Perl - passing an array to subroutine

Question

I'm in the process of learning Perl and am trying to write a script that takes a pattern and list of files as command line arguments and passes them to a subroutine, the subroutine then opens each file and prints the lines that match the pattern. The code below works; however, it stops after printing the lines from the first file and doesn't even touch the second file. What am I missing here?

#!/usr/bin/perl
use strict;
use warnings;

 sub grep_file 
 {
  my $pattern = shift;
  my @files = shift;

  foreach my $doc (@files) 
  {
    open FILE, $doc;
    while (my $line = <FILE>) 
    {
      if ($line =~ m/$pattern/) 
      {
        print $line;
      }
    }
  }

grep_file @ARGV;

Stefan · Accepted Answer · 2013-12-13 15:42:25Z

8

Shift pops an element from your parameter (see: http://perldoc.perl.org/functions/shift.html).

So @files can only contain one value.

Try

sub foo
{
 my $one = shift @_;
 my @files = @_;
 print $one."\n";
 print @files;
}

foo(@ARGV);

answered Dec 13, 2013 at 15:42

Stefan

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4 Comments

Jukodan Over a year ago

Of course! I had a similar but different understanding of shift. Thanks a lot, it worked.

Stefan Over a year ago

Borodin: You are right, so I removed the information that he may feel to accept this as answer if it helped him (as he had shown). Regarding his point stats I just wanted to show him that if this helped like he explained he can accept this answer to set the problem as solved.

Borodin Over a year ago

I think that, after a day or so, it is fine to post a comment under the question to explain that an answer should be accepted if the problem has been resolved.

Borodin Over a year ago

What I mean is that your foo is very unlike the OP's grep_file, and it prints file1file2file3file4file5file6 on a line after the regex.

Borodin · Accepted Answer · 2013-12-13 15:53:57Z

1

There is little reason to use a subroutine here. You are just putting the whole program inside a function and then calling it.

The empty <> operator will read from all the files in @ARGV in sequence, without you having to open them explicitly.

I would code your program like this

use strict;
use warnings;

my $pattern = shift;
$pattern = qr/$pattern/; # Compile the regex

while (<>) {
  print if $_ =~ $pattern;
}

answered Dec 13, 2013 at 15:53

Borodin

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4 Comments

Stefan Over a year ago

I agree by not needing a subroutine, but maybe he just built a simple example to understand his problem and solve it.

Borodin Over a year ago

Perhaps. But your own answer explains why the OP is getting unexpected results (with an oddly-irrelevant subroutine!)

Stefan Over a year ago

I can't understand why a simple and minimalistic example demonstrating the way shift works is oddly-irrelevant? In such a case, your answer should be discarded the same way. Just leaving a reference to the perl shift documentation uncommented must then be the only correct answer.

Borodin Over a year ago

@Stefan This is about your own answer so I have responded there.

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