Group by Key - Perl DSA

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this Perl DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.pl

use strict;
use warnings;

my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);
my %groups;
my @order;
for my $pair (@pairs) {
    my ($key, $value) = @$pair;
    if (!exists $groups{$key}) {
        $groups{$key} = [];
        push @order, $key;
    }
    push @{$groups{$key}}, $value;
}
my @parts;
for my $key (@order) {
    push @parts, "$key: [" . join(", ", @{$groups{$key}}) . "]";
}
print "{" . join(", ", @parts) . "}\n";

Complexity

Time: O(n) average
Space: O(k + n) for buckets and values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.