Perl for Web Site Management

Creating Anonymous Hashes and Arrays

If we look at Dumper’s output more closely, we can see that something interesting is going on. Here’s a simplified version of it:

$HoH = {
    key1 => {
                key_a => 'value_a',
                key_b => 'value_b',
            },
    key2 => {
                key_a => 'value_a',
                key_b => 'value_b',
            },
};

What’s up with the curly braces? We haven’t seen them used like this before.

What’s up is the fourth and final piece of reference magic we need to know: how to initialize an anonymous hash and return a reference to it at the same time. We do that by putting curly braces around it, where we would normally put the enclosing parentheses:

$hash_ref = { 
              key1 => 'value1',
              key2 => 'value2',
            };

Here we have created an anonymous hash with two keys and two corresponding values, and returned a reference to it that we’ve then stuck in the scalar variable $hash_ref. If we take a bunch of those anonymous hashes and make the references that they return the values in an all-encompassing hash, and if we make that all-encompassing hash an anonymous hash and return a reference to it, we get a reference to a hash of hashes, or HoH. That’s what Data::Dumper’s Dumper routine printed out for us.

We can do similar reference magic with square brackets to create and return an anonymous array reference:

my $ary_ref = [ 'item1', 'item2', 'item3' ];

If we nest those we get a reference to an LoL, or List of Lists:

my $LoL = [ 
            [ 'item1', 'item2', 'item3' ],
            [ 'item1', 'item2', 'item3' ],
            [ 'item1', 'item2', 'item3' ],
          ];

And we can ...