Question

I am trying to use List::MoreUtils methods. But, need some clarity on its usage it in some scenarios.

Please let me know, if it can be used with a map. For example:

#!/usr/bin/perl

use strict;
use warnings;
use List::Util;
use List::MoreUtils;
use Data::Dumper;


my @udData1 = qw(WILL SMITH TOMMY LEE JONES);

my @arr = qw(WILL TOMMY);
my %output = map{$_=>List::MoreUtils::firstidx{/$_/} @udData1} @arr;
print Dumper %output;
print List::MoreUtils::firstidx{/TOMMY/} @udData1;
print "\n";

Output:

$VAR1 = 'TOMMY';
$VAR2 = 0;
$VAR3 = 'WILL';
$VAR4 = 0;
2

As observed I am not getting the values correctly when using map, but getting it fine when used in the later command.
I intend to use $_ as an element of @arr. This may be incorrect. So, please suggest me an alternative. Shall i have to use foreach?

Answer 1

The problem is this bit right here:

List::MoreUtils::firstidx{/$_/} @udData1

In this bit of code, you’re expecting $_ to be both the pattern taken from @arr and the string taken from @udData1 at the same time. (Remember that firstidx{/TOMMY/} means firstidx{$_ =~ /TOMMY/}, and likewise firstidx{/$_/} means firstidx{$_ =~ /$_/}.)

What actually happens is that $_ is the value from @udData1 (since that’s the innermost loop) and you wind up matching that against itself. Because it’s a simple alphabetic string, it always matches itself, and firstidx correctly returns 0.

Here’s one solution using a temporary lexical variable:

my %output = map{ my $p = $_;
                  $p => List::MoreUtils::firstidx{/$p/} @udData1 } @arr;