TIL: substr and -1
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.