a huge success, probably because it's incredibly simple and anyone can pick it up in ten
minutes. As a result, all the important database systems support it in some fashion or another. This
includes the big players, like Oracle and Sybase, high-quality free or inexpensive database systems
like MySQL, and funny hacks like Perl's DBD::CSV module, which we'll see later.
There are four important things one can do with a table:
Those are the four most important SQL commands, also called queries . Suppose that the
example table above is named people . Here are examples of each of the four important
kinds of queries:
SELECT firstname FROM people WHERE lastname = 'Smith'
(Locate the first names of all the Smiths.)
DELETE FROM people WHERE id = 3
(Delete Mark Smith from the table)
UPDATE people SET age = age+1 WHERE id = 247
(William Hamilton just had a birthday.)
(Add Leonhard Euler to the table.)
There are a bunch of other SQL commands for creating and discarding tables, for granting and revoking
access permissions, for committing and abandoning transactions, and so forth. But these four are
the important ones. Congratulations; you are now a SQL programmer. For the details, go to any reasonable
bookstore and pick up a SQL quick reference.
Every database system is a little different. You talk to some databases over the network and make
requests of the database engine; other databases you talk to through files or something else.
Typically when you buy a commercial database, you get a library with it. The vendor has
written some functions for talking to the database in some language like C, compiled the functions,
and the compiled code is the library. You can write a C program that calls the functions in the library
when it wants to talk to the database.
There's a saying that any software problem can be solved by adding a layer of indirection. That's
what Perl's DBI (`Database Interface') module is all about. It was written by Tim Bunce.
DBI is designed to protect you from the details of the vendor libraries. It has a
very simple interface for saying what SQL queries you want to make, and for getting the results back.
DBI doesn't know how to talk to any particular database, but it does know how to locate
and load in DBD modules have the vendor libraries in them and know how to talk to the
real databases; there is one DBD module for every different database.
When you ask DBI module, which spins around three times or drinks out of its sneaker
or whatever is necessary to communicate with the real database. When it gets the results back, it
passes them to DBI . Then DBI gives you the results. Since your program
only has to deal with DBI , and not with the real database, you don't have to worry
about barking like a chicken.
Here's your program talking to the DBI library. You are using two databases at once.
One is an Oracle database server on some other machine, and another is a DBD::CSV database
that stores the data in a bunch of plain text files on the local disk.
Your program sends a query to DBI , which forwards it to the appropriate DBD
module; let's say it's DBD::Oracle . DBD::Oracle knows how to translate
what it gets from DBI into the format demanded by the Oracle library, which is built
into it. The library forwards the request across the network, gets the results back, and returns
them to DBD::Oracle . DBD::Oracle returns the results to DBI
as a Perl data structure. Finally, your program can get the results from DBI
.
On the other hand, suppose that your program was querying the text files. It would prepare the
same sort of query in exactly the same way, and send it to DBI in exactly the same way.
DBI would see that you were trying to talk to the DBD::CSV database and
forward the request to the DBD::CSV module. The DBD::CSV module has Perl
functions in it that tell it how to parse SQL and how to hunt around in the text files to find the
information you asked for. It then returns the results to DBI as a Perl data structure.
Finally, your program gets the results from DBI in exactly the same way that it would
have if you were talking to Oracle instead.
There are two big wins that result from this organization. First, you don't have to worry about
the details of hunting around in text files or talking on the network to the Oracle server or dealing
with Oracle's library. You just have to know how to talk to DBI .
Second, if you build your program to use Oracle, and then the following week upper management
signs a new Strategic Partnership with Sybase, it's easy to convert your code to use Sybase instead
of Oracle. You change exactly one line in your program, the line that tells DBI to talk
to DBD::Oracle , and have it use DBD::Sybase instead. Or you might build
your program to talk to a cheap, crappy database like MS Access, and then next year when the application
is doing well and getting more use than you expected, you can upgrade to a better database next year
without changing any of your code.
There are DBD modules for talking to every important kind of SQL database.
DBD::Oracle will talk to Oracle, and DBD::Sybase will talk to Sybase. DBD::ODBC
will talk to any ODBC database including Microsoft Acesss. (ODBC is a Microsoft invention
that is analogous to DBI itself. There is no DBD module for talking to
Access directly.) DBD::CSV allows SQL queries on plain text files. DBD::mysql
talks to the excellent MySQL database from TCX DataKonsultAB in Sweden. (MySQL is a tremendous
bargain: It's $200 for commercial use, and free for noncommerical use.)
Here's a typical program. When you run it, it waits for you to type a last name. Then it searches
the database for people with that last name and prints out the full name and ID number for each person
it finds. For example:
Enter name> Noether
118: Emmy Noether
Enter name> Smith
3: Mark Smith
28: Jeff Smith
Enter name> Snonkopus
No names matched `Snonkopus'.
Enter name> ^D
Here is the code:
use DBI;
my $dbh = DBI->connect('DBI:Oracle:payroll')
or die "Couldn't connect to database: " . DBI->errstr;
my $sth = $dbh->prepare('SELECT * FROM people WHERE lastname = ?')
or die "Couldn't prepare statement: " . $dbh->errstr;
print "Enter name> ";
while ($lastname = <>) { # Read input from the user
my @data;
chomp $lastname;
$sth->execute($lastname) # Execute the query
or die "Couldn't execute statement: " . $sth->errstr;
# Read the matching records and print them out
while (@data = $sth->fetchrow_array()) {
my $firstname = $data[1];
my $id = $data[2];
print "\t$id: $firstname $lastname\n";
}
if ($sth->rows == 0) {
print "No names matched `$lastname'.\n\n";
}
$sth->finish;
print "\n";
print "Enter name> ";
}
$dbh->disconnect;
This loads in the DBI module. Notice that we don't have to load in any DBD
module. DBI will do that for us when it needs to.
my $dbh = DBI->connect('DBI:Oracle:payroll');
or die "Couldn't connect to database: " . DBI->errstr;
The connect call tries to connect to a database. The first argument, DBI:Oracle:payroll
, tells DBI what kind of database it is connecting to. The Oracle
part tells it to load DBD::Oracle and to use that to communicate with the database.
If we had to switch to Sybase next week, this is the one line of the program that we would change.
We would have to change Oracle to Sybase .
payroll is the name of the database we will be searching. If we were going to supply
a username and password to the database, we would do it in the connect call:
my $dbh = DBI->connect('DBI:Oracle:payroll', 'username', 'password')
or die "Couldn't connect to database: " . DBI->errstr;
If DBI connects to the database, it returns a database handle object, which
we store into $dbh . This object represents the database connection. We can be connected
to many databases at once and have many such database connection objects.
If DBI can't connect, it returns an undefined value. In this case, we use die
to abort the program with an error message. DBI->errstr returns the reason why
we couldn't connect-``Bad password'' for example.
my $sth = $dbh->prepare('SELECT * FROM people WHERE lastname = ?')
or die "Couldn't prepare statement: " . $dbh->errstr;
The prepare call prepares a query to be executed by the database. The argument is
any SQL at all. On high-end databases, prepare will send the SQL to the database server,
which will compile it. If prepare is successful, it returns a statement handle
object which represents the statement; otherwise it returns an undefined value and we abort
the program. $dbh->errstr will return the reason for failure, which might be ``Syntax
error in SQL''. It gets this reason from the actual database, if possible.
The ? in the SQL will be filled in later. Most databases can handle this. For some
databases that don't understand the ? , the DBD module will emulate it for you and will
pretend that the database understands how to fill values in later, even though it doesn't.
print "Enter name> ";
Here we just print a prompt for the user.
while ($lastname = <>) { # Read input from the user
...
}
This loop will repeat over and over again as long as the user enters a last name. If they type
a blank line, it will exit. The Perl <> symbol means to read from the terminal or from
files named on the command line if there were any.
my @data;
This declares a variable to hold the data that we will get back from the database.
chomp $lastname;
This trims the newline character off the end of the user's input.
$sth->execute($lastname) # Execute the query
or die "Couldn't execute statement: " . $sth->errstr;
execute executes the statement that we prepared before. The argument $lastname
is substituted into the SQL in place of the ? that we saw earlier. execute
returns a true value if it succeeds and a false value otherwise, so we abort if for some reason
the execution fails.
while (@data = $sth->fetchrow_array()) {
...
}
fetchrow_array returns one of the selected rows from the database. You get back an
array whose elements contain the data from the selected row. In this case, the array you get back
has six elements. The first element is the person's last name; the second element is the first name;
the third element is the ID, and then the other elements are the postal code, age, and sex.
Each time we call fetchrow_array , we get back a different record from the database.
When there are no more matching records, fetchrow_array returns the empty list and the
while loop exits.
my $firstname = $data[1];
my $id = $data[2];
These lines extract the first name and the ID number from the record data.
print "\t$id: $firstname $lastname\n";
This prints out the result.
if ($sth->rows == 0) {
print "No names matched `$lastname'.\n\n";
}
The rows method returns the number of rows of the database that were selected. If
no rows were selected, then there is nobody in the database with the last name that the user is looking
for. In that case, we print out a message. We have to do this after the while
loop that fetches whatever rows were available, because with some databases you don't know
how many rows there were until after you've gotten them all.
$sth->finish;
print "\n";
print "Enter name> ";
Once we're done reporting about the result of the query, we print another prompt so that the user
can enter another name. finish tells the database that we have finished retrieving all
the data for this query and allows it to reinitialize the handle so that we can execute it again
for the next query.
$dbh->disconnect;
When the user has finished querying the database, they type a blank line and the main while
loop exits. disconnect closes the connection to the database.
Here's a function which looks up someone in the example table, given their ID number, and returns
their age:
sub age_by_id {
# Arguments: database handle, person ID number
my ($dbh, $id) = @_;
my $sth = $dbh->prepare('SELECT age FROM people WHERE id = ?')
or die "Couldn't prepare statement: " . $dbh->errstr;
$sth->execute($id)
or die "Couldn't execute statement: " . $sth->errstr;
my ($age) = $sth->fetchrow_array();
return $age;
}
It prepares the query, executes it, and retrieves the result.
There's a problem here though. Even though the function works correctly, it's inefficient. Every
time it's called, it prepares a new query. Typically, preparing a query is a relatively expensive
operation. For example, the database engine may parse and understand the SQL and translate it into
an internal format. Since the query is the same every time, it's wasteful to throw away this work
when the function returns.
Here's one solution:
{ my $sth;
sub age_by_id {
# Arguments: database handle, person ID number
my ($dbh, $id) = @_;
if (! defined $sth) {
$sth = $dbh->prepare('SELECT age FROM people WHERE id = ?')
or die "Couldn't prepare statement: " . $dbh->errstr;
}
$sth->execute($id)
or die "Couldn't execute statement: " . $sth->errstr;
my ($age) = $sth->fetchrow_array();
return $age;
}
}
There are two big changes to this function from the previous version. First, the $sth
variable has moved outside of the function; this tells Perl that its value should persist
even after the function returns. Next time the function is called, $sth will have the
same value as before.
Second, the prepare code is in a conditional block. It's only executed if $sth
does not yet have a value. The first time the function is called, the prepare
code is executed and the statement handle is stored into $sth . This value persists
after the function returns, and the next time the function is called, $sth still contains
the statement handle and the prepare code is skipped.
Here's another solution:
sub age_by_id {
# Arguments: database handle, person ID number
my ($dbh, $id) = @_;
my $sth = $dbh->prepare_cached('SELECT age FROM people WHERE id = ?')
or die "Couldn't prepare statement: " . $dbh->errstr;
$sth->execute($id)
or die "Couldn't execute statement: " . $sth->errstr;
my ($age) = $sth->fetchrow_array();
return $age;
}
Here the only change to to replace prepare with prepare_cached . The
prepare_cached call is just like prepare , except that it looks to see
if the query is the same as last time. If so, it gives you the statement handle that it gave you
before.
Many databases support transactions . This means that you can make a whole bunch of queries
which would modify the databases, but none of the changes are actually made. Then at the end you
issue the special SQL query COMMIT , and all the changes are made simultaneously. Alternatively,
you can issue the query ROLLBACK , in which case all the queries are thrown away.
As an example of this, consider a function to add a new employee to a database. The database has
a table called employees that looks like this:
FIRSTNAME LASTNAME DEPARTMENT_ID
Gauss Karl 17
Smith Mark 19
Noether Emmy 17
Smith Jeff 666
Hamilton William 17
and a table called departments that looks like this:
ID NAME NUM_MEMBERS
17 Mathematics 3
666 Legal 1
19 Grounds Crew 1
The mathematics department is department #17 and has three members: Karl Gauss, Emmy Noether,
and William Hamilton.
Here's our first cut at a function to insert a new employee. It will return true or false depending
on whether or not it was successful:
sub new_employee {
# Arguments: database handle; first and last names of new employee;
# department ID number for new employee's work assignment
my ($dbh, $first, $last, $department) = @_;
my ($insert_handle, $update_handle);
my $insert_handle =
$dbh->prepare_cached('INSERT INTO employees VALUES (?,?,?)');
my $update_handle =
$dbh->prepare_cached('UPDATE departments
SET num_members = num_members + 1
WHERE id = ?');
die "Couldn't prepare queries; aborting"
unless defined $insert_handle && defined $update_handle;
$insert_handle->execute($first, $last, $department) or return 0;
$update_handle->execute($department) or return 0;
return 1; # Success
}
We create two handles, one for an insert query that will insert the new employee's
name and department number into the employees table, and an update query
that will increment the number of members in the new employee's department in the department
table. Then we execute the two queries with the appropriate arguments.
There's a big problem here: Suppose, for some reason, the second query fails. Our function returns
a failure code, but it's too late, it has already added the employee to the employees
table, and that means that the count in the departments table is wrong. The database
now has corrupted data in it.
The solution is to make both updates part of the same transaction. Most databases will do this
automatically, but without an explicit instruction about whether or not to commit the changes, some
databases will commit the changes when we disconnect from the database, and others will roll them
back. We should specify the behavior explicitly.
Typically, no changes will actually be made to the database until we issue a commit
. The version of our program with commit looks like this:
sub new_employee {
# Arguments: database handle; first and last names of new employee;
# department ID number for new employee's work assignment
my ($dbh, $first, $last, $department) = @_;
my ($insert_handle, $update_handle);
my $insert_handle =
$dbh->prepare_cached('INSERT INTO employees VALUES (?,?,?)');
my $update_handle =
$dbh->prepare_cached('UPDATE departments
SET num_members = num_members + 1
WHERE id = ?');
die "Couldn't prepare queries; aborting"
unless defined $insert_handle && defined $update_handle;
We perform both queries, and record in $success whether they both succeeded.
$success will be true if both queries succeeded, false otherwise. If the queries succeded,
we commit the transaction; otherwise, we roll it back, cancelling all our changes.
The problem of concurrent database access is also solved by transactions. Suppose that queries
were executed immediately, and that some other program came along and examined the database after
our insert but before our update. It would see inconsistent data in the database, even if our update
would eventually have succeeded. But with transactions, all the changes happen simultaneously when
we do the commit , and the changes are committed automatically, which means that any
other program looking at the database either sees all of them or none.
If you're doing an UPDATE , INSERT , or DELETE there is
no data that comes back from the database, so there is a short cut. You can say
$dbh->do('DELETE FROM people WHERE age > 65');
for example, and DBI will prepare the statement, execute it, and finish it.
do returns a true value if it succeeded, and a false value if it failed. Actually, if it succeeds
it returns the number of affected rows. In the example it would return the number of rows that were
actually deleted. ( DBI plays a magic trick so that the value it turns is true even
when it is 0. This is bizarre, because 0 is usually false in Perl. But it's convenient because you
can use it either as a number or as a true-or-false success code, and it works both ways.)
If your transactions are simple, you can save yourself the trouble of having to issue a lot of
commit s. When you make the connect call, you can specify an AutoCommit
option which will perform an automatic commit operation after every successful
query. Here's what it looks like:
my $dbh = DBI->connect('DBI:Oracle:payroll',
{AutoCommit => 1},
)
or die "Couldn't connect to database: " . DBI->errstr;
When you make the connect call, you can specify a RaiseErrors option
that handles errors for you automatically. When an error occurs, DBI will abort your
program instead of returning a failure code. If all you want is to abort the program on an error,
this can be convenient:
my $dbh = DBI->connect('DBI:Oracle:payroll',
{RaiseError => 1},
)
or die "Couldn't connect to database: " . DBI->errstr;
while ($lastname = <>) {
my $sth = $dbh->prepare("SELECT * FROM people
WHERE lastname = '$lastname'");
$sth->execute();
# and so on ...
}
Here we interpolated the value of $lastname directly into the SQL in the prepare
call.
This is a bad thing to do for three reasons.
First, prepare calls can take a long time. The database server has to compile the
SQL and figure out how it is going to run the query. If you have many similar queries, that is a
waste of time.
Second, it will not work if $lastname contains a name like O'Malley or D'Amico or
some other name with an ' . The ' has a special meaning in SQL, and the
database will not understand when you ask it to prepare a statement that looks like
SELECT * FROM people WHERE lastname = 'O'Malley'
It will see that you have three ' s and complain that you don't have a fourth matching
' somewhere else.
Finally, if you're going to be constructing your query based on a user input, as we did in the
example program, it's unsafe to simply interpolate the input directly into the query, because the
user can construct a strange input in an attempt to trick your program into doing something it didn't
expect. For example, suppose the user enters the following bizarre value for $input
:
x' or lastname = lastname or lastname = 'y
Now our query has become something very surprising:
SELECT * FROM people WHERE lastname = 'x'
or lastname = lastname or lastname = 'y'
The part of this query that our sneaky user is interested in is the second or clause.
This clause selects all the records for which lastname is equal to lastname
; that is, all of them. We thought that the user was only going to be able to see a few records
at a time, and now they've found a way to get them all at once. This probably wasn't what we wanted.
References
• A complete list of DBD modules are available
here
• You can download these modules
here
• DBI modules are available
here
• You can get MySQL from www.tcx.se
People go to all sorts of trouble to get around these problems with interpolation. They write
a function that puts the last name in quotes and then backslashes any apostrophes that appear in
it. Then it breaks because they forgot to backslash backslashes. Then they make their escape function
better. Then their code is a big message because they are calling the backslashing function every
other line. They put a lot of work into it the backslashing function, and it was all for nothing,
because the whole problem is solved by just putting a ? into the query, like this
SELECT * FROM people WHERE lastname = ?
All my examples look like this. It is safer and more convenient and more efficient to
do it this way.
Damian Conway is the author of the newly released
Object Oriented Perl
, the first of a new series of Perl books from Manning.
Object-oriented programming in Perl is easy. Forget the heavy theory and the sesquipedalian jargon:
classes in Perl are just regular packages, objects are just variables, methods are just subroutines.
The syntax and semantics are a little different from regular Perl, but the basic building blocks
are completely familiar.
The one problem most newcomers to object-oriented Perl seem to stumble over is the notion of references
and referents, and how the two combine to create objects in Perl. So let's look at how references
and referents relate to Perl objects, and see who gets to be blessed and who just gets to point the
finger.
Let's start with a short detour down a dark alley...
References and referents
Sometimes it's important to be able to access a variable indirectly- to be able to use it without
specifying its name. There are two obvious motivations: the variable you want may not have
a name (it may be an anonymous array or hash), or you may only know which variable you want at run-time
(so you don't have a name to offer the compiler).
To handle such cases, Perl provides a special scalar datatype called a reference . A reference
is like the traditional Zen idea of the "finger pointing at the moon". It's something that identifies
a variable, and allows us to locate it. And that's the stumbling block most people need to get over:
the finger (reference) isn't the moon (variable); it's merely a means of working out where the moon
is.
Making a reference
When you prefix an existing variable or value with the unary \ operator you get a reference to
the original variable or value. That original is then known as the referent to which the reference
refers.
For example, if $s is a scalar variable, then \$s is a reference to that scalar variable (i.e.
a finger pointing at it) and $s is that finger's referent. Likewise, if @a in an array, then \@a
is a reference to it.
In Perl, a reference to any kind of variable can be stored in another scalar variable. For example:
$slr_ref = \$s;
# scalar $slr_ref stores a reference to scalar $s
$arr_ref = \@a;
# scalar $arr_ref stores a reference to array @a
$hsh_ref = \%h;
# scalar $hsh_ref stores a reference to hash %h
Figure 1 shows the relationships produced by those assignments.
Note that the references are separate entities from the referents at which they point. The only
time that isn't the case is when a variable happens to contain a reference to itself:
$self_ref = \$self_ref;
# $self_ref stores a reference to itself!
That (highly unusual) situation produces an arrangement shown in Figure 2.
Once you have a reference, you can get back to the original thing it refers to-it's referent-simply
by prefixing the variable containing the reference (optionally in curly braces) with the appropriate
variable symbol. Hence to access $s , you could write $$slr_ref or ${$slr_ref} . At first glance,
that might look like one too many dollar signs, but it isn't. The $slr_ref tells Perl which variable
has the reference; the extra $ tells Perl to follow that reference and treat the referent as a scalar.
Similarly, you could access the array @a as @{$arr_ref} , or the hash %h as %{$hsh_ref} . In each
case, the $whatever_ref is the name of the scalar containing the reference, and the leading @ or
% indicates what type of variable the referent is. That type is important: if you attempt to prefix
a reference with the wrong symbol (for example, @{$slr_ref} or ${$hsh_ref} ), Perl produces a fatal
run-time error.
Figure 1: References and their referents
Figure 2: A reference that is its own referent
The "arrow" operator
Accessing the elements of an array or a hash through a reference can be awkward using the syntax
shown above. You end up with a confusing tangle of dollar signs and brackets:
${$arr_ref}[0] = ${$hsh_ref}{"first"};
# i.e. $a[0] = $h{"first"}
So Perl provides a little extra syntax to make life just a little less cluttered:
$arr_ref->[0] = $hsh_ref->{"first"};
# i.e. $a[0] = $h{"first"}
The "arrow" operator ( -> ) takes a reference on its left and either an array index (in square brackets)
or a hash key (in curly braces) on its right. It locates the array or hash that the reference refers
to, and then accesses the appropriate element of it.
Identifying a referent
Because a scalar variable can store a reference to any kind of data, and because dereferencing
a reference with the wrong prefix leads to fatal errors, it's sometimes important to be able to determine
what type of referent a specific reference refers to. Perl provides a built-in function called ref
that takes a scalar and returns a description of the kind of reference it contains. Table 1 summarizes
the string that is returned for each type of reference.
If $slr_ref contains...
then ref($slr_ref) returns...
undef
a reference to a scalar
a reference to an array
"ARRAY"
a reference to a hash
"HASH"
a reference to a subroutine
"CODE"
a reference to a filehandle
"IO" or "IO::Handle"
a reference to a typeglob
"GLOB"
a reference to a precompiled pattern
"Regexp"
a reference to another reference
"REF"
Table 1: What ref returns
As Table 1 indicates, you can create references to many kinds of Perl constructs, apart from variables.
If a reference is used in a context where a string is expected, then the ref function is called
automatically to produce the expected string, and a unique hexadecimal value (the internal memory
address of the thing being referred to) is appended. That means that printing out a reference:
print $hsh_ref, "\n";
produces something like:
HASH(0x10027588)
since each element of print 's argument list is stringified before printing.
The ref function has a vital additional role in object-oriented Perl, where it can be used to
identify the class to which a particular object belongs. More on that in a moment.
References, referents, and objects
References and referents matter because they're both required when you come to build objects in
Perl. In fact, Perl objects are just referents (i.e. variables or values) that have a special relationship
with a particular package. References come into the picture because Perl objects are always accessed
via a reference, using an extension of the "arrow" notation.
But that doesn't mean that Perl's object-oriented features are difficult to use (even if you're
still unsure of references and referents). To do real, useful, production-strength, object-oriented
programming in Perl you only need to learn about one extra function, one straightforward piece of
additional syntax, and three very simple rules. Let's start with the rules...
Rule 1: To create a class, build a package
Perl packages already have a number of class-like features:
They collect related code together;
They distinguish that code from unrelated code;
They provide a separate namespace within the program, which keeps subroutine names from clashing
with those in other packages;
They have a name, which can be used to identify data and subroutines defined in the package.
In Perl, those features are sufficient to allow a package to act like a class.
Suppose you wanted to build an application to track faults in a system. Here's how to declare
a class named "Bug" in Perl:
package Bug;
That's it! In Perl, classes are packages. No magic, no extra syntax, just plain, ordinary packages.
Of course, a class like the one declared above isn't very interesting or useful, since its objects
will have no attributes or behaviour.
That brings us to the second rule...
Rule 2: To create a method, write a subroutine
In object-oriented theory, methods are just subroutines that are associated with a particular
class and exist specifically to operate on objects that are instances of that class. In Perl, a subroutine
that is declared in a particular package is already associated with that package. So to write
a Perl method, you just write a subroutine within the package that is acting as your class.
For example, here's how to provide an object method to print Bug objects:
package Bug;
sub print_me
{
# The code needed to print the Bug goes here
}
Again, that's it. The subroutine print_me is now associated with the package Bug, so whenever Bug
is used as a class, Perl automatically treats Bug::print_me as a method.
Invoking the Bug::print_me method involves that one extra piece of syntax mentioned above-an extension
to the existing Perl "arrow" notation. If you have a reference to an object of class Bug, you can
access any method of that object by using a -> symbol, followed by the name of the method.
For example, if the variable $nextbug holds a reference to a Bug object, you could call Bug::print_me
on that object by writing:
$nextbug->print_me();
Calling a method through an arrow should be very familiar to any C++ programmers; for the rest of
us, it's at least consistent with other Perl usages:
$hsh_ref->{"key"};
# Access the hash referred to by $hashref
$arr_ref->[$index];
# Access the array referred to by $arrayref
$sub_ref->(@args);
# Access the sub referred to by $subref
$obj_ref->method(@args);
# Access the object referred to by $objref
The only difference with the last case is that the referent (i.e. the object) pointed to by $objref
has many ways of being accessed (namely, its various methods). So, when you want to access that object,
you have to specify which particular way-which method-should be used. Hence, the method name after
the arrow.
When a method like Bug::print_me is called, the argument list that it receives begins with the
reference through which it was called, followed by any arguments that were explicitly given to the
method. That means that calling Bug::print_me("logfile") is not the same as calling $nextbug->print_me("logfile")
. In the first case, print_me is treated as a regular subroutine so the argument list passed to Bug::print_me
is equivalent to:
( "logfile" )
In the second case, print_me is treated as a method so the argument list is equivalent to:
( $objref, "logfile" )
Having a reference to the object passed as the first parameter is vital, because it means that the
method then has access to the object on which it's supposed to operate. Hence you'll find that most
methods in Perl start with something equivalent to this:
package Bug;
sub print_me
{
my ($self) = shift;
# The @_ array now stores the arguments passed to &Bug::print_me
# The rest of &print_me uses the data referred to by $self
# and the explicit arguments (still in @_)
}
or, better still:
package Bug;
sub print_me
{
my ($self, @args) = @_;
# The @args array now stores the arguments passed to &Bug::print_me
# The rest of &print_me uses the data referred to by $self
# and the explicit arguments (now in @args)
}
This second version is better because it provides a lexically scoped copy of the argument list (
@args ). Remember that the @_ array is "magical"-changing any element of it actually changes the
caller's version of the corresponding argument. Copying argument values to a lexical array
like @args prevents nasty surprises of this kind, as well as improving the internal documentation
of the subroutine (especially if a more meaningful name than @args is chosen).
The only remaining question is: how do you create the invoking object in the first place?
Rule 3: To create an object, bless a referent
Unlike other object-oriented languages, Perl doesn't require that an object be a special kind
of record-like data structure. In fact, you can use any existing type of Perl variable-a scalar,
an array, a hash, etc.-as an object in Perl.
Hence, the issue isn't how to create the object, because you create them exactly like any
other Perl variable: declare them with a my , or generate them anonymously with a [ ... ] or { ...
} . The real problem is how to tell Perl that such an object belongs to a particular class.
That brings us to the one extra built-in Perl function you need to know about. It's called bless
, and its only job is to mark a variable as belonging to a particular class.
The bless function takes two arguments: a reference to the variable to be marked, and a string
containing the name of the class. It then sets an internal flag on the variable, indicating that
it now belongs to the class.
For example, suppose that $nextbug actually stores a reference to an anonymous hash:
$nextbug = {
id => "00001",
type => "fatal",
descr => "application does not compile",
};
To turn that anonymous hash into an object of class Bug you write:
bless $nextbug, "Bug";
And, once again, that's it! The anonymous array referred to by $nextbug is now marked as being an
object of class Bug. Note that the variable $nextbug itself hasn't been altered in any way; only
the nameless hash it refers to has been marked. In other words, bless sanctified the referent,
not the reference. Figure 3 illustrates where the new class membership flag is set.
You can check that the blessing succeeded by applying the built-in ref function to $nextbug .
As explained above, when ref is applied to a reference, it normally returns the type of that reference.
Hence, before $nextbug was blessed, ref($nextbug) would have returned the string 'HASH' .
Once an object is blessed, ref returns the name of its class instead. So after the blessing, ref($nextbug)
will return 'Bug' . Of course the object itself still is a hash, but now it's a hash that
belongs to the Bug class. The various entries of the hash become the attributes of the newly
created Bug object.
Figure 3: What changes when an object is blessed
Creating a constructor
Given that you're likely to want to create many such Bug objects, it would be convenient to have
a subroutine that took care of all the messy, blessy details. You could pass it the necessary information,
and it would then wrap it in an anonymous hash, bless the hash, and give you back a reference to
the resulting object.
And, of course, you might as well put such a subroutine in the Bug package itself, and call it
something that indicates its role. Such a subroutine is known as a constructor, and it generally
looks like this:
package Bug;
sub new
{
my $class = $_[0];
my $objref = {
id => $_[1],
type => $_[2],
descr => $_[3],
};
bless $objref, $class;
return $objref;
}
Note that the middle bits of the subroutine (in bold) look just like the raw blessing that was handed
out to $nextbug in the previous example.
The bless function is set up to make writing constructors like this a little easier. Specifically,
it returns the reference that's passed as its first argument (i.e. the reference to whatever referent
it just blessed into object-hood). And since Perl subroutines automatically return the value of their
last evaluated statement, that means that you could condense the definition of Bug::new to this:
sub Bug::new
{
bless { id => $_[1], type => $_[2], descr => $_[3] }, $_[0];
}
This version has exactly the same effects: slot the data into an anonymous hash, bless the hash into
the class specified first argument, and return a reference to the hash.
Regardless of which version you use, now whenever you want to create a new Bug object, you can
just call:
That's a little redundant, since you have to type "Bug" twice. Fortunately, there's another feature
of the "arrow" method-call syntax that solves this problem. If the operand to the left of the arrow
is the name of a class -rather than an object reference-then the appropriate method of that class
is called. More importantly, if the arrow notation is used, the first argument passed to the method
is a string containing the class name. That means that you could rewrite the previous call to Bug::new
like this:
$nextbug = Bug->new($id, $type, $description);
There are other benefits to this notation when your class uses inheritance, so you should always
call constructors and other class methods this way.
Method enacting
Apart from encapsulating the gory details of object creation within the class itself, using a
class method like this to create objects has another big advantage. If you abide by the convention
of only ever creating new Bug objects by calling Bug::new , you're guaranteed that all such objects
will always be hashes. Of course, there's nothing to prevent us from "manually" blessing arrays,
or scalars as Bug objects, but it turns out to make life much easier if you stick to blessing
one type of object into each class.
For example, if you can be confident that any Bug object is going to be a blessed hash, you can
(finally!) fill in the missing code in the Bug:: print_me method:
package Bug;
sub print_me
{
my ($self) = @_;
print "ID: $self->{id}\n";
print "$self->{descr}\n";
print "(Note: problem is fatal)\n" if $self->{type} eq "fatal";
}
Now, whenever the print_me method is called via a reference to any hash that's been blessed into
the Bug class, the $self variable extracts the reference that was passed as the first argument and
then the print statements access the various entries of the blessed hash.
Till death us do part...
Objects sometimes require special attention at the other end of their lifespan too. Most object-oriented
languages provide the ability to specify a subroutine that is called automatically when an object
ceases to exist. Such subroutines are usually called destructors , and are used to undo any
side-effects caused by the previous existence of an object. That may include:
deallocating related memory (although in Perl that's almost never necessary since reference
counting usually takes care of it for you);
closing file or directory handles stored in the object;
closing pipes to other processes;
closing databases used by the object;
updating class-wide information;
anything else that the object should do before it ceases to exist (such as logging the
fact of its own demise, or storing its data away to provide persistence, etc.)
In Perl, you can set up a destructor for a class by defining a subroutine named DESTROY in the class's
package. Any such subroutine is automatically called on an object of that class, just before that
object's memory is reclaimed. Typically, this happens when the last variable holding a reference
to the object goes out of scope, or has another value assigned to it.
For example, you could provide a destructor for the Bug class like this:
package Bug;
# other stuff as before
sub DESTROY
{
my ($self) = @_;
print "<< Squashed the bug: $self->{id} >>\n\n";
}
Now, every time an object of class Bug is about to cease to exist, that object will automatically
have its DESTROY method called, which will print an epitaph for the object. For example, the following
code:
package main;
use Bug;
open BUGDATA, "Bug.dat" or die "Couldn't find Bug data";
while (<BUGDATA>)
{
my @data = split ',', $_;
# extract comma-separated Bug data
my $bug = Bug->new(@data);
# create a new Bug object
$bug->print_me();
# print it out
}
print "(end of list)\n";
prints out something like this:
ID: HW000761
"Cup holder" broken
Note: problem is fatal
<< Squashed the bug HW000761 >>
ID: SW000214
Word processor trashing disk after 20 saves.
<< Squashed the bug SW000214 >>
ID: OS000633
Can't change background colour (blue) on blue screen of death.
<< Squashed the bug OS000633 >>
(end of list)
That's because, at the end of each iteration of the while loop, the lexical variable $bug goes out
of scope, taking with it the only reference to the Bug object created earlier in the same loop. That
object's reference count immediately becomes zero and, because it was blessed, the corresponding
DESTROY method (i.e. Bug::DESTROY ) is automatically called on the object.
Where to from here?
Of course, these fundamental techniques only scratch the surface of object-oriented programming
in Perl. Simple hash-based classes with methods, constructors, and destructors may be enough to let
you solve real problems in Perl, but there's a vast array of powerful and labor-saving techniques
you can add to those basic components: autoloaded methods, class methods and class attributes, inheritance
and multiple inheritance, polymorphism, multiple dispatch, enforced encapsulation, operator overloading,
tied objects, genericity, and persistence.
Perl's standard documentation includes plenty of good material- perlref , perlreftut
, perlobj , perltoot , perltootc , and perlbot to get you started.
But if you're looking for a comprehensive tutorial on everything you need to know, you may also like
to consider my new book, Object Oriented
Perl , from which this article has been adapted.
NOTE : The following is an excerpt from the draft manuscript of Programming Perl , 4ᵗʰ
edition
Calling sort without a comparison function is quite often the wrong thing to do,
even on plain text. That's because if you use a bare sort, you can get really strange results. It's
not just Perl either: almost all programming languages work this way, even the shell command. You
might be surprised to find that with this sort of nonsense sort, ‹B› comes before ‹a› not after it,
‹é› comes before ‹ｄ›, and ‹ff› comes after ‹zz›. There's no end to such silliness, either; see the
default sort tables at the end of this article to see what I mean.
There are situations when a bare sort is appropriate, but fewer than you think. One
scenario is when every string you're sorting contains nothing but the 26 lowercase (or uppercase,
but not both) Latin letters from ‹a-z›, without any whitespace or punctuation.
Another occasion when a simple, unadorned sort is appropriate is when you have no
other goal but to iterate in an order that is merely repeatable, even if that order should happen
to be completely arbitrary. In other words, yes, it's garbage, but it's the same garbage this time
as it was last time. That's because the default sort resorts to an unmediated
cmp operator, which has the "predictable garbage" characteristics I just mentioned.
The last situation is much less frequent than the first two. It requires that the things you're
sorting be special‐purpose, dedicated binary keys whose bit sequences have with excruciating care
been arranged to sort in some prescribed fashion. This is also the strategy for any reasonable use
of the cmp operator.
So what's wrong with sort anyway?
I know, I know. I can hear everyone saying, "But it's called sort , so how could
that ever be wrong?" Sure it's called sort , but you still have to know how to use it
to get useful results out. Probably the most surprising thing about sort is that
it does not by default do an alphabetic, an alphanumeric, or a numeric sort. What it actually
does is something else altogether, and that something else is of surprisingly limited usefulness.
Imagine you have an array of records. It does you virtually no good to write:
@sorted_recs = sort @recs;
Because Perl's cmp operator does only a bit comparison not an alphabetic one, it
does nearly as little good to write your record sort this way:
The problem is that that cmp for the record's SURNAME field is not
an alphabetic comparison. It's merely a code point comparison. That means it works like C's
strcmp function or Java's String.compareTo method. Although commonly referred
to as a "lexicographic" comparison, this is a gross misnomer: it's about as far away from the way
real lexicographers sort dictionary entries as you can get without flipping a coin.
Fortunately, you don't have to come up with your own algorithm for dictionary sorting, because
Perl provides a standard class to do this for you:
Unicode::Collate .
Don't let the name throw you, because while it was first invented for Unicode, it works great on
regular ASCII text, too, and does a better job at making lexicographers happy than a plain old
sort ever manages.
If you have code that purports to sort text that looks like this:
@sorted_lines = sort @lines;
Then all you have to get a dictionary sort is write instead:
use Unicode::Collate;
@sorted_lines = Unicode::Collate::->new->sort(@lines);
For structured records, like those with ages and surnames in them, you have to be a bit fancier.
One way to fix it would be to use the class's own cmp operator instead of the built‐in
one.
use Unicode::Collate;
my $collator = Unicode::Collate::->new();
@srecs = sort {
$b->{AGE} <=> $b->{AGE}
||
$collator->cmp( $a->{SURNAME}, $b->{SURNAME} )
} @recs;
However, that makes a fairly expensive method call for every possible comparison. Because Perl's
adaptive merge sort algorithm usually runs in O(n · log n) time given n items,
and because each comparison requires two different computed keys, that can be a lot of duplicate
effort. Our sorting class therefore provide a convenient getSortKey method that calculates
a special binary key which you can cache and later pass to the normal cmp operator on
your own. This trick lets you use cmp yet get a truly alphabetic sort out of it for
a change.
Here is a simple but sufficient example of how to do that:
use Unicode::Collate;
my $collator = Unicode::Collate::->new();
# first calculate the magic sort key for each text field, and cache it
for my $rec (@recs) {
$rec->{SURNAME_key} = $collator->getSortKey( $rec->{SURNAME} );
}
# now sort the records as before, but for the surname field,
# use the cached sort key instead
@srecs = sort {
$b->{AGE} <=> $b->{AGE}
||
$a->{SURNAME_key} cmp $b->{SURNAME_key}
} @recs;
That's what I meant about very carefully preparing a mediated sort key that contains the precomputed
binary key.
English Card Catalogue Sorts
The simple code just demonstrated assumes you want to sort names the same way you do regular text.
That isn't a good assumption, however. Many countries, languages, institutions, and sometimes even
librarians have their own notions about how a card catalogue or a phonebook ought to be sorted.
For example, in the English language, surnames with Scottish patronymics starting with ‹Mc› or
‹Mac›, like MacKinley and McKinley , not only count as completely identical synonyms
for sorting purposes, they go before any other surname that begins with ‹M›, and so precede surnames
like Mables or Machado .
Yes, really.
That means that the following names are sorted correctly -- for English:
Lewis, C.S.
McKinley, Bill
MacKinley, Ron
Mables, Martha
Machado, José
Macon, Bacon
Yes, it's true. Check out your local large English‐language bookseller or library -- presuming
you can find one. If you do, best make sure to blow the dust off first.
Sorting Spanish Names
It's a good thing those names follow English rules for sorting names. If this were Spanish, we
would have to deal with double‐barrelled surnames, where the patronym sorts before the matronym,
which in turn sorts before any given names. That means that if Señor Machado's full name were, like
the poet's, Antonio Cipriano José María y Francisco de Santa Ana Machado y Ruiz , then you
would have to sort him with the other Machados but then consider Ruiz before Antonio
if there were any other Machados . Similarly, the poet Federico del Sagrado Corazón
de Jesús García Lorca sorts before the writer Gabriel José de la Concordia García Márquez
.
On the other hand, if your records are not full multifield hashes but only simple text that don't
happen to be surnames, your task is a lot simpler, since now all you have to is get the cmp
operator to behave sensibly. That you can do easily enough this way:
use Unicode::Collate;
@sorted_text = Unicode::Collate::->new->sort(@text);
Sorting Text, Not Binary
Imagine you had this list of German‐language authors:
If you just sorted them with an unmediated sort operator, you would get this utter
nonsense:
Bobrowski
Bodmer
Borchert
Born
Brandis
Brant
Böhme
Böll
Böttcher
Or maybe this equally nonsensical answer:
Bobrowski
Bodmer
Borchert
Born
Böll
Brandis
Brant
Böhme
Böttcher
Or even this still completely nonsensical answer:
Bobrowski
Bodmer
Borchert
Born
Böhme
Böll
Brandis
Brant
Böttcher
The crucial point to all that is that it's text not binary , so not only can you never
judge what its bit patterns hold just by eyeballing it, more importantly, it has special rules to
make it sort alphabetically (some might say sanely), an ordering no naïve code‐point sort will never
come even close to getting right, especially on Unicode.
The correct ordering is:
Bobrowski
Bodmer
Böhme
Böll
Borchert
Born
Böttcher
Brandis
Brant
And that is precisely what
use Unicode::Collate;
@sorted_germans = Unicode::Collate::->new->sort(@german_names);
gives you: a correctly sorted list of those Germans' names.
Sorting German Names
Hold on, though.
Correct in what language? In English, yes, the order given is now correct. But considering that
these authors wrote in the German language, it is quite conceivable that you should be following
the rules for ordering German names in German , not in English. That produces this ordering:
Bobrowski
Bodmer
Böhme
Böll
Böttcher
Borchert
Born
Brandis
Brant
How come Böttcher now came before Borchert ? Because Böttcher is supposed
to be the same as Boettcher . In a German phonebook or other German list of German names,
things like ‹ö› and ‹oe› are considered synonyms, which is not at all how it works in English. To
get the German phonebook sort, you merely have to modify your constructor this way:
use Unicode::Collate::Locale;
@sorted_germans = Unicode::Collate::Locale::
->new(locale => "de_phonebook")
->sort(@german_names);
Isn't this fun?
Be glad you're not sorting names. Sorting names is hard.
Default Sort Tables
Here are most of the Latin letters, ordered using the default sort :
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j
k l m n o p q r s t u v w x y z ª º À Á Â Ã Ä Å Æ Ç È É Ê Ë Ì Í Î Ï Ð Ñ
Ò Ó Ô Õ Ö Ø Ù Ú Û Ü Ý Þ ß à á â ã ä å æ ç è é ê ë ì í î ï ð ñ ò ó ô õ ö
ø ù ú û ü ý þ ÿ Ā ā Ă ă Ą ą Ć ć Ĉ ĉ Ċ ċ Č č Ď ď Đ đ Ē ē Ĕ ĕ Ė ė Ę ę Ě ě
Ĝ ĝ Ğ ğ Ġ ġ Ģ ģ Ĥ ĥ Ħ ħ Ĩ ĩ Ī ī Ĭ ĭ Į į İ ı IJ ij Ĵ ĵ Ķ ķ ĸ Ĺ ĺ Ļ ļ Ľ ľ Ŀ
ŀ Ł ł Ń ń Ņ ņ Ň ň Ŋ ŋ Ō ō Ŏ ŏ Ő ő Œ œ Ŕ ŕ Ŗ ŗ Ř ř Ś ś Ŝ ŝ Ş ş Š š Ţ ţ Ť
ť Ŧ ŧ Ũ ũ Ū ū Ŭ ŭ Ů ů Ű ű Ų ų Ŵ ŵ Ŷ ŷ Ÿ Ź ź Ż ż Ž ž ſ ƀ Ɓ Ƃ ƃ Ƈ ƈ Ɖ Ɗ Ƌ
ƌ ƍ Ǝ Ə Ɛ Ƒ ƒ Ɠ Ɣ ƕ Ɩ Ɨ Ƙ ƙ ƚ ƛ Ɯ Ɲ ƞ Ƥ ƥ Ʀ ƫ Ƭ ƭ Ʈ Ư ư Ʊ Ʋ Ƴ ƴ Ƶ ƶ Ʒ Ƹ
ƹ ƺ ƾ ƿ DŽ Dž dž LJ Lj lj NJ Nj nj Ǎ ǎ Ǐ ǐ Ǒ ǒ Ǔ ǔ Ǖ ǖ Ǘ ǘ Ǚ ǚ Ǜ ǜ ǝ Ǟ ǟ Ǡ ǡ Ǣ ǣ
Ǥ ǥ Ǧ ǧ Ǩ ǩ Ǫ ǫ Ǭ ǭ Ǯ ǯ ǰ DZ Dz dz Ǵ ǵ Ƿ Ǹ ǹ Ǻ ǻ Ǽ ǽ Ǿ ǿ Ȁ ȁ Ȃ ȃ Ȅ ȅ Ȇ ȇ Ȉ
ȉ Ȋ ȋ Ȍ ȍ Ȏ ȏ Ȑ ȑ Ȓ ȓ Ȕ ȕ Ȗ ȗ Ș ș Ț ț Ȝ ȝ Ȟ ȟ Ƞ ȡ Ȥ ȥ Ȧ ȧ Ȩ ȩ Ȫ ȫ Ȭ ȭ Ȯ
ȯ Ȱ ȱ Ȳ ȳ ȴ ȵ ȶ ȷ Ⱥ Ȼ ȼ Ƚ Ⱦ ɐ ɑ ɒ ɓ ɕ ɖ ɗ ɘ ə ɚ ɛ ɜ ɝ ɞ ɟ ɠ ɡ ɢ ɣ ɤ ɥ ɦ
ɧ ɨ ɩ ɪ ɫ ɬ ɭ ɮ ɯ ɰ ɱ ɲ ɳ ɴ ɶ ɹ ɺ ɻ ɼ ɽ ɾ ɿ ʀ ʁ ʂ ʃ ʄ ʅ ʆ ʇ ʈ ʉ ʊ ʋ ʌ ʍ
ʎ ʏ ʐ ʑ ʒ ʓ ʙ ʚ ʛ ʜ ʝ ʞ ʟ ʠ ʣ ʤ ʥ ʦ ʧ ʨ ʩ ʪ ʫ ˡ ˢ ˣ ᴀ ᴁ ᴂ ᴃ ᴄ ᴅ ᴆ ᴇ ᴈ ᴉ
ᴊ ᴋ ᴌ ᴍ ᴎ ᴏ ᴑ ᴓ ᴔ ᴘ ᴙ ᴚ ᴛ ᴜ ᴝ ᴞ ᴟ ᴠ ᴡ ᴢ ᴣ ᴬ ᴭ ᴮ ᴯ ᴰ ᴱ ᴲ ᴳ ᴴ ᴵ ᴶ ᴷ ᴸ ᴹ ᴺ
ᴻ ᴼ ᴾ ᴿ ᵀ ᵁ ᵂ ᵃ ᵄ ᵅ ᵆ ᵇ ᵈ ᵉ ᵊ ᵋ ᵌ ᵍ ᵎ ᵏ ᵐ ᵑ ᵒ ᵖ ᵗ ᵘ ᵙ ᵚ ᵛ ᵢ ᵣ ᵤ ᵥ ᵫ ᵬ ᵭ
ᵮ ᵯ ᵰ ᵱ ᵲ ᵳ ᵴ ᵵ ᵶ Ḁ ḁ Ḃ ḃ Ḅ ḅ Ḇ ḇ Ḉ ḉ Ḋ ḋ Ḍ ḍ Ḏ ḏ Ḑ ḑ Ḓ ḓ Ḕ ḕ Ḗ ḗ Ḙ ḙ Ḛ
ḛ Ḝ ḝ Ḟ ḟ Ḡ ḡ Ḣ ḣ Ḥ ḥ Ḧ ḧ Ḩ ḩ Ḫ ḫ Ḭ ḭ Ḯ ḯ Ḱ ḱ Ḳ ḳ Ḵ ḵ Ḷ ḷ Ḹ ḹ Ḻ ḻ Ḽ ḽ Ḿ
ḿ Ṁ ṁ Ṃ ṃ Ṅ ṅ Ṇ ṇ Ṉ ṉ Ṋ ṋ Ṍ ṍ Ṏ ṏ Ṑ ṑ Ṓ ṓ Ṕ ṕ Ṗ ṗ Ṙ ṙ Ṛ ṛ Ṝ ṝ Ṟ ṟ Ṡ ṡ Ṣ
ṣ Ṥ ṥ Ṧ ṧ Ṩ ṩ Ṫ ṫ Ṭ ṭ Ṯ ṯ Ṱ ṱ Ṳ ṳ Ṵ ṵ Ṷ ṷ Ṹ ṹ Ṻ ṻ Ṽ ṽ Ṿ ṿ Ẁ ẁ Ẃ ẃ Ẅ ẅ Ẇ
ẇ Ẉ ẉ Ẋ ẋ Ẍ ẍ Ẏ ẏ Ẑ ẑ Ẓ ẓ Ẕ ẕ ẖ ẗ ẘ ẙ ẚ ẛ ẞ ẟ Ạ ạ Ả ả Ấ ấ Ầ ầ Ẩ ẩ Ẫ ẫ Ậ
ậ Ắ ắ Ằ ằ Ẳ ẳ Ẵ ẵ Ặ ặ Ẹ ẹ Ẻ ẻ Ẽ ẽ Ế ế Ề ề Ể ể Ễ ễ Ệ ệ Ỉ ỉ Ị ị Ọ ọ Ỏ ỏ Ố
ố Ồ ồ Ổ ổ Ỗ ỗ Ộ ộ Ớ ớ Ờ ờ Ở ở Ỡ ỡ Ợ ợ Ụ ụ Ủ ủ Ứ ứ Ừ ừ Ử ử Ữ ữ Ự ự Ỳ ỳ Ỵ
ỵ Ỷ ỷ Ỹ ỹ K Å Ⅎ ⅎ Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ Ⅶ Ⅷ Ⅸ Ⅹ Ⅺ Ⅻ Ⅼ Ⅽ Ⅾ Ⅿ ⅰ ⅱ ⅲ ⅳ ⅴ
ⅵ ⅶ ⅷ ⅸ ⅹ ⅺ ⅻ ⅼ ⅽ ⅾ ⅿ ff fi fl ffi ffl ſt st Ａ Ｂ Ｃ Ｄ Ｅ Ｆ Ｇ Ｈ Ｉ
Ｊ Ｋ Ｌ Ｍ Ｎ Ｏ Ｐ Ｑ Ｒ Ｓ Ｔ Ｕ Ｖ Ｗ Ｘ Ｙ Ｚ ａ ｂ ｃ ｄ ｅ ｆ ｇ ｈ ｉ
ｊ ｋ ｌ ｍ ｎ ｏ ｐ ｑ ｒ ｓ ｔ ｕ ｖ ｗ ｘ ｙ ｚ
As you can see, those letters are scattered all over the place. Sure, it's not completely random,
but it's not useful either, because it is full of arbitrary placement that makes no alphabetical
sense. That's because it is not an alphabetic sort at all. However, with the special kind of sort
I've just shown you above, the ones that call the sort method from the Unicode::Collate
class, you do get an alphabetic sort. Using that method, the Latin letters I just showed you
now come out in alphabetical order, which is like this:
a ａ A Ａ ª ᵃ ᴬ á Á à À ă Ă ắ Ắ ằ Ằ ẵ Ẵ ẳ Ẳ â Â ấ Ấ ầ Ầ ẫ Ẫ ẩ Ẩ ǎ Ǎ å Å
Å ǻ Ǻ ä Ä ǟ Ǟ ã Ã ȧ Ȧ ǡ Ǡ ą Ą ā Ā ả Ả ȁ Ȁ ȃ Ȃ ạ Ạ ặ Ặ ậ Ậ ḁ Ḁ æ Æ ᴭ ǽ Ǽ
ǣ Ǣ ẚ ᴀ Ⱥ ᴁ ᴂ ᵆ ɐ ᵄ ɑ ᵅ ɒ b ｂ B Ｂ ᵇ ᴮ ḃ Ḃ ḅ Ḅ ḇ Ḇ ʙ ƀ ᴯ ᴃ ᵬ ɓ Ɓ ƃ Ƃ c
ｃ ⅽ C Ｃ Ⅽ ć Ć ĉ Ĉ č Č ċ Ċ ç Ç ḉ Ḉ ᴄ ȼ Ȼ ƈ Ƈ ɕ d ｄ ⅾ D Ｄ Ⅾ ᵈ ᴰ ď Ď ḋ
Ḋ ḑ Ḑ ḍ Ḍ ḓ Ḓ ḏ Ḏ đ Đ ð Ð dz ʣ Dz DZ dž Dž DŽ ʥ ʤ ᴅ ᴆ ᵭ ɖ Ɖ ɗ Ɗ ƌ Ƌ ȡ ẟ e ｅ E
Ｅ ᵉ ᴱ é É è È ĕ Ĕ ê Ê ế Ế ề Ề ễ Ễ ể Ể ě Ě ë Ë ẽ Ẽ ė Ė ȩ Ȩ ḝ Ḝ ę Ę ē Ē ḗ
Ḗ ḕ Ḕ ẻ Ẻ ȅ Ȅ ȇ Ȇ ẹ Ẹ ệ Ệ ḙ Ḙ ḛ Ḛ ᴇ ǝ Ǝ ᴲ ə Ə ᵊ ɛ Ɛ ᵋ ɘ ɚ ɜ ᴈ ᵌ ɝ ɞ ʚ ɤ
f ｆ F Ｆ ḟ Ḟ ff ffi ffl fi fl ʩ ᵮ ƒ Ƒ ⅎ Ⅎ g ｇ G Ｇ ᵍ ᴳ ǵ Ǵ ğ Ğ ĝ Ĝ ǧ Ǧ ġ Ġ ģ
Ģ ḡ Ḡ ɡ ɢ ǥ Ǥ ɠ Ɠ ʛ ɣ Ɣ h ｈ H Ｈ ᴴ ĥ Ĥ ȟ Ȟ ḧ Ḧ ḣ Ḣ ḩ Ḩ ḥ Ḥ ḫ Ḫ ẖ ħ Ħ ʜ
ƕ ɦ ɧ i ｉ ⅰ I Ｉ Ⅰ ᵢ ᴵ í Í ì Ì ĭ Ĭ î Î ǐ Ǐ ï Ï ḯ Ḯ ĩ Ĩ İ į Į ī Ī ỉ Ỉ ȉ
Ȉ ȋ Ȋ ị Ị ḭ Ḭ ⅱ Ⅱ ⅲ Ⅲ ij IJ ⅳ Ⅳ ⅸ Ⅸ ı ɪ ᴉ ᵎ ɨ Ɨ ɩ Ɩ j ｊ J Ｊ ᴶ ĵ Ĵ ǰ ȷ ᴊ
ʝ ɟ ʄ k ｋ K K Ｋ ᵏ ᴷ ḱ Ḱ ǩ Ǩ ķ Ķ ḳ Ḳ ḵ Ḵ ᴋ ƙ Ƙ ʞ l ｌ ⅼ L Ｌ Ⅼ ˡ ᴸ ĺ Ĺ
ľ Ľ ļ Ļ ḷ Ḷ ḹ Ḹ ḽ Ḽ ḻ Ḻ ł Ł ŀ Ŀ lj Lj LJ ʪ ʫ ʟ ᴌ ƚ Ƚ ɫ ɬ ɭ ȴ ɮ ƛ ʎ m ｍ ⅿ M
Ｍ Ⅿ ᵐ ᴹ ḿ Ḿ ṁ Ṁ ṃ Ṃ ᴍ ᵯ ɱ n ｎ N Ｎ ᴺ ń Ń ǹ Ǹ ň Ň ñ Ñ ṅ Ṅ ņ Ņ ṇ Ṇ ṋ Ṋ ṉ
Ṉ nj Nj NJ ɴ ᴻ ᴎ ᵰ ɲ Ɲ ƞ Ƞ ɳ ȵ ŋ Ŋ ᵑ o ｏ O Ｏ º ᵒ ᴼ ó Ó ò Ò ŏ Ŏ ô Ô ố Ố ồ
Ồ ỗ Ỗ ổ Ổ ǒ Ǒ ö Ö ȫ Ȫ ő Ő õ Õ ṍ Ṍ ṏ Ṏ ȭ Ȭ ȯ Ȯ ȱ Ȱ ø Ø ǿ Ǿ ǫ Ǫ ǭ Ǭ ō Ō ṓ
Ṓ ṑ Ṑ ỏ Ỏ ȍ Ȍ ȏ Ȏ ớ Ớ ờ Ờ ỡ Ỡ ở Ở ợ Ợ ọ Ọ ộ Ộ œ Œ ᴏ ᴑ ɶ ᴔ ᴓ p ｐ P Ｐ ᵖ
ᴾ ṕ Ṕ ṗ Ṗ ᴘ ᵱ ƥ Ƥ q ｑ Q Ｑ ʠ ĸ r ｒ R Ｒ ᵣ ᴿ ŕ Ŕ ř Ř ṙ Ṙ ŗ Ŗ ȑ Ȑ ȓ Ȓ ṛ
Ṛ ṝ Ṝ ṟ Ṟ ʀ Ʀ ᴙ ᵲ ɹ ᴚ ɺ ɻ ɼ ɽ ɾ ᵳ ɿ ʁ s ｓ S Ｓ ˢ ś Ś ṥ Ṥ ŝ Ŝ š Š ṧ Ṧ ṡ
Ṡ ş Ş ṣ Ṣ ṩ Ṩ ș Ș ſ ẛ ß ẞ st ſt ᵴ ʂ ʃ ʅ ʆ t ｔ T Ｔ ᵗ ᵀ ť Ť ẗ ṫ Ṫ ţ Ţ ṭ Ṭ
ț Ț ṱ Ṱ ṯ Ṯ ʨ ƾ ʦ ʧ ᴛ ŧ Ŧ Ⱦ ᵵ ƫ ƭ Ƭ ʈ Ʈ ȶ ʇ u ｕ U Ｕ ᵘ ᵤ ᵁ ú Ú ù Ù ŭ Ŭ
û Û ǔ Ǔ ů Ů ü Ü ǘ Ǘ ǜ Ǜ ǚ Ǚ ǖ Ǖ ű Ű ũ Ũ ṹ Ṹ ų Ų ū Ū ṻ Ṻ ủ Ủ ȕ Ȕ ȗ Ȗ ư Ư
ứ Ứ ừ Ừ ữ Ữ ử Ử ự Ự ụ Ụ ṳ Ṳ ṷ Ṷ ṵ Ṵ ᴜ ᴝ ᵙ ᴞ ᵫ ʉ ɥ ɯ Ɯ ᵚ ᴟ ɰ ʊ Ʊ v ｖ ⅴ V
Ｖ Ⅴ ᵛ ᵥ ṽ Ṽ ṿ Ṿ ⅵ Ⅵ ⅶ Ⅶ ⅷ Ⅷ ᴠ ʋ Ʋ ʌ w ｗ W Ｗ ᵂ ẃ Ẃ ẁ Ẁ ŵ Ŵ ẘ ẅ Ẅ ẇ Ẇ ẉ
Ẉ ᴡ ʍ x ｘ ⅹ X Ｘ Ⅹ ˣ ẍ Ẍ ẋ Ẋ ⅺ Ⅺ ⅻ Ⅻ y ｙ Y Ｙ ý Ý ỳ Ỳ ŷ Ŷ ẙ ÿ Ÿ ỹ Ỹ ẏ
Ẏ ȳ Ȳ ỷ Ỷ ỵ Ỵ ʏ ƴ Ƴ z ｚ Z Ｚ ź Ź ẑ Ẑ ž Ž ż Ż ẓ Ẓ ẕ Ẕ ƍ ᴢ ƶ Ƶ ᵶ ȥ Ȥ ʐ ʑ
ʒ Ʒ ǯ Ǯ ᴣ ƹ Ƹ ƺ ʓ ȝ Ȝ þ Þ ƿ Ƿ
Isn't that much nicer?
Romani Ite Domum
In case you're wondering what that last row of distinctly un‐Roman Latin letters might possibly
be, they're called respectively
ezh ʒ,
yogh ȝ,
thorn þ, and
wynn ƿ. They had to go somewhere,
so they ended up getting stuck after ‹z›
Some are still used in certain non‐English (but still Latin) alphabets today, such as Icelandic,
and even though you probably won't bump into them in contemporary English texts, you might see some
if you're reading the original texts of famous medieval English poems like Beowulf , Sir
Gawain and the Green Knight , or Brut .
The last of those, Brut , was written by a fellow named Laȝamon , a name whose third
letter is a yogh. Famous though he was, I wouldn't suggest changing your name to ‹Laȝamon› in his
honor, as I doubt the phone company would be amused.
By
Andy Sylvester
on
August 7,
2007 12:00 AM
Perl software development can occur at several levels. When first
developing the idea for an application, a Perl developer may start
with a short program to flesh out the necessary algorithms. After
that, the next step might be to create a package to support
object-oriented development. The final work is often to create a Perl
module for the package to make the logic available to all parts of the
application. Andy Sylvester explores this topic with a simple
mathematical function.
Creating a Perl Subroutine
I am working on ideas for implementing some mathematical concepts
for a method of composing music. The ideas come from the work of
Joseph
Schillinger
. At the heart of the method is being able to generate
patterns using mathematical operations and using those patterns in
music composition. One of the basic operations described by
Schillinger is creating a "resultant," or series of numbers, based on
two integers (or "generators"). Figure 1 shows a diagram of how to
create the resultant of the integers 5 and 3.
Figure 1. Creating the resultant of 5 and 3
Figure 1 shows two line patterns with units of 5 and units of 3.
The lines continue until both lines come down (or "close") at the same
time. The length of each line corresponds to the product of the two
generators (5 x 3 = 15). If you draw dotted lines down from where each
of the two generator lines change state, you can create a third line
that changes state at each of the dotted line points. The lengths of
the segments of the third line make up the resultant of the integers 5
and 3 (3, 2, 1, 3, 1, 2, 3).
Schillinger used graph paper to create resultants in his
System of
Musical Composition
. However, another convenient way of creating a
resultant is to calculate the modulus of a counter and then calculate
a term in the resultant series based on the state of the counter. An
algorithm to create the terms in a resultant might resemble:
Read generators from command line
Determine total number of counts for resultant
(major_generator * minor_generator)
Initialize resultant counter = 0
For MyCounts from 1 to the total number of counts
Get the modulus of MyCounts to the major and minor generators
Increment the resultant counter
If either modulus = 0
Save the resultant counter to the resultant array
Re-initialize resultant counter = 0
End if
End for
From this design, I wrote a short program using the Perl modulus
operator (
%
):
#!/usr/bin/perl
#*******************************************************
#
# FILENAME: result01.pl
#
# USAGE: perl result01.pl major_generator minor_generator
#
# DESCRIPTION:
# This Perl script will generate a Schillinger resultant
# based on two integers for the major generator and minor
# generator.
#
# In normal usage, the user will input the two integers
# via the command line. The sequence of numbers representing
# the resultant will be sent to standard output (the console
# window).
#
# INPUTS:
# major_generator - First generator for the resultant, input
# as the first calling argument on the
# command line.
#
# minor_generator - Second generator for the resultant, input
# as the second calling argument on the
# command line.
#
# OUTPUTS:
# resultant - Sequence of numbers written to the console window
#
#**************************************************************
use strict;
use warnings;
my $major_generator = $ARGV[0];
my $minor_generator = $ARGV[1];
my $total_counts = $major_generator * $minor_generator;
my $result_counter = 0;
my $major_mod = 0;
my $minor_mod = 0;
my $i = 0;
my $j = 0;
my @resultant;
print "Generator Total = $total_counts\n";
while ($i < $total_counts) {
$i++;
$result_counter++;
$major_mod = $i % $major_generator;
$minor_mod = $i % $minor_generator;
if (($major_mod == 0) || ($minor_mod == 0)) {
push(@resultant, $result_counter);
$result_counter = 0;
}
print "$i \n";
print "Modulus of $major_generator is $major_mod \n";
print "Modulus of $minor_generator is $minor_mod \n";
}
print "\n";
print "The resultant is @resultant \n";
Run the program with 5 and 3 as the inputs (
perl result01.pl
5 3
):
Generator Total = 15
1
Modulus of 5 is 1
Modulus of 3 is 1
2
Modulus of 5 is 2
Modulus of 3 is 2
3
Modulus of 5 is 3
Modulus of 3 is 0
4
Modulus of 5 is 4
Modulus of 3 is 1
5
Modulus of 5 is 0
Modulus of 3 is 2
6
Modulus of 5 is 1
Modulus of 3 is 0
7
Modulus of 5 is 2
Modulus of 3 is 1
8
Modulus of 5 is 3
Modulus of 3 is 2
9
Modulus of 5 is 4
Modulus of 3 is 0
10
Modulus of 5 is 0
Modulus of 3 is 1
11
Modulus of 5 is 1
Modulus of 3 is 2
12
Modulus of 5 is 2
Modulus of 3 is 0
13
Modulus of 5 is 3
Modulus of 3 is 1
14
Modulus of 5 is 4
Modulus of 3 is 2
15
Modulus of 5 is 0
Modulus of 3 is 0
The resultant is 3 2 1 3 1 2 3
This result matches the resultant terms as shown in the graph in
Figure 1, so it looks like the program generates the correct output.
Creating a Perl Package from a Program
With a working program, you can create a Perl package as a step
toward being able to reuse code in a larger application. The initial
program has two pieces of input data (the major generator and the
minor generator). The single output is the list of numbers that make
up the resultant. These three pieces of data could be combined in an
object. The program could easily become a subroutine to generate the
terms in the resultant. This could be a method in the class contained
in the package. Creating a class implies adding a constructor method
to create a new object. Finally, there should be some methods to get
the major generator and minor generator from the object to use in
generating the resultant (see the
perlboot
and
perltoot
tutorials
for background on object-oriented programming in Perl).
From these requirements, the resulting package might be:
#!/usr/bin/perl
#*******************************************************
#
# Filename: result01a.pl
#
# Description:
# This Perl script creates a class for a Schillinger resultant
# based on two integers for the major generator and the
# minor generator.
#
# Class Name: Resultant
#
# Synopsis:
#
# use Resultant;
#
# Class Methods:
#
# $seq1 = Resultant ->new(5, 3)
#
# Creates a new object with a major generator of 5 and
# a minor generator of 3. These parameters need to be
# initialized when a new object is created, as there
# are no methods to set these elements within the object.
#
# $seq1->generate()
#
# Generates a resultant and saves it in the ResultList array
#
# Object Data Methods:
#
# $major_generator = $seq1->get_major()
#
# Returns the major generator
#
# $minor_generator = $seq1->get_minor()
#
# Returns the minor generator
#
#
#**************************************************************
{ package Resultant;
use strict;
sub new {
my $class = shift;
my $major_generator = shift;
my $minor_generator = shift;
my $self = {Major => $major_generator,
Minor => $minor_generator,
ResultantList => []};
bless $self, $class;
return $self;
}
sub get_major {
my $self = shift;
return $self->{Major};
}
sub get_minor {
my $self = shift;
return $self->{Minor};
}
sub generate {
my $self = shift;
my $total_counts = $self->get_major * $self->get_minor;
my $i = 0;
my $major_mod;
my $minor_mod;
my @result;
my $result_counter = 0;
while ($i < $total_counts) {
$i++;
$result_counter++;
$major_mod = $i % $self->get_major;
$minor_mod = $i % $self->get_minor;
if (($major_mod == 0) || ($minor_mod == 0)) {
push(@result, $result_counter);
$result_counter = 0;
}
}
@{$self->{ResultList}} = @result;
}
}
#
# Test code to check out class methods
#
# Counter declaration
my $j;
# Create new object and initialize major and minor generators
my $seq1 = Resultant->new(5, 3);
# Print major and minor generators
print "The major generator is ", $seq1->get_major(), "\n";
print "The minor generator is ", $seq1->get_minor(), "\n";
# Generate a resultant
$seq1->generate();
# Print the resultant
print "The resultant is ";
foreach $j (@{$seq1->{ResultList}}) {
print "$j ";
}
print "\n";
Execute the file (
perl result01a.pl
):
The major generator is 5
The minor generator is 3
The resultant is 3 2 1 3 1 2 3
This output text shows the same resultant terms as produced by the
first program.
Creating a Perl Module
From a package, you can create a Perl module to make the package
fully reusable in an application. Also, you can modify our original
test code into a series of module tests to show that the module works
the same as the standalone package and the original program.
I like to use the Perl module
Module::Starter
to create a skeleton module for the package code.
To start, install the
Module::Starter
module and its
associated modules from CPAN, using the Perl Package Manager, or some
other package manager. To see if you already have the
Module::Starter
module installed, type
perldoc
Module::Starter
in a terminal window. If the man page does not
appear, you probably do not have the module installed.
Select a working directory to create the module directory. This can
be the same directory that you have been using to develop your Perl
program. Type the following command (though with your own name and
email address):
In the working directory, you should see a folder or directory
called
Music-Resultant
. Change your current directory to
Music-Resultant
, then type the commands:
$
perl Makefile.PL
$
make
These commands will create the full directory structure for the
module. Now paste the text from the package into the module template
at
Music-Resultant/lib/Music/Resultant.pm
. Open
Resultant.pm
in a text editor and paste the subroutines from the
package after the lines:
=head1 FUNCTIONS
=head2 function1
=cut
When you paste the package source code, remove the opening brace
from the package, so that the first lines appear as:
package Resultant;
sub new {
use strict;
my $class = shift;
and the last lines of the source appears without the the final
closing brace as:
@{$self->{ResultList}} = @result;
}
After making the above changes, save
Resultant.pm
. This is
all that you need to do to create a module for your own use. If you
eventually release your module to the Perl community or upload it to
CPAN
, you should do some more work
to prepare the module and its documentation (see the
perlmod
and
perlmodlib
documentation for more information).
After modifying
Resultant.pm
, you need to install the
module to make it available for other Perl applications. To avoid
configuration issues, install the module in your home directory,
separate from your main Perl installation.
In your home directory, create a
lib/
directory, then
create a
perl/
directory within the
lib/
directory. The result should resemble:
/home/myname/lib/perl
Go to your module directory (
Music-Resultant
) and
re-run the build process with a directory path to tell Perl where
to install the module:
$
perl Makefile.PL LIB=/home/myname/lib/perl
$
make install
Once this is complete, the module will be installed in the
directory.
The final step in module development is to add tests to the
.t
file templates created in the module directory. The Perl distribution
includes several built-in test modules, such as
Test::Simple
and
Test::More
to help test Perl subroutines and modules.
To test the module, open the file
Music-Resultant/t/00-load.t
.
The initial text in this file is:
You can run this test file from the
t/
directory using the
command:
perl -I/home/myname/lib/perl -T 00-load.t
The
-I
switch tells the Perl interpreter to look for
the module
Resultant.pm
in your alternate installation
directory. The directory path must immediately follow the
-I
switch, or Perl may not search your alternate directory for your
module. The
-T
switch is necessary because there is a
-T
switch in the first line of the test script, which
turns on taint checking. (Taint checking only works when enabled at
Perl startup;
perl
will exit with an error if you try to
enable it later.) Your results should resemble the following(your Perl
version may be different).
1..1
ok 1 - use Music::Resultant;
# Testing Music::Resultant 0.01, Perl 5.008006, perl
The test code from the second listing is easy to convert to the
format used by
Test::More
. Change the number at the end
of the tests line from 1 to 4, as you will be adding three more tests
to this file. The template file has an initial test to show that the
module exists. Next, add tests after the
BEGIN
block in
the file:
# Test 2:
my $seq1 = Resultant->new(5, 3); # create an object
isa_ok ($seq1, Resultant); # check object definition
# Test 3: check major generator
my $local_major_generator = $seq1->get_major();
is ($local_major_generator, 5, 'major generator is correct' );
# Test 4: check minor generator
my $local_minor_generator = $seq1->get_minor();
is ($local_minor_generator, 3, 'minor generator is correct' );
To run the tests, retype the earlier command line in the
Music-Resultant/
directory:
$
perl -I/home/myname/lib/perl -T t/00-load.t
You should see the results:
1..4
ok 1 - use Music::Resultant;
ok 2 - The object isa Resultant
ok 3 - major generator is correct
ok 4 - minor generator is correct
# Testing Music::Resultant 0.01, Perl 5.008006, perl
These tests create a Resultant object with a major generator of 5
and a minor generator of 3 (Test 2), and check to see that the major
generator in the object is correct (Test 3), and that the minor
generator is correct (Test 4). They do
not
cover the
resultant terms. One way to check the resultant is to add the test
code used in the second listing to the
.t
file:
# Generate a resultant
$seq1->generate();
# Print the resultant
my $j;
print "The resultant is ";
foreach $j (@{$seq1->{ResultList}}) {
print "$j ";
}
print "\n";
You should get the following results:
1..4
ok 1 - use Music::Resultant;
ok 2 - The object isa Resultant
ok 3 - major generator is correct
ok 4 - minor generator is correct
The resultant is 3 2 1 3 1 2 3
# Testing Music::Resultant 0.01, Perl 5.008006, perl
That's not valid test output, so it needs a little bit of
manipulation. To check the elements of a list using a testing
function, install the
Test::Differences
module and its associated modules from CPAN,
using the Perl Package Manager, or some other package manager. To see
if you already have the
Test::Differences
module
installed, type
perldoc Test::Differences
in a terminal
window. If the man page does not appear, you probably do not have the
module installed.
Once that module is part of your Perl installation, change the
number of tests from 4 to 5 on the
Test::More
statement
line and add a following statement after the
use Test::More
statement:
use Test::Differences;
Finally, replace the code that prints the resultant with:
# Test 5: (uses Test::Differences and associated modules)
$seq1->generate();
my @result = @{$seq1->{ResultList}};
my @expected = (3, 2, 1, 3, 1, 2, 3);
eq_or_diff \@result, \@expected, "resultant terms are correct";
Now when the test file runs, you can confirm that the resultant is
correct:
1..5
ok 1 - use Music::Resultant;
ok 2 - The object isa Resultant
ok 3 - major generator is correct
ok 4 - minor generator is correct
ok 5 - resultant terms are correct
# Testing Music::Resultant 0.01, Perl 5.008006, perl
Summary
There are multiple levels of Perl software development. Once you
start to create modules to enable reuse of your Perl code, you will be
able to leverage your effort into larger applications. By using Perl
testing modules, you can ensure that your code works the way you
expect and provide a way to ensure that the modules continue to work
as you add more features.
Resources
Here are some other good resources on creating Perl modules:
Perl Module Mechanics
goes into detail about the various files
created when you create a module directory.
While example are genome sequencing specific most code is good illustiontion of string processing
in Perl and as such has a wider appeal. See also
molecularevolution.org
This page contains an uncompressed copy of example code from your course text, downloaded on January
15, 2003. Please see the official
Beginning Perl for Bioinformatics
Website under the heading " Examples and Exercises " for any updates to this code.
General files
readme.txt
- overview of code provided by text author
NOTE: Examples 4-5 to 4-7 also require the protein sequence data file:
NM_021964fragment.pep.txt - To match the example in your book, save the file out with the name:
NM_021964fragment.pep
NOTE: Example 5-3 also requires the protein sequence data file:
NM_021964fragment.pep.txt - To match the example in your book, save the file out with the name:
NM_021964fragment.pep NOTE: Example 5-4, 5-6 and 5-7 also require the DNA file:
small.dna.txt
- To match the example in your book, save the file out with the name: small.dna
NOTE: BeginPerlBioinfo.pm
may be needed to execute some code examples from this chapter. Place this file in the same directory
as your .pl files.
NOTE: Example 8-2,8-3 and 8-4 also require the DNA file:
sample.dna.txt
- To match the example in your book, save the file out with the name: sample.dna
Perl is remarkably good for slicing, dicing, twisting, wringing, smoothing, summarizing
and otherwise mangling text. Although the biological sciences do involve a good deal of
numeric analysis now, most of the primary data is still text: clone names, annotations, comments,
bibliographic references. Even DNA sequences are textlike. Interconverting incompatible data
formats is a matter of text mangling combined with some creative guesswork. Perl's powerful
regular expression matching and string manipulation operators simplify this job in a way that
isn't equalled by any other modern language.
Perl is forgiving. Biological data is often incomplete, fields can be missing, or
a field that is expected to be present once occurs several times (because, for example, an
experiment was run in duplicate), or the data was entered by hand and doesn't quite fit the
expected format. Perl doesn't particularly mind if a value is empty or contains odd characters.
Regular expressions can be written to pick up and correct a variety of common errors in data
entry. Of course this flexibility can be also be a curse. I talk more about the problems with
Perl below.
Perl is component-oriented. Perl encourages people to write their software in small
modules, either using Perl library modules or with the classic Unix tool-oriented approach.
External programs can easily be incorporated into a Perl script using a pipe, system call or
socket. The dynamic loader introduced with Perl5 allows people to extend the Perl language
with C routines or to make entire compiled libraries available for the Perl interpreter. An
effort is currently under way to gather all the world's collected wisdom about biological data
into a set of modules called "bioPerl" (discussed at length in an article to be published later
in the Perl Journal).
Perl is easy to write and fast to develop in. The interpreter doesn't require you
to declare all your function prototypes and data types in advance, new variables spring into
existence as needed, calls to undefined functions only cause an error when the function is
needed. The debugger works well with Emacs and allows a comfortable interactive style of development.
Perl is a good prototyping language. Because Perl is quick and dirty, it often makes
sense to prototype new algorithms in Perl before moving them to a fast compiled language. Sometimes
it turns out that Perl is fast enough so that of the algorithm doesn't have to be ported; more
frequently one can write a small core of the algorithm in C, compile it as a dynamically loaded
module or external executable, and leave the rest of the application in Perl (for an example
of a complex genome mapping application implemented in this way, see
http://waldo.wi.mit.edu/ftp/distribution/software/rhmapper/
).
Perl is a good language for Web CGI scripting, and is growing in importance as more
labs turn to the Web for publishing their data.
I use lots of Perl for dealing with qualitative and quantitative data in social science research.
In terms of getting things done (largely with text) quickly, finding libraries on CPAN (nice central
location), and generally just getting things done quickly, it can't be surpassed.
Perl is also excellent glue, so if you have some instrumental records, and you need to glue
them to data analysis routines, then Perl is your language. Perl is very powerful when it comes
to deal with text and it's present in almost every Linux/Unix distribution. In bioinformatics,
not only are sequence data very easy to manipulate with Perl, but also most of the bionformatics
algorithms will output some kind of text results.
Then, the biggest bioinformatics centers like the
EBI
had that great guy, Ewan Birney, who was leading the
BioPerl project. That library
has lots of parsers for every kind of popular bioinformatics algorithms' results, and for manipulating
the different sequence formats used in major sequence databases.
Nowadays, however, Perl is not the only language used by bioinformaticians: along with sequence
data, labs produce more and more different kinds of data types and other languages are more often
used in those areas.
The R
statistics programming language for example, is widely used for statistical analysis of microarray
and qPCR data (among others). Again, why are we using it so much? Because it has great libraries
for that kind of data (see bioconductor
project).
Now when it comes to web development,
CGI is not
really state of the art today, but people who know Perl may stick to it. In my company though
it is no longer used...
I hope this helps.
Bioinformatics deals primarily in text parsing and Perl is the best programming language for the
job as it is made for string parsing. As the O'Reilly book (Beginning Perl for Bioinformatics) says
that "With [Perl]s highly developed capacity to detect patterns in data, Perl has become one of the
most popular languages for biological data analysis." This seems to be a pretty comprehensive response.
Perhaps one thing missing, however, is that most biologists (until recently, perhaps) don't have
much programming experience at all. The learning curve for Perl is much lower than for compiled languages
(like C or Java), and yet Perl still provides a ton of features when it comes to text processing.
So what if it takes longer to run? Biologists can definitely handle that. Lab experiments routinely
take one hour or more finish, so waiting a few extra minutes for that data processing to finish isn't
going to kill them!
Just note that I am talking here about biologists that program out of necessity. I understand
that there are some very skilled programmers and computer scientists out there that use Perl as well,
and these comments may not apply to them.
===
People missed out DBI , the
Perl abstract database interface that makes it really easy to work with bioinformatic databases.
There is also the one-liner
angle. You can write something to reformat data in a single line in Perl and just use the
-pe flag to embed that at the command line. Many people using
AWK and
sed moved to Perl. Even in full programs,
file I/O is incredibly easy and quick to write, and text transformation is expressive at a high level
compared to any engineering language around. People who use Java or even Python for one-off text
transformation are just too lazy to learn another language. Java especially has a high dependence
on the JVM implementation
and its I/O performance.
At least you know how fast or slow Perl will be everywhere, slightly slower than C I/O. Don't
learn grep ,
cut ,
sed , or
AWK ; just learn Perl as your command
line tool, even if you don't produce large programs with it. Regarding CGI, Perl has plenty of better
web frameworks such as
Catalyst and
Mojolicious , but the mindshare
definitely came from CGI and bioinformatics being one of the earliest heavy users of the Internet.
===
Perl is very easy to learn as compared to other languages. It can fully exploit the biological
data which is becoming the big data. It can manipulate big data and perform good for manipulation
data curation and all type of DNA programming, automation of biology has become easy due languages
like Perl, Python and
Ruby .
It is very easy for those who are knowing biology, but not knowing how to program that in other programming
languages.
Personally, and I know this will date me, but it's because I learned Perl first. I was being asked
to take FASTA files and
mix with other FASTA files. Perl was the recommended tool when I asked around.
At the time I'd been through a few computer science classes, but I didn't really know programming
all that well.
Perl proved fairly easy to learn. Once I'd gotten
regular expressions
into my head I was parsing and making new FASTA files within a day.
As has been suggested, I was not a programmer. I was a biochemistry graduate working in a lab,
and I'd made the mistake of setting up a Linux server where everyone could see me. This was back
in the day when that was an all-day project.
Anyway, Perl became my goto for anything I needed to do around the lab. It was awesome, easy to
use, super flexible, other Perl guys in other labs we're a lot like me.
So, to cut it short, Perl is easy to learn, flexible and forgiving, and it did what I needed.
Once I really got into bioinformatics I picked up R, Python, and even Java. Perl is not that great
at helping to create maintainable code, mostly because it is so flexible. Now I just use the language
for the job, but Perl is still one of my favorite languages, like a first kiss or something.
To reiterate, most bioinformatics folks learned coding by just kluging stuff together, and most
of the time you're just trying to get an answer for the
principal investigator
(PI), so you can't spend days on code design. Perl is superb at just getting an answer, it probably
won't work a second time, and you will not understand anything in your own code if you see it six
months later; BUT if you need something now, then it is a good choice even though I mostly use Python
now.
I hope that gives you an answer from someone who lived it.
Perl subst function can used as pseudo function on the left side of assignment, That
allow to insert a substring into arbitrary point of the string
For example, the code fragment:
$test_string='<cite>xxx<blockquote>test to show to insert substring into string using substr as pseudo-function</blockquote>';
print "Before: $test_string\n";
substr($test_string,length('<cite>xxx'),0)='</cite>';
print "After: $test_string\n";
will print
Before: <cite>xxx<blockquote>test to show to insert substring into string using substr as pseudo-function</blockquote>
After: <cite>xxx</cite><blockquote>test to show to insert substring into string using substr as pseudo-function</blockquote>
Please note that is you found the symbol of string bafore which you need to insert the string
you need to substrac one from the found position
This article, as regular readers may have guessed, is the sequel to "
One-liners
101
," which appeared in a previous installment of "Cultured Perl".
The earlier article is an absolute requirement for understanding the
material here, so please take a look at it before you continue.
The goal of this article, as with its predecessor, is to show legible
and reusable code, not necessarily the shortest or most efficient version
of a program. With that in mind, let's get to the code!
Tom Christiansen's list
Tom Christiansen posted a list of one-liners on Usenet years ago, and
that list is still interesting and useful for any Perl programmer. We
will look at the more complex one-liners from the list; the full list is
available in the file tomc.txt (see
Related topics
to download this file). The list overlaps slightly
with the "
One-liners
101
" article, and I will try to point out those intersections.
Awk is commonly used for basic tasks such as breaking up text into
fields; Perl excels at text manipulation by design. Thus, we come to our
first one-liner, intended to add two columns in the text input to the
script.
Listing 1. Like awk?
1
2
3
4
# add first and
penultimate columns
# NOTE the equivalent
awk script:
# awk '{i = NF - 1;
print $1 + $i}'
perl -lane 'print
$F[0] + $F[-2]'
So what does it do? The magic is in the switches. The
-n
and
-a
switches make the script a wrapper around input that
splits the input on whitespace into the
@F
array; the
-e
switch adds an extra statement into the wrapper. The code of
interest actually produced is:
Listing 2: The full Monty
1
2
3
4
5
while (<>)
{
@F
= split(' ');
print
$F[0] + $F[-2]; # offset -2 means "2nd to last
element of the array"
}
Another common task is to print the contents of a file between two
markers or between two line numbers.
Listing 3: Printing a range of lines
1
2
3
4
5
6
7
8
9
10
11
# 1. just lines 15 to
17
perl -ne 'print if 15
.. 17'
# 2. just lines NOT
between line 10 and 20
perl -ne 'print
unless 10 .. 20'
# 3. lines between
START and END
perl -ne 'print if
/^START$/ .. /^END$/'
# 4. lines NOT
between START and END
perl -ne 'print
unless /^START$/ .. /^END$/'
A problem with the first one-liner in Listing 3 is that it will go
through the
whole
file, even if the necessary range has already
been covered. The third one-liner does
not
have that problem,
because it will print all the lines between the
START
and
END
markers. If there are eight sets of
START/END
markers, the third one-liner will print the lines inside all eight sets.
Preventing the inefficiency of the first one-liner is easy: just use
the
$.
variable, which tells you the current line. Start
printing if
$.
is over 15 and exit if
$.
is
greater than 17.
Listing 4: Printing a numeric range of
lines more efficiently
1
2
# just lines 15 to
17, efficiently
perl -ne 'print if $.
>= 15; exit if $. >= 17;'
Enough printing, let's do some editing. Needless to say, if you are
experimenting with one-liners, especially ones
intended
to
modify data, you should keep backups. You wouldn't be the first
programmer to think a minor modification couldn't possibly make a
difference to a one-liner program; just don't make that assumption while
editing the Sendmail configuration or your mailbox.
Listing 5: In-place editing
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
# 1. in-place edit of
*.c files changing all foo to bar
perl -p -i.bak -e
's/\bfoo\b/bar/g' *.c
# 2. delete first 10
lines
perl -i.old -ne
'print unless 1 .. 10' foo.txt
# 3. change all the
isolated oldvar occurrences to newvar
perl -i.old -pe
's{\boldvar\b}{newvar}g' *.[chy]
# 4. increment all
numbers found in these files
perl -i.tiny -pe
's/(\d+)/ 1 + $1 /ge' file1 file2 ....
# 5. delete all but
lines between START and END
perl -i.old -ne
'print unless /^START$/ .. /^END$/' foo.txt
# 6. binary edit
(careful!)
perl -i.bak -pe
's/Mozilla/Slopoke/g' /usr/local/bin/netscape
Why does
1 .. 10
specify line numbers 1 through 10? Read
the "perldoc perlop" manual page. Basically, the
..
operator
iterates through a range. Thus, the script does not count 10
lines
,
it counts 10 iterations of the loop generated by the
-n
switch (see "perldoc perlrun" and Listing 2 for an example of that loop).
The magic of the
-i
switch is that it replaces each file
in
@ARGV
with the version produced by the script's output on
that file. Thus, the
-i
switch makes Perl into an editing
text filter. Do
not
forget to use the backup option to the
-i
switch. Following the
i
with an extension will
make a backup of the edited file using that extension.
Note how the
-p
and
-n
switch are used. The
-n
switch is used when you want explicitly to print out
data. The
-p
switch implicitly inserts a
print $_
statement in the loop produced by the
-n
switch. Thus, the
-p
switch is better for
full
processing of a file,
while the
-n
switch is better for
selective
file
processing, where only specific data needs to be printed.
Examples of in-place editing can also be found in the "
One-liners
101
" article.
Reversing the contents of a file is not a common task, but the
following one-liners show than the
-n
and
-p
switches are not always the best choice when processing an entire file.
Listing 6: Reversal of files' fortunes
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
# 1. command-line
that reverses the whole input by lines
# (printing each
line in reverse order)
perl -e 'print
reverse <>' file1 file2 file3 ....
# 2. command-line
that shows each line with its characters backwards
perl -nle 'print
scalar reverse $_' file1 file2 file3 ....
# 3. find palindromes
in the /usr/dict/words dictionary file
perl -lne '$_ = lc
$_; print if $_ eq reverse' /usr/dict/words
# 4. command-line
that reverses all the bytes in a file
perl -0777e 'print
scalar reverse <>' f1 f2 f3 ...
# 5. command-line
that reverses each paragraph in the file but prints
# them in order
perl -00 -e 'print
reverse <>' file1 file2 file3 ....
The
-0
(zero) flag is very useful if you want to read a
full paragraph or a full file into a single string. (It also works with
any character number, so you can use a special character as a marker.) Be
careful when reading a full file in one command (
-0777
),
because a large file will use up all your memory. If you need to read the
contents of a file backwards (for instance, to analyze a log in reverse
order), use the CPAN module File::ReadBackwards. Also see "
One-liners
101
," which shows an example of log analysis with
File::ReadBackwards.
Note the similarity between the first and second scripts in Listing 6.
The first one, however, is completely different from the second one. The
difference lies in using <> in scalar context (as
-n
does in
the second script) or list context (as the first script does).
The third script, the palindrome detector, did not originally have the
$_ = lc $_;
segment. I added that to catch those palindromes
like "Bob" that are not the same backwards.
My addition can be written as
$_ = lc;
as well, but
explicitly stating the subject of the
lc()
function makes
the one-liner more legible, in my opinion.
Paul Joslin's list
Paul Joslin was kind enough to send me some of his one-liners for this
article.
Listing 7: Rewrite with a random number
1
2
# replace string XYZ
with a random number less than 611 in these files
perl -i.bak -pe
"s/XYZ/int rand(611)/e" f1 f2 f3
This is a filter that replaces
XYZ
with a random number
less than 611 (that number is arbitrarily chosen). Remember the
rand()
function returns a random number between 0 and its
argument.
Note that
XYZ
will be replaced by a
different
random number every time, because the substitution evaluates "int
rand(611)" every time.
Listing 8: Revealing the files' base nature
1
2
3
4
5
6
7
8
9
10
11
# 1. Run basename on
contents of file
perl -pe "s@.*/@@gio"
INDEX
# 2. Run dirname on
contents of file
perl -pe
's@^(.*/)[^/]+@$1\n@' INDEX
# 3. Run basename on
contents of file
perl -MFile::Basename
-ne 'print basename $_' INDEX
# 4. Run dirname on
contents of file
perl -MFile::Basename
-ne 'print dirname $_' INDEX
One-liners 1 and 2 came from Paul, while 3 and 4 were my rewrites of
them with the File::Basename module. Their purpose is simple, but any
system administrator will find these one-liners useful.
Listing 9: Moving or renaming, it's all the
same in UNIX
1
2
3
4
5
6
# 1. write command to
mv dirs XYZ_asd to Asd
# (you may have to
preface each '!' with a '\' depending on your shell)
ls | perl -pe
's!([^_]+)_(.)(.*)!mv $1_$2$3 \u$2\E$3!gio'
# 2. Write a shell
script to move input from xyz to Xyz
ls | perl -ne 'chop;
printf "mv $_ %s\n", ucfirst $_;'
For regular users or system administrators, renaming files based on a
pattern is a very common task. The scripts above will do two kinds of
job: either remove the file name portion up to the
_
character, or change each filename so that its first letter is uppercased
according to the Perl
ucfirst()
function.
There is a UNIX utility called "mmv" by Vladimir Lanin that may also
be of interest. It allows you to rename files based on simple patterns,
and it's surprisingly powerful. See the
Related topics
section for a link to this utility.
Some of mine
The following is not a one-liner, but it's a pretty useful script that
started as a one-liner. It is similar to Listing 7 in that it replaces a
fixed string, but the trick is that the replacement itself for the fixed
string becomes the fixed string the next time.
The idea came from a newsgroup posting a long time ago, but I haven't
been able to find original version. The script is useful in case you need
to replace one IP address with another in all your system files -- for
instance, if your default router has changed. The script includes
$0
(in UNIX, usually the name of the script) in the list of files
to rewrite.
As a one-liner it ultimately proved too complex, and the messages
regarding what is about to be executed are necessary when system files
are going to be modified.
Listing 10: Replace one IP address with
another one
#!/usr/bin/perl -w
use Regexp::Common qw/net/;
# provides the regular expressions for IP matching
my $replacement =
shift @ARGV; # get the new IP address
die "You must provide
$0 with a replacement string for the IP
111.111.111.111"
unless
$replacement;
# we require that
$replacement be JUST a valid IP address
die "Invalid IP
address provided: [$replacement]"
unless
$replacement =~ m/^$RE{net}{IPv4}$/;
# replace the string
in each file
foreach my $file ($0,
qw[/etc/hosts /etc/defaultrouter /etc/ethers], @ARGV)
{
#
note that we know $replacement is a valid IP
address, so this is
#
not a dangerous invocation
my
$command = "perl -p -i.bak -e
's/111.111.111.111/$replacement/g' $file";
print
"Executing [$command]\n";
system($command);
}
Note the use of the Regexp::Common module, an indispensable resource
for any Perl programmer today. Without Regexp::Common, you will be
wasting a lot of time trying to match a number or other common patterns
manually, and you're likely to get it wrong.
Conclusion
Thanks to Paul Joslin for sending me his list of one-liners. And in
the spirit of conciseness that one-liners inspire, I'll refer you to "
One-liners
101
" for some closing thoughts on one-line Perl scripts.
Earlier this year, ActiveState conducted
a survey of users
who had downloaded our distribution of Perl over the prior year and a half. We received 356 responses–99
commercial users and 257 individual users. I've been using Perl for a long time, and I expected that
lengthy experience would be typical of the Perl community. Our survey results, however, tell a different
story.
Almost one-third of the respondents have three or fewer years of experience. Nearly half of all
respondents reported using Perl for fewer than five years, a statistic that could be attributed to
Perl's outstanding, inclusive community. The powerful and pragmatic nature of Perl and its supportive
community make it a great choice for a wide array of uses across a variety of industries.
For a deeper dive, check out this video of
my talk at YAPC
North America this year.
Perl careers
Right now you can search online and find Perl jobs related to Amazon and BBC, not to mention several
positions at Boeing. A quick search on Dice.com, an IT and engineering career website, yielded 3,575
listings containing the word Perl at companies like Amazon, Athena Health, and Northrop Grumman.
Perl is also found in the finance industry, where it's primarily used to pull data from databases
and process it.
Perl benefits
Perl's consistent utilization is the result of myriad factors, but its open source background
is a powerful attribute.
Projects using Perl reduce upfront costs and downstream risks, and when you factor in how clean
and powerful Perl is, it becomes quite a compelling option. Add to this that Perl sees yearly releases
(more than that, even, as Perl has seen seven releases since 2012), and you can begin to understand
why Perl still runs large parts of the web.
Mojolicious, Dancer, and Catalyst are just a few of the powerful web frameworks built for Perl.
Designed for simplicity and scalability, these frameworks provide aspiring Perl developers an easy
entry point to the language, which might explain some of the numbers from the survey I mentioned
above. The inclusive nature of the Perl community draws developers, as well. It's hard to find a
more welcoming or active community, and you can see evidence of that in the online groups, open source
projects, and regular worldwide conferences and workshops.
Perl modules
Perl also has a mature installation tool chain and a strong testing culture. Anyone who wants
to create automated test suites for Perl projects has the assistance of the over 400 testing and
quality modules available on CPAN (Comprehensive Perl Archive Network). They won't have to sort through
all 400 to choose the best, though: Test::Most is a one-stop shop for the most commonly used test
modules. CPAN is one of Perl's biggest advantages over other programming languages. The archive hosts
tens of thousands of ready-to-use modules for Perl, and the breadth and variety of those modules
is astounding.
Even with a quick search you can find hardcore numerical modules, ODE (ordinary differential equations)
solvers, and countless other types of modules written over the last 20 years by thousands of contributors.
This contribution-based archive network helps keep Perl fresh and relevant, proliferating modules
like pollen that will blow around to the incredible number of Perl projects out in the world.
You might think that community modules aren't the most reliable, but every distribution of modules
on CPAN has been tested on myriad platforms and Perl configurations. As a testament to the determination
of Perl users, the community has constructed a testing network and they spend time to make sure each
Perl module works well on every available platform. They also maintain extensively-checked libraries
that help Perl developers with big data projects.
What we're seeing today is a significant, dedicated community of Perl developers. This is not
only because the language is pragmatic, effective, and powerful, but also because of the incredible
community that these developers compose. The Perl community doesn't appear to be going anywhere,
which means neither is Perl.
Posted by EditorDavid on Saturday December 03, 2016 @10:34AM
An anonymous reader writes: Thursday brought this year's first new posts on
the Perl Advent Calendar , a
geeky tradition first started back in 2000.
Friday's post described Santa's need for fast, efficient
code, and the day that a Christmas miracle occurred during Santa's annual code review (involving
the is_hashref subroutine from Perl's reference utility library). And for the last five
years, the calendar has also had its own
Twitter feed .
But in another corner of the North Pole, you can also unwrap the
Perl 6 Advent Calendar , which this
year celebrates the one-year anniversary of the official launch of Perl 6. Friday's post was by brian
d foy, a writer on the classic Perl textbooks Learning Perl and Intermediate Perl (who's now also
crowdfunding
his next O'Reilly book , Learning Perl 6 ).
foy's post talked about Perl 6's object
hashes, while the calendar kicked off its new season Thursday with a discussion about creating Docker
images using webhooks triggered by GitHub commits as an example of Perl 6's "whipupitude".
[Nov 16, 2015] undef can be used as a dummy variable in split function
LWP (short for "Library for WWW in Perl") is a popular group of Perl modules for accessing data
on the Web. Like most Perl module-distributions, each of LWP's component modules comes with documentation
that is a complete reference to its interface. However, there are so many modules in LWP that it's
hard to know where to look for information on doing even the simplest things.
Introducing you to using LWP would require a whole book--a book that just happens to exist, called
Perl & LWP. This article
offers a sampling of recipes that let you perform common tasks with LWP.
Getting Documents with LWP::Simple
If you just want to access what's at a particular URL,
the simplest way to do it is to use LWP::Simple's functions.
In a Perl program, you can call its get($url) function. It will try getting that
URL's content. If it works, then it'll return the content; but if there's some error, it'll return
undef.
my $url = 'http://freshair.npr.org/dayFA.cfm?todayDate=current';
# Just an example: the URL for the most recent /Fresh Air/ show
use LWP::Simple;
my $content = get $url;
die "Couldn't get $url" unless defined $content;
# Then go do things with $content, like this:
if($content =~ m/jazz/i) {
print "They're talking about jazz today on Fresh Air!\n";
} else {
print "Fresh Air is apparently jazzless today.\n";
}
The handiest variant on get is getprint, which is useful in Perl one-liners.
If it can get the page whose URL you provide, it sends it to STDOUT; otherwise it complains
to STDERR.
This is the URL of a plain-text file. It lists new files in CPAN in the past two weeks. You can
easily make it part of a tidy little shell command, like this one that mails you the list of new
Acme:: modules:
% perl -MLWP::Simple -e "getprint 'http://cpan.org/RECENT'" \
| grep "/by-module/Acme" | mail -s "New Acme modules! Joy!" $USER
There are other useful functions in LWP::Simple, including one function for running
a HEAD request on a URL (useful for checking links, or getting the last-revised time
of a URL), and two functions for saving and mirroring a URL to a local file. See the
LWP::Simple
documentation for the full details, or Chapter 2, "Web Basics" of Perl & LWP for more
examples.
The Basics of the LWP Class Model
LWP::Simple's functions are handy for simple
cases, but its functions don't support cookies or authorization; they don't support setting header
lines in the HTTP request; and generally, they don't support reading header lines in the HTTP response
(most notably the full HTTP error message, in case of an error). To get at all those features, you'll
have to use the full LWP class model.
While LWP consists of dozens of classes, the two that you have to understand are LWP::UserAgent
and HTTP::Response. LWP::UserAgent is a class for "virtual browsers," which
you use for performing requests, and HTTP::Response is a class for the responses (or
error messages) that you get back from those requests.
The basic idiom is $response = $browser->get($url), or fully illustrated:
# Early in your program:
use LWP 5.64; # Loads all important LWP classes, and makes
# sure your version is reasonably recent.
my $browser = LWP::UserAgent->new;
...
# Then later, whenever you need to make a get request:
my $url = 'http://freshair.npr.org/dayFA.cfm?todayDate=current';
my $response = $browser->get( $url );
die "Can't get $url -- ", $response->status_line
unless $response->is_success;
die "Hey, I was expecting HTML, not ", $response->content_type
unless $response->content_type eq 'text/html';
# or whatever content-type you're equipped to deal with
# Otherwise, process the content somehow:
if($response->content =~ m/jazz/i) {
print "They're talking about jazz today on Fresh Air!\n";
} else {
print "Fresh Air is apparently jazzless today.\n";
}
There are two objects involved: $browser, which holds an object of the class LWP::UserAgent,
and then the $response object, which is of the class HTTP::Response. You
really need only one browser object per program; but every time you make a request, you get back
a new HTTP::Response object, which will have some interesting attributes:
A status code indicating success or failure (which you can test with $response->is_success).
An HTTP status line, which I hope is informative if there is a failure (which you can see
with $response->status_line, and which returns something like "404 Not Found").
A MIME content-type like "text/html", "image/gif", "application/xml", and so on, which you
can see with $response->content_type
The actual content of the response, in $response->content. If the response is
HTML, that's where the HTML source will be; if it's a GIF, then $response->content
will be the binary GIF data.
And dozens of other convenient and more specific methods that are documented in the docs for
HTTP::Response, and its superclasses, HTTP::Message and HTTP::Headers.
Adding Other HTTP Request Headers
The most commonly used syntax for requests is $response
= $browser->get($url), but in truth, you can add extra HTTP header lines to the request by
adding a list of key-value pairs after the URL, like so:
If you were only going to change the 'User-Agent' line, you could just change the $browser
object's default line from "libwww-perl/5.65" (or the like) to whatever you like, using LWP::UserAgent's
agent method:
$browser->agent('Mozilla/4.76 [en] (Win98; U)');
Enabling Cookies
A default LWP::UserAgent object acts like a browser with its
cookies support turned off. There are various ways of turning it on, by setting its cookie_jar
attribute. A "cookie jar" is an object representing a little database of all the HTTP cookies that
a browser can know about. It can correspond to a file on disk (the way Netscape uses its cookies.txt
file), or it can be just an in-memory object that starts out empty, and whose collection of cookies
will disappear once the program is finished running.
To give a browser an in-memory empty cookie jar, you set its cookie_jar attribute
like so:
$browser->cookie_jar({});
To give it a copy that will be read from a file on disk, and will be saved to it when the program
is finished running, set the cookie_jar attribute like this:
use HTTP::Cookies;
$browser->cookie_jar( HTTP::Cookies->new(
'file' => '/some/where/cookies.lwp',
# where to read/write cookies
'autosave' => 1,
# save it to disk when done
));
That file will be an LWP-specific format. If you want to access the cookies in your Netscape cookies
file, you can use the HTTP::Cookies::Netscape class:
use HTTP::Cookies;
# yes, loads HTTP::Cookies::Netscape too
$browser->cookie_jar( HTTP::Cookies::Netscape->new(
'file' => 'c:/Program Files/Netscape/Users/DIR-NAME-HERE/cookies.txt',
# where to read cookies
));
You could add an 'autosave' => 1 line as we did earlier, but at time of writing,
it's uncertain whether Netscape might discard some of the cookies you could be writing back to disk.
Posting Form Data
Many HTML forms send data to their server using an HTTP POST request,
which you can send with this syntax:
For example, the following program makes a search request to AltaVista (by sending some form data
via an HTTP POST request), and extracts from the HTML the report of the number of matches:
use strict;
use warnings;
use LWP 5.64;
my $browser = LWP::UserAgent->new;
my $word = 'tarragon';
my $url = 'http://www.altavista.com/sites/search/web';
my $response = $browser->post( $url,
[ 'q' => $word, # the Altavista query string
'pg' => 'q', 'avkw' => 'tgz', 'kl' => 'XX',
]
);
die "$url error: ", $response->status_line
unless $response->is_success;
die "Weird content type at $url -- ", $response->content_type
unless $response->content_type eq 'text/html';
if( $response->content =~ m{AltaVista found ([0-9,]+) results} ) {
# The substring will be like "AltaVista found 2,345 results"
print "$word: $1\n";
} else {
print "Couldn't find the match-string in the response\n";
}
Sending GET Form Data
Some HTML forms convey their form data not by sending the data in
an HTTP POST request, but by making a normal GET request with the data stuck on the
end of the URL. For example, if you went to imdb.com
and ran a search on Blade Runner, the URL you'd see in your browser window would be:
To run the same search with LWP, you'd use this idiom, which involves the URI class:
use URI;
my $url = URI->new( 'http://us.imdb.com/Tsearch' );
# makes an object representing the URL
$url->query_form( # And here the form data pairs:
'title' => 'Blade Runner',
'restrict' => 'Movies and TV',
);
my $response = $browser->get($url);
See Chapter 5, "Forms" of Perl & LWP for a longer discussion of HTML forms and of form
data, as well as Chapter 6 through Chapter 9 for a longer discussion of extracting data from HTML.
Absolutizing URLs
The URI class that we just mentioned above provides all sorts of methods
for accessing and modifying parts of URLs (such as asking sort of URL it is with $url->scheme,
and asking what host it refers to with $url->host, and so on, as described in
the docs for the URI class.
However, the methods of most immediate interest are the query_form method seen above,
and now the new_abs method for taking a probably relative URL string (like "../foo.html")
and getting back an absolute URL (like "http://www.perl.com/stuff/foo.html"), as shown here:
use URI;
$abs = URI->new_abs($maybe_relative, $base);
For example, consider this program that matches URLs in the HTML list of new modules in CPAN:
use strict;
use warnings;
use LWP 5.64;
my $browser = LWP::UserAgent->new;
my $url = 'http://www.cpan.org/RECENT.html';
my $response = $browser->get($url);
die "Can't get $url -- ", $response->status_line
unless $response->is_success;
my $html = $response->content;
while( $html =~ m/<A HREF=\"(.*?)\"/g ) {
print "$1\n";
}
When run, it emits output that starts out something like this:
(The $response->base method from HTTP::Message is for returning the
URL that should be used for resolving relative URLs--it's usually just the same as the URL that you
requested.)
See Chapter 4, "URLs", of Perl & LWP for a longer discussion of URI objects.
Of course, using a regexp to match hrefs is a bit simplistic, and for more robust programs, you'll
probably want to use an HTML-parsing module like HTML::LinkExtor, or HTML::TokeParser,
or even maybe HTML::TreeBuilder.
Other Browser Attributes
LWP::UserAgent objects have many attributes for controlling
how they work. Here are a few notable ones:
$browser->timeout(15): This sets this browser object to give up on requests that
don't answer within 15 seconds.
$browser->protocols_allowed( [ 'http', 'gopher'] ): This sets this browser object
to not speak any protocols other than HTTP and gopher. If it tries accessing any other kind of
URL (like an "ftp:" or "mailto:" or "news:" URL), then it won't actually try connecting, but instead
will immediately return an error code 500, with a message like "Access to ftp URIs has been disabled".
use LWP::ConnCache;
$browser->conn_cache(LWP::ConnCache->new()): This tells the browser object to try using
the HTTP/1.1 "Keep-Alive" feature, which speeds up requests by reusing the same socket connection
for multiple requests to the same server.
$browser->agent( 'SomeName/1.23 (more info here maybe)' ): This changes how the
browser object will identify itself in the default "User-Agent" line is its HTTP requests. By
default, it'll send "libwww-perl/versionnumber", like "libwww-perl/5.65". You can change
that to something more descriptive like this:
push @{ $ua->requests_redirectable }, 'POST': This tells this browser to obey
redirection responses to POST requests (like most modern interactive browsers), even though the
HTTP RFC says that should not normally be done.
If you want to make sure that your LWP-based program respects robots.txt
files and doesn't make too many requests too fast, you can use the LWP::RobotUA class
instead of the LWP::UserAgent class.
LWP::RobotUA class is just like LWP::UserAgent, and you can use it like
so:
use LWP::RobotUA;
my $browser = LWP::RobotUA->new(
'YourSuperBot/1.34', '[email protected]');
# Your bot's name and your email address
my $response = $browser->get($url);
But HTTP::RobotUA adds these features:
If the robots.txt on $url's server forbids you from accessing $url,
then the $browser object (assuming it's of the class LWP::RobotUA) won't
actually request it, but instead will give you back (in $response) a 403 error with
a message "Forbidden by robots.txt". That is, if you have this line:
die "$url -- ", $response->status_line, "\nAborted"
unless $response->is_success;
then the program would die with an error message like this:
http://whatever.site.int/pith/x.html -- 403 Forbidden
by robots.txt
Aborted at whateverprogram.pl line 1234
If this $browser object sees that the last time it talked to $url's
server was too recently, then it will pause (via sleep) to avoid making too many
requests too often. How long it will pause for, is by default one minute--but you can control
it with the $browser->delay( minutes ) attribute.
For example, this code:
$browser->delay( 7/60 );
means that this browser will pause when it needs to avoid talking to any given server more
than once every 7 seconds.
In some cases, you will want to (or will have to) use proxies for accessing
certain sites or for using certain protocols. This is most commonly the case when your LWP program
is running (or could be running) on a machine that is behind a firewall.
To make a browser object use proxies that are defined in the usual environment variables (HTTP_PROXY),
just call the env_proxy on a user-agent object before you go making any requests on
it. Specifically:
use LWP::UserAgent;
my $browser = LWP::UserAgent->new;
# And before you go making any requests:
$browser->env_proxy;
For more information on proxy parameters, see
the
LWP::UserAgent documentation, specifically the proxy, env_proxy, and
no_proxy methods.
HTTP Authentication
Many Web sites restrict access to documents by using "HTTP Authentication".
This isn't just any form of "enter your password" restriction, but is a specific mechanism where
the HTTP server sends the browser an HTTP code that says "That document is part of a protected 'realm',
and you can access it only if you re-request it and add some special authorization headers to your
request".
For example, the Unicode.org administrators stop email-harvesting bots from harvesting the contents
of their mailing list archives by protecting them with HTTP Authentication, and then publicly stating
the username and password (at http://www.unicode.org/mail-arch/)--namely username "unicode-ml"
and password "unicode".
For example, consider this URL, which is part of the protected area of the Web site:
If you access that with a browser, you'll get a prompt like "Enter username and password for 'Unicode-MailList-Archives'
at server 'www.unicode.org'", or in a graphical browser, something like this:
In LWP, if you just request that URL, like this:
use LWP 5.64;
my $browser = LWP::UserAgent->new;
my $url =
'http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html';
my $response = $browser->get($url);
die "Error: ", $response->header('WWW-Authenticate') ||
'Error accessing',
# ('WWW-Authenticate' is the realm-name)
"\n ", $response->status_line, "\n at $url\n Aborting"
unless $response->is_success;
Then you'll get this error:
Error: Basic realm="Unicode-MailList-Archives"
401 Authorization Required
at http://www.unicode.org/mail-arch/unicode-ml/y2002-m08/0067.html
Aborting at auth1.pl line 9. [or wherever]
because the $browser doesn't know any the username and password for that realm ("Unicode-MailList-Archives")
at that host ("www.unicode.org"). The simplest way to let the browser know about this is to use the
credentials method to let it know about a username and password that it can try using
for that realm at that host. The syntax is:
In most cases, the port number is 80, the default TCP/IP port for HTTP; and you usually call the
credentials method before you make any requests. For example:
So if we add the following to the program above, right after the $browser = LWP::UserAgent->new;
line:
$browser->credentials( # add this to our $browser 's "key ring"
'www.unicode.org:80',
'Unicode-MailList-Archives',
'unicode-ml' => 'unicode'
);
and then when we run it, the request succeeds, instead of causing the die to be called.
Accessing HTTPS URLs
When you access an HTTPS URL, it'll work for you just like an HTTP
URL would--if your LWP installation has HTTPS support (via an appropriate Secure Sockets Layer library).
For example:
use LWP 5.64;
my $url = 'https://www.paypal.com/'; # Yes, HTTPS!
my $browser = LWP::UserAgent->new;
my $response = $browser->get($url);
die "Error at $url\n ", $response->status_line, "\n Aborting"
unless $response->is_success;
print "Whee, it worked! I got that ",
$response->content_type, " document!\n";
If your LWP installation doesn't have HTTPS support set up, then the response will be unsuccessful,
and you'll get this error message:
Error at https://www.paypal.com/
501 Protocol scheme 'https' is not supported
Aborting at paypal.pl line 7. [or whatever program and line]
If your LWP installation does have HTTPS support installed, then the response should be
successful, and you should be able to consult $response just like with any normal HTTP
response.
For information about installing HTTPS support for your LWP installation, see the helpful README.SSL
file that comes in the libwww-perl distribution.
Getting Large Documents
When you're requesting a large (or at least potentially large) document,
a problem with the normal way of using the request methods (like $response = $browser->get($url))
is that the response object in memory will have to hold the whole document--in memory. If
the response is a 30-megabyte file, this is likely to be quite an imposition on this process's memory
usage.
A notable alternative is to have LWP save the content to a file on disk, instead of saving it
up in memory. This is the syntax to use:
When you use this :content_file option, the $response will have all
the normal header lines, but $response->content will be empty.
Note that this ":content_file" option isn't supported under older versions of LWP, so you should
consider adding use LWP 5.66; to check the LWP version, if you think your program might
run on systems with older versions.
If you need to be compatible with older LWP versions, then use this syntax, which does the same
thing:
use HTTP::Request::Common;
$response = $ua->request( GET($url), $filespec );
Resources
Remember, this article is just the most rudimentary introduction to LWP--to learn more about LWP
and LWP-related tasks, you really must read from the following:
LWP::Simple:
Simple functions for getting, heading, and mirroring URLs.
The extended whitespace feature of most regex implementations allows programmers
to extend their regular expressions over several lines, with comments at the
end of each. Why do so few programmers use this feature? Perl 6 regular expressions,
for example, will be in extended whitespace mode by default. Until your language
makes extended whitespace the default, turn it on yourself.
The only trick to remember with extended whitespace is that the regex engine
ignores whitespace. So if you are hoping to match whitespace, you have to say
so explicitly, often with \s.
In Perl, add an x to the end of the regex, so m/foo|bar/
becomes:
m/
foo
|
bar
/x
...
The value of whitespace and comments becomes more important when working
with more complex regular expressions. Consider the following regular expression
to match a U.S. phone number:
\(?\d{3}\)? ?\d{3}[-.]\d{4}
This regex matches phone numbers like "(314)555-4000". Ask yourself if the
regex would match "314-555-4000" or "555-4000". The answer is no in both cases.
Writing this pattern on one line conceals both flaws and design decisions. The
area code is required and the regex fails to account for a separator between
the area code and prefix.
Spreading the pattern out over several lines makes the flaws more visible
and the necessary modifications easier.
In Perl this would look like:
/
\(? # optional parentheses
\d{3} # area code required
\)? # optional parentheses
[-\s.]? # separator is either a dash, a space, or a period.
\d{3} # 3-digit prefix
[-.] # another separator
\d{4} # 4-digit line number
/x
The rewritten regex now has an optional separator after the area code so
that it matches "314-555-4000." The area code is still required. However, a
new programmer who wants to make the area code optional can quickly see that
it is not optional now, and that a small change will fix that.
2. Write Tests
There are three levels of testing, each adding a higher level of reliability
to your code. First, you need to think hard about what you want to match and
whether you can deal with false matches. Second, you need to test the regex
on example data. Third, you need to formalize the tests into a test suite.
Deciding what to match is a trade-off between making false matches and missing
valid matches. If your regex is too strict, it will miss valid matches. If it
is too loose, it will generate false matches. Once the regex is released into
live code, you probably will not notice either way. Consider the phone regex
example above; it would match the text "800-555-4000 = -5355". False matches
are hard to catch, so it's important to plan ahead and test.
Sticking with the phone number example, if you are validating a phone number
on a web form, you may settle for ten digits in any format. However, if you
are trying to extract phone numbers from a large amount of text, you might want
to be more exact to avoid a unacceptable numbers of false matches.
When thinking about what you want to match, write down example cases. Then
write some code that tests your regular expression against the example cases.
Any complicated regular expression is best written in a small test program,
as the examples below demonstrate:
In Perl:
#!/usr/bin/perl
my @tests = ( "314-555-4000",
"800-555-4400",
"(314)555-4000",
"314.555.4000",
"555-4000",
"aasdklfjklas",
"1234-123-12345"
);
foreach my $test (@tests) {
if ( $test =~ m/
\(? # optional parentheses
\d{3} # area code required
\)? # optional parentheses
[-\s.]? # separator is either a dash, a space, or a period.
\d{3} # 3-digit prefix
[-\s.] # another separator
\d{4} # 4-digit line number
/x ) {
print "Matched on $test\n";
}
else {
print "Failed match on $test\n";
}
}
.... ... ...
Running the test script exposes yet another problem in the phone number regex:
it matched "1234-123-12345". Include tests that you expect to fail as well as
those you expect to match.
Ideally, you would incorporate these tests into the test suite for your entire
program. Even if you do not have a test suite already, your regular expression
tests are a good foundation for a suite, and now is the perfect opportunity
to start on one. Even if now is not the right time (really, it is!), you should
make a habit to run your regex tests after every modification. A little extra
time here could save you many headaches.
3. Group the Alternation Operator
The alternation operator (|) has a low precedence. This means that
it often alternates over more than the programmer intended. For example, a regex
to extract email addresses out of a mail file might look like:
^CC:|To:(.*)
The above attempt is incorrect, but the bugs often go unnoticed. The intent
of the above regex is to find lines starting with "CC:" or "To:" and then capture
any email addresses on the rest of the line.
Unfortunately, the regex doesn't actually capture anything from lines starting
with "CC:" and may capture random text if "To:" appears in the middle of a line.
In plain English, the regular expression matches lines beginning with "CC:"
and captures nothing, or matches any line containing the text "To:" and then
captures the rest of the line. Usually, it will capture plenty of addresses
and nobody will notice the failings.
If that were the real intent, you should add parentheses to say it explicitly,
like this:
(^CC:)|(To:(.*))
However, the real intent of the regex is to match lines starting with "CC:"
or "To:" and then capture the rest of the line. The following regex does that:
^(CC:|To:)(.*)
This is a common and hard-to-catch bug. If you develop the habit of wrapping
your alternations in parentheses (or non-capturing parentheses -- (?:...))
you can avoid this error.
4. Use Lazy Quantifiers
Most people avoid using the lazy quantifiers *?, +?, and
??, even though they are easy to understand and make many regular expressions
easier to write.
Lazy quantifiers match as little text as possible while still aiding the
success of the overall match. If you write foo(.*?)bar, the quantifier
will stop matching the first time it sees "bar", not the last time. This may
be important if you are trying to capture "###" in the text "foo###bar+++bar".
A regular quantifier would have captured "###bar+++".
Let's say you want to capture all of the phone numbers from an HTML file.
You could use the phone number regular expression example we discussed earlier
in this article. However, if you know that the file contains all of the phone
numbers in the first column of a table, you can write a much simpler regex using
lazy quantifiers:
<tr><td>(.+?)<td>
Many beginning regular expression programmers avoid lazy quantifiers with
negated character classes. They write the above code as:
<tr><td>([^>]+)</td>
That works in this case, but leads to trouble if the text you are trying
to capture contains common characters from your delimiter (in this case,
</td>). If you use lazy quantifiers, you will spend less time kludging
character classes and produce clearer regular expressions.
Lazy quantifiers are most valuable when you know the structure surrounding
the text you want to capture.
5. Use Available Delimiters
Perl and PHP often use the forward slash to mark the start and end of a regular
expression. Python uses a variety of quotes to mark the start and end of a string,
which may then be used as a regular expression. If you stick with the slash
delimiter in Perl and PHP, you will have to escape any slashes in your regex.
If you use regular quotes in Python, you will have to escape all of your backslashes.
Choosing different delimiters or quotes allows to avoid escaping half of your
regex. This makes the regex easier to read and reduces the potential for bugs
when you forget to escape something.
Perl and PHP allow you to use any non-alphanumeric or whitespace character
as a delimiter. If you switch to a new delimiter, you can avoid having to escape
the forward slashes when you are trying to match URLs or HTML tags such as "http://"
or "<br />".
For example:
/http:\/\/(\S)*/
could be rewritten as:
#http://(\S)*#
Common delimiters are #, !, |. If you use square
brackets, angle brackets, or curly braces, the opening and closing brackets
must match. Here are some common uses of delimiters:
#…#
!…!
{…}
s|…|…| (Perl only)
s[…][…] (Perl only)
s<…>/…/ (Perl only)
In Python, regular expressions are treated as strings first. If you use quotes
-- the regular string delimiter -- you will have to escape all of your backslashes.
However, you can use raw strings, r'', to avoid this. If you use raw
triple-quoted strings with the re.VERBOSE option, it allows you
to include newlines.
For example:
regex = "(\\w+)(\\d+)"
could be rewritten as:
regex = r'''
(\w+)
(\d+)
'''
[Aug 19, 2009] Checking perl syntax in VIM on each save
May 10, 2004
au BufWritePost *.pl,*.pm !perl -c %
Every time you save a .pl or .pm file, it executes perl -c and shows you
the output.
I was just reminded about this small thing, which is so easy to forget –
regular expressions that have markers of line start (^) and/or line end($) are
so much faster than those regexps that don't have these markers. The thing is
that with line start/end marker regexp engine needs to make only one match/substution,
whereas when there is no such markers, it has to repeat the match/substitution
operation at every character of the string.
In practice, it's unbelievable how much difference this can make. Especially
when using complex regular expressions over large data sets.
P.S.: I understand that it is not always possible to use these markers, but
I think that they can be used much more often than they are. Everywhere.
The Last but not LeastTechnology is dominated by
two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt.
Ph.D
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will be inlined by the compiler:
Corresponding
author. # Contributed equally. Received 2015 Nov 26; Accepted 2016 Jun 1.
Copyright © The Author(s) 2016
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Earlier we discussed
the basics of how to write and execute a perl program using 
Written by
![[A series of scalar variables with arrows pointing to
other variables]](http://www.perl.com/pub/1999/09/graphics/refs.gif)
![[A scalar variable with an arrow pointing back to
itself]](http://www.perl.com/pub/1999/09/graphics/selfref.gif)
![[A picture of an anonymous hash having a flag set within it]](http://www.perl.com/pub/1999/09/graphics/blessing.gif)
Teodor Zlatanov
0
