blindluke has asked for the wisdom of the Perl Monks concerning the following question:

Hello, enlightened Monks!

A few days ago, a colleague of mine faced a simple problem - he had a list containing all users, and another one, containing disabled users. He wanted to find active users.

This, of course is a problem of finding a difference between two arrays, and perlfaq4 shows a simple way of solving it. This is my initial solution, based upon the perlfaq info:

#!/usr/bin/perl -w use strict; my @list1 = (1, 2, 3, 4, 5); my @list2 = (2, 3, 4); my @diff; my %repeats; for (@list1, @list2) { $repeats{$_}++ } for (keys %repeats) { push @diff, $_ unless $repeats{$_} > 1; }

A short while after I send him this code, my colleague found another way of solving the problem. This is his approach:

#!/usr/bin/perl -w use strict; my @list1 = (1, 2, 3, 4, 5); my @list2 = (2, 3, 4); my %diff; @diff{ @list1 } = undef; delete @diff{ @list2 };

I'm wondering, is there another clever way of getting the difference between two arrays? Could it be done in some way using map or grep?

The second array is a subset of the first one, so this is not a general case of computing a difference between arrays.