Mastering Modern Perl: Writing Efficient and Maintainable Code/

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Nested Data Structures

Nested Data Structures

Learn about nested data structures in Perl.

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Storing an array in an array

Perl’s aggregate data types—arrays and hashes—store scalars indexed by integer or string keys. Note the word scalar. If we try to store an array in an array, Perl’s automatic list flattening will make everything into a single array:

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my @counts = qw( eenie miney moe );
my @ducks = qw( huey dewey louie );
my @game = qw( duck duck grayduck );
my @famous_triplets = (
    @counts, @ducks, @game
);
say "@famous_triplets";

Use references

Perl’s solution to this is references, which are special scalars that can refer to other variables. Nested data structures in Perl, such as an array of arrays or a hash of hashes, are possible through the use of references. Unfortunately, the reference syntax isn’t the most visually appealing.

We can use the reference operator \ to produce a reference to a named variable:

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my @counts = qw( eenie miney moe );
my @ducks = qw( huey dewey louie );
my @game = qw( duck duck grayduck );
my @famous_triplets = ( 
    \@counts, \@ducks, \@game
);
say "@famous_triplets";

Alternatively, we can use the anonymous reference declaration syntax to avoid the use of named variables:

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my @famous_triplets = (
    [qw( eenie miney moe )],
    [qw( huey dewey louie )],
    [qw( duck duck goose )],
);
my %meals = (
    breakfast => { entree => 'eggs',
                side => 'hash browns' },
    lunch => { entree => 'panini',
                side => 'apple' },
    dinner => { entree => 'steak',
                side => 'avocado salad' },
);

Note: Perl allows an optional trailing ...