Perl Cookbook

Hashes with Multiple Values Per Key

Problem

You want to store more than one value for each key.

Solution

Store an array reference in $hash{$key}, and put the values into that array.

Discussion

You can only store scalar values in a hash. References, however, are scalars. This solves the problem of storing multiple values for one key by making $hash{$key} a reference to an array containing values for $key. The normal hash operations—insertion, deletion, iteration, and testing for existence—can now be written in terms of array operations like push, splice, and foreach.

This code shows simple insertion into the hash. It processes the output of who(1) on Unix machines and outputs a terse listing of users and the ttys they’re logged in on:

%ttys = ();

open(WHO, "who|")                   or die "can't open who: $!";
while (<WHO>) {
    ($user, $tty) = split;
    push( @{$ttys{$user}}, $tty );
}

foreach $user (sort keys %ttys) {
    print "$user: @{$ttys{$user}}\n";
}

The heart of the code is the push line, the multihash version of $ttys{$user} = $tty. We interpolate all the tty names in the print line with @{$ttys{$user}}. We’d loop over the anonymous array if, for instance, we wanted to print the owner of each tty:

foreach $user (sort keys %ttys) {
    print "$user: ", scalar( @{$ttys{$user}} ), " ttys.\n";
    foreach $tty (sort @{$ttys{$user}}) {
        @stat = stat("/dev/$tty");
        $user = @stat ? ( getpwuid($stat[4]) )[0] : "(not available)";
        print "\t$tty (owned by $user)\n";
    }
}

The exists function can have two meanings: “Is there ...