Why do I need a local variable in map function

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

Hi Monks,

I have a function with should select files by a given list of extension. I wrote a program, which did not work. After playing a little around I found a working solution (using a local variable: my $x). But I do not understand, why my first version is not working.

Can you help me explaining the difference between this two versions:

In the first version the expression(@ext) is replaces by '1';

In the second version the expression(@ext) is correct. ('f.*\\.txt$', 'f.*\\.txt.*$')

Thanks for your help !!!


use strict;
use warnings;
use Data::Dumper qw(Dumper);
$\="\n"; 

my @ext;
my @list=('f1.txt','f2.txtx','f3.xtxt','x1.txt');
my  $ex="f*.txt,f*.txt*";

print " Version 1: Not working--------------------------------";
@ext=map { $_=quotemeta($_).'$'; s/\\\*/.*/;} split(/,/,$ex);         
+  # <<<<<< compare this line
print "qr=".Dumper(\@ext);
foreach my $e(@ext)
{ foreach my $f(@list) {print "Match: ($e) => $f" if ($f=~m/$e/); }}

print " Version 2: working--------------------------------";
@ext=map { my $x=quotemeta($_).'$'; $x=~s/\\\*/.*/g; $_=$x;} split(/,/
+,$ex);  # <<<<<< compare this line
print "qr=".Dumper(\@ext);
foreach my $e(@ext)
{ foreach my $f(@list) {print "Match: ($e) => $f" if ($f=~m/$e/); }}
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Comment on Why do I need a local variable in map function Download Code

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Re: Why do I need a local variable in map function by haukex (Archbishop) on May 23, 2019 at 07:41 UTC
`s///` returns the number of substitutions made, so that's what that `map` block is returning. Although it's generally not considered good practice to modify the original list by modifying or assigning to `$_` in a map block, the first version could be written as `map { $_ = quotemeta($_) . '$'; s/\\\/./g; $_ }` to achieve the desired result. Alternatively, as of Perl 5.14, you can say `s///r` to have the regex return the modified string, e.g. `map { my $x = quotemeta($_) . '$'; $x =~ s/\\\/./gr }`. Update: Note that your second regex includes `/g`, while your first one does not; I've added it to both my examples. Update 2: Or even just `map { quotemeta($_).'$' =~ s/\\\/./gr }` or `map { "\Q$_\E\$" =~ s/\\\/./gr }`	[reply] [d/l] [select]
Re^2: Why do I need a local variable in map function by Anonymous Monk on May 23, 2019 at 09:35 UTC
Thanks for the perfect answer.	[reply]


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