if(defined $cache[$num]) {return $cache[$num];}
if($num < 0) {return 0;}
In Perl, -1 is a valid array index, meaning the last element of the array (-2 is the second-to-last, and so on). Move the if($num < 0) check before the if(defined $cache[$num]) check and your code produces the expected output. Another option would be to use a hash instead of an array as the cache.
A couple more tips:
- Instead of a global, you could define your cache as what is known as a "static" variable in other languages, using state.
- Update: While implementing the cache yourself like that is fine, note that Perl has a module for that, Memoize.
- The &find() style of calling subs is outdated, call your subs without the &.
- The if($num == 0) check isn't necessary if you pre-populate your array with that index.
- You can use Test::More instead of inputting your test cases manually:
use warnings;
use strict;
use feature 'state';
sub find {
state @cache = (1);
my $num = shift;
return 0 if $num < 0;
if ( defined $cache[$num] ) { return $cache[$num] }
$cache[$num] = find($num-1) + find($num-2) + find($num-3);
return $cache[$num];
}
use Test::More tests=>3;
is find(2), 2;
is find(3), 4;
is find(4), 7;