I like the Perl Weekly Challenges because I like to teach and I like to learn. This is why I like to blog my results; to allow others see the tools I reach for regularly.

For Challenge #115, I used this subroutine to make it easier to compare the first and last characters of strings.

sub l_char( $str ) {
    return substr( $str, -1 + length $str, 1 );
}

This is where I get to learn. @adherzog on Twitter responded:

I think substr() can count backwards from the end of the string, eg. substr( $str, -1, 1 )

Hrmm. That would make thinks so much easier. I don’t think I understood that substr behaved like that. I strongly considered something like my $last = reverse split //, $str or my @str = split //, $str; my last = $str[-1] but decided that substr was sufficient, if a bit clunky.

But was it really clunky? Or did I just not know how to use it?

I keep a file, test.pl, around so I can take ideas I want to boil down to the basics, then keep them around for future use.

#!/usr/bin/env perl

use strict;
use warnings;
use feature qw{ say signatures state };
no warnings qw{ experimental };

if (1) {
    my $str = 'testing';
    for my $i ( 1 .. length($str) ) {
        my $noti = 0 - $i;
        my $j    = length($str) - $noti;
        say join "\t", '', $i, $noti, substr( $str, $noti, $j );
    }
    exit;
}

        1       -1      g
        2       -2      ng
        3       -3      ing
        4       -4      ting
        5       -5      sting
        6       -6      esting
        7       -7      testing

So, -1, as well as the other start from the end formulations, work in substr as well as list context.

Today I Learned.

If you have any questions or comments, I would be glad to hear it. Ask me on Twitter or make an issue on my blog repo.