strange thing in perl's eval function(solved))

a369 has asked for the wisdom of the Perl Monks concerning the following question:

I have paste the same question on StackOverflow It is solved.

The following script get warning: "Use of uninitialized value in concatenation (.) or string at C:\tools\test.pl line 17, <DATA> line 1." But the next line of __DATA__ will be processed without any warning and get these: test1b.txt:test test1c.txt:test :test More strange thing is that when I add a line: print "$line:".$'."\n"; The warning disappeared. Anybody have some clues?

#file: test.pl
#!/usr/bin/perl -w
use strict;
my $pattern='test';
my $output='$&';
while(<DATA>)
{
    chomp;
    my $line=$_;
    chomp($line);
    $line=~/$pattern/;
    #print "$line:".$'."\n";   #why uncommenting this line make the fo
+llowing line pass without no warning.
    my $result="$line:".eval($output)."\n";
    print $result;
}

__DATA__
test1a.txt
test1b.txt
test1c.txt
[download]

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Re: strange thing in perl's eval function by 2teez (Vicar) on Jun 07, 2013 at 07:39 UTC
There are several things that are not efficient in your script as shown in your OP. why assign "matched" string before going over the string? Only to check it using an "eval" string. You are chomping the string twice for what reason? Why not do: `while (my $line=<DATA>) { chomp($line);..` [download] finally, you don't want to be using $&, $' and $` in your program. Check perlfaq6 on your system as perldoc perlfaq6 OR check Here Something like this could help: `#file: test.pl #!/usr/bin/perl -w use strict; use v5.10; my $pattern = qr/test/; while (my $line=<DATA>) { chomp($line); $line =~ /($pattern)/p; my $result = $line. ' => matched:'.${^MATCH}.$/; print $result; } __DATA__ test1a.txt test1b.txt test1c.txt` [download] If you tell me, I'll forget. If you show me, I'll remember. if you involve me, I'll understand. --- Author unknown to me	[reply] [d/l] [select]
Re^2: strange thing in perl's eval function by a369 (Initiate) on Jun 07, 2013 at 07:59 UTC
1. Just because if removing the first one, warning appears, an answer on StackOverflow has cleared this problem. 2. Because sometimes $_ changes unpredictable, I used another variable but forgot to remove the first chomp. 3. Good suggestion. I hope perl6 will mature soon, not just on VM.	[reply]
Re: strange thing in perl's eval function by choroba (Cardinal) on Jun 07, 2013 at 06:49 UTC
Crossposted (and answered) at StackOverflow. It is considered polite to inform about crossposting, so that people not attending both sites do not waste their efforts solving a problem already solved at the other end of the Internets. لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply]
Re^2: strange thing in perl's eval function by a369 (Initiate) on Jun 07, 2013 at 08:07 UTC
thanks for pointing that. Corrected.	[reply]
Re: strange thing in perl's eval function (re match vars) by Anonymous Monk on Jun 07, 2013 at 07:17 UTC
Its because $' and $& are magic, and come with a performance penalty, and in general you shouldn't use them Also, you shouldn't use them without testing for sucess, without writing `if ( $line=~/$pattern/ ){ ... }` Change `my $output= q{" $& \| $'"};` and you'll get `Use of uninitialized value $& in concatenation (.) or string at (eval +1) line 1, <DATA> line 1. Use of uninitialized value $' in concatenation (.) or string at (eval +1) line 1, <DATA> line 1.` [download] Add `local $&;` on top of your program, that is mention $& in your code, and the penalty kicks in and the warning goes away Also, there might be a bug in there too, for example this doesn't trigger this warning `#!perl -w $o = q{qq{ $& \| $'}}; $p = q{.}; $_=1234; /$p/; print eval $o; __END__ 1 \| 234` [download] But this does `#!perl -w $o = q{qq{ $& \| $'}}; $p = q{2}; $_=1234; /$p/; print eval $o; /$p/; print eval $o; __END__ Use of uninitialized value $& in concatenation (.) or string at (eval +1) line 1. Use of uninitialized value $' in concatenation (.) or string at (eval +1) line 1. \| 2 \| 34` [download] On a scale of importance from 1 to 10, I'd rate this bug a -3000 :)	[reply] [d/l] [select]
Re^2: strange thing in perl's eval function (re match vars) by a369 (Initiate) on Jun 07, 2013 at 07:42 UTC
As choroba pointed out, this question is solved on StackOverflow	[reply]
Re^3: strange thing in perl's eval function (re match vars) by Anonymous Monk on Jun 07, 2013 at 08:18 UTC
As choroba pointed out, this question is solved on StackOverflow Its a good thing this here is perlmonks, where relevant discussion is welcome :)	[reply]