http://www.perlmonks.org?node_id=1079022

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

Hi, I am trying to use strict perl coding for this simple task, but my variable returns empty. Why?
use strict;
my $final_res='';
open FILE, $infile;
while(my $final_line=<FILE>)
{
    if($final_line=~/^SeqID/)
    {
        my $final_line=<FILE>; chomp $final_line;
        
        my @split_final=split(/\t/, $final_line);

        my $final_res = $split_final[7];
        $final_res=~s/D/M/g;
        $final_res=~s/U/M/g;

#print $final_res."\n";
    }
}
close FILE;

print $final_res."\n";

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If I print inside the loop, in the comment, it prints fine. what am I missing?

Replies are listed 'Best First'.
Re: Why is my result empty?
by toolic (Bishop) on Mar 20, 2014 at 00:14 UTC
[reply]
[d/l]
      Damn! Silly mistake :)
      Thank you!
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Re: Why is my result empty?
by Anonymous Monk on Mar 20, 2014 at 00:18 UTC

    Hi, I am trying to use strict perl coding for this simple task, but my variable returns empty. Why?

    Because you're not Coping with Scoping (you're not Lexical scoping like a fox)

    Consider this program, then ask yourself why you do this in your program 

    $ perl -le " use warnings; use strict; my $one = 1; { my $one = 2; pri
+nt $one; } print $one; "
2
1

    [download]

    what am I missing?

    Sample input (and sample output) portion of How do I post a question effectively?

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[d/l]
Re: Why is my result empty?
by Anonymous Monk on Mar 20, 2014 at 00:20 UTC

    Because my $final_res = $split_final[7]; creates a new variable that is local to the if block, and it hides the other variable named $final_res that you defined before the loop. Write $final_res = $split_final[7]; (remove the my) and you'll only have one variable named $final_res and your code will do what you intend.

    In general, you should use warnings; in addition to use strict; to help you catch some potential problems. Also, take a look at Perl::Critic, which can help you catch this kind of error.

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Re: Why is my result empty?
by Marshall (Canon) on Mar 20, 2014 at 05:49 UTC
    Another idea:
    use strict;
my $final_res='';
open FILE, $infile;
while(my $final_line=<FILE>)
{
    if($final_line=~/^SeqID/)
    {
        my $final_line=<FILE>; chomp $final_line;
        
        my @split_final=split(/\t/, $final_line);

        my $final_res = $split_final[7];
        $final_res=~s/D/M/g;
        $final_res=~s/U/M/g;

#print $final_res."\n";
    }
}
close FILE;

print $final_res."\n";

################
# different coding
################

use strict;
my $infile = ''; #some file
open (FILE, "<", $infile) or die "unable to open $infile $!";

while(my $line=<FILE>)
{
    chomp $line;

    my ($token) = (split(/\t/, $line))[7];
    $token=~s/D/M/g;
    print "Token= ", $token, "\n";
}

    [download]
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[d/l]
Re: OT: Why is my result empty?
by AnomalousMonk (Archbishop) on Mar 20, 2014 at 16:27 UTC

    Slightly OT, but if you have a lot of such character-for-character translation to do (a bio-app?), consider using  tr/// instead of  s/// (pattern substitution) for faster operation (see  tr/// in Quote-Like Operators in perlop): 

    c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'xxxDxUxxDDxxUUxDUDxUDUx';
 print qq{'$s'};
 ;;
 $s =~ tr/DU/M/;
 print qq{'$s'};
"
'xxxDxUxxDDxxUUxDUDxUDUx'
'xxxMxMxxMMxxMMxMMMxMMMx'

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