...and here's the 2D array method I tried. You can see why I prefer the hash, as it's a good bit more involved, since you need to map the animal names into array indices. Take out the lines that start with "next" and you'll see another reason I prefer the hash.

And now for the code....

#!/usr/bin/perl -w
use warnings;
use strict;

my @animals = ("cat", "dog", "fish", "horse", "pig");

# Hash to map an animal to an array index.  I'm sure there's a
# better way, but I don't know it...
my %a2i;
my $i=0;
for my $a (@animals) {
        $a2i{$a}=$i++;
}


my @count;
while (<DATA>) {
        my ($x,$y) = split /\s/;
        ($x,$y) = ($y,$x) if ($x gt $y);
        $count[$a2i{$x}][$a2i{$y}]++;
}

for my $x (sort keys %a2i) {
        next if !defined $count[$a2i{$x}];
        for my $y (sort keys %a2i) {
                next if !defined $count[$a2i{$x}][$a2i{$y}];
                print $count[$a2i{$x}][$a2i{$y}],
                        " people have both a ", $x,
                        " and a ", $y, "\n";
        }
}

__DATA__
cat dog
cat fish
pig dog
horse fish
pig fish
dog fish
fish cat

[download]
--roboticus

In reply to Re^2: help with 2D arrays with perl requested by roboticus
in thread help with 2D arrays with perl requested by Anonymous Monk

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