Issues using List::MoreUtils::firstidx

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 "
"; ``` 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?

Accepted Answer

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;
```

Issues using List::MoreUtils::firstidx

Solution

Problem