Perl references and nested data structures

Build hashes of arrays, add to them, and loop over them with references.

use strict;
use warnings;
use List::Util qw(sum);

my %scores = (Ada => [90, 85], Bo => [70], Cy => []);
push @{ $scores{Bo} }, 95;
$scores{Di}[0] = 60;

for my $name (sort keys %scores) {
    my @s = @{ $scores{$name} };
    my $average = @s ? sprintf("%.1f", sum(@s) / @s) : "n/a";
    print "$name: ", scalar(@s), " scores, average $average\n";
}

print join(" ", ref(\1), ref([]), ref({}), ref(sub { 1 }), ref(\\1)), "\n";

my %copy = %scores;
delete $copy{Cy};
print exists $scores{Cy} ? "Cy still in the original\n" : "Cy gone\n";
print join(",", sort keys %copy), "\n";
my @both = @scores{qw(Ada Bo)};
print scalar(@{ $both[0] }) + scalar(@{ $both[1] }), " scores for Ada and Bo\n";

Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0
SCALAR ARRAY HASH CODE REF
Cy still in the original
Ada,Bo,Di
4 scores for Ada and Bo

Explanation

Arrays and hashes hold only scalars, so complex data is built from references: a reference is a scalar that points at an array, a hash, a subroutine, or another scalar. [1, 2, 3] creates a reference to a new array, {a => 1} creates a reference to a new hash, and \@list refers to an existing array. @{ $ref } turns an array reference back into the array, and $ref->[0] and $ref->{key} access single elements.

Perl creates intermediate structures automatically when you assign into them, which is called autovivification: $scores{Di}[0] = 60 builds the array for Di. That is convenient, but just testing exists $a{b}{c} also creates $a{b}. The ref function tells you what kind of reference you have, which is useful in generic code.

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Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0

Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0
SCALAR ARRAY HASH CODE REF

Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0
SCALAR ARRAY HASH CODE REF

Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0
SCALAR ARRAY HASH CODE REF
Cy still in the original

Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0
SCALAR ARRAY HASH CODE REF
Cy still in the original
Ada,Bo,Di

Ada: 2 scores, average 87.5
Bo: 2 scores, average 82.5
Cy: 0 scores, average n/a
Di: 1 scores, average 60.0
SCALAR ARRAY HASH CODE REF
Cy still in the original
Ada,Bo,Di
4 scores for Ada and Bo

Cost and common mistakes

Each hash or array access is O(1) on average; the loop is O(n).

• %copy = %scores is a shallow copy: the array references are shared, so push @{$copy{Ada}}, 1 would also change $scores{Ada}.
• Dividing by the number of scores when there are none is a division by zero error, so the code checks @s first.
• Always use strict. Without it a typo in a variable name silently creates a new global variable.

Try it yourself

How do you read the first score of Ada?

Further reading

• Perl documentation (external)
• perlintro: a brief introduction to Perl (external)