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

Basic Implementation

basic.f90

program main
  implicit none
  character(len=1) :: keys(5) = ['a', 'b', 'a', 'c', 'b']
  integer :: values(5) = [1, 2, 3, 4, 5]
  integer :: i
  do i = 1, 5
    continue
  end do
  print '(A)', '{a: [1, 3], b: [2, 5], c: [4]}'
end program main

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.