Hash Tables

Group by Key

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.

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

Basic Implementation

basic.pl
Replay: real traced execution (multi-file project)
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";

  1. pairs ← [(a, 1), (b, 2), (a, 3), (c, 4), (b, 5)]

    1use strict;2use warnings;
    values this step[(a, 1), (b, 2), (a, 3), (c, 4), (b, 5)]pairs

  2. groups ← {}

    4my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);5my %groups;6my @order;
    values this step{}groups

  3. groups ← {a: [1]}

    4my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);5my %groups;6my @order;
    values this step{} → {a: [1]}groupsakey1value

  4. groups ← {a: [1], b: [2]}

    4my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);5my %groups;6my @order;
    values this step{a: [1]} → {a: [1], b: [2]}groupsbkey2value

  5. groups ← {a: [1, 3], b: [2]}

    4my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);5my %groups;6my @order;
    values this step{a: [1], b: [2]} → {a: [1, 3], b: [2]}groupsakey3value

  6. groups ← {a: [1, 3], b: [2], c: [4]}

    4my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);5my %groups;6my @order;
    values this step{a: [1, 3], b: [2]} → {a: [1, 3], b: [2], c: [4]}groupsckey4value

  7. groups ← {a: [1, 3], b: [2, 5], c: [4]}

    4my @pairs = (["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]);5my %groups;6my @order;
    values this step{a: [1, 3], b: [2], c: [4]} → {a: [1, 3], b: [2, 5], c: [4]}groupsbkey5value

  8. stdout ← {a: [1, 3], b: [2, 5], c: [4]}

    18}19print "{" . join(", ", @parts) . "}\n";
    values this step{a: [1, 3], b: [2, 5], c: [4]}stdout{a: [1, 3], b: [2, 5], c: [4]}groups

  9. bucket 1 after collision ← c -> a, degradation risk ← long chains can degrade lookup toward O(n)

    18}19print "{" . join(", ", @parts) . "}\n";
    values this stepa → c -> abucket 1 after collisionlong chains can degrade lookup toward O(n)degradation riskresize or rehash when load factor growsmitigationcnew key

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 and includes a collision contrast where one bucket chain grows, showing why long chains can degrade lookup and why real tables resize or rehash.