Making Hashes of Arrays (Perl Cookbook, 2nd Edition)

11.2.3. 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 the values for $key. Normal hash operations acting on individual scalar values (insertion, deletion, iteration, and testing for existence) are now written with array operations acting on lists of values (like push, splice, and foreach).

Here's how to give a key many values:

$hash{"a key"} = [ 3, 4, 5 ];       # anonymous array

Once you have a key with many values, here's how to use them:

@values = @{ $hash{"a key"} };

To append a new value to the array of values associated with a particular key, use push:

push @{ $hash{"a key"} }, $value;

One common application of such data structures is inverting a hash that may have several keys with the same associated value. When inverted, you end up with a hash that has many values for the same key. This is addressed in Recipe 5.9.

Be warned that this:

@residents = @{ $phone2name{$number} };

causes a runtime exception under use strict because you're dereferencing an undefined reference where autovivification won't occur. You must do this instead:

@residents = exists( $phone2name{$number} )
                       ? @{ $phone2name{$number} }
                       : ( );

11.2. Making Hashes of Arrays

11.2.1. Problem

11.2.2. Solution

11.2.3. Discussion

11.2.4. See Also