The k-permutations of a set are the permutations of the combinations of size k. They are also known as sequences without repetitions.

There are 24 3-permutations of the 4-set {0, 1, 2, 3}:

[0, 1, 2] [0, 2, 1] [1, 0, 2] [1, 2, 0] [2, 0, 1] [2, 1, 0] [0, 1, 3] [0, 3, 1] [1, 0, 3] [1, 3, 0] [3, 0, 1] [3, 1, 0] [0, 2, 3] [0, 3, 2] [2, 0, 3] [2, 3, 0] [3, 0, 2] [3, 2, 0] [1, 2, 3] [1, 3, 2] [2, 1, 3] [2, 3, 1] [3, 1, 2] [3, 2, 1]

The algorithm simply finds the next permutation of the array. If the array is at the last permutation, the next combination from the set is constructed.

Note that this algorithm does not produce the k-permutations in lexicographic order.

unsigned int next_k_permutation(unsigned int *ar, size_t n, unsigned int k) { unsigned int result = next_permutation(ar, k); if (result == 0) { result = next_combination(ar, n, k); } return result; }