Group by Key - Ruby 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 Ruby DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.rb

pairs = [["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]]
groups = {}
pairs.each do |key, value|
  groups[key] ||= []
  groups[key] << value
end
parts = groups.map { |key, values| "#{key}: [#{values.join(', ')}]" }
puts "{#{parts.join(', ')}}"

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.