Question

Let’s say I have these arrays

my @new = qw/a b c d e/;
my @old = qw/a b   d e f/;

and I would like to them compared, so I get 3 new arrays containing the differences

• an array with the elements that are in @new and not in @old : c
• an array with the elements that are not in @new and in @old : f
• an array with the elements that are in both @new and @old : a b d e

I am thinking about the exists function, but that only works for hashes I suppose.

Update: I messed up the letter examples.

Answer 1

Here’s a function that I’ve used many, many times.

sub compute_sets {
    my ($ra, $rb) = @_;
    my (@a, @b, @ab, %a, %b, %seen);

    @a{@$ra} = ();
    @b{@$rb} = ();

    foreach (keys %a, keys %b) {
        next if $seen{$_}++;

        if (exists $a{$_} && exists $b{$_}) {
            push(@ab, $_);
        }
        elsif (exists $a{$_}) {
            push(@a, $_);
        }
        else {
            push(@b, $_);
        }
    }

    return(\@a, \@b, \@ab);
}

It returns references to arrays containing the elements in the first/second/both lists:

my @new = qw/a b c d e/;
my @old = qw/a b   d e f/;

my ($new_only, $old_only, $both) = compute_sets(\@new, \@old);

say 'new only: ', join ' ', @$new_only; # c
say 'old only: ', join ' ', @$old_only; # f
say 'both: ', join ' ', @$both;         # e a b d