Enumerate every length-K combination of { 1, 2, ..., N } in ascending lexicographic order.

enumerate-combinations.hxx

#include <vector>
#include <cassert>

// Call |f| with every length-K combination of { 1, 2, ..., N }, in ascending
// lexicographic order.
template<typename F>
void enumerate_combinations(size_t N, size_t K, const F& f) {
	assert(K <= N);
	std::vector<size_t> position;
	// Initialize with initial position.
	position.reserve(K);
	for (size_t i = 1; i <= K; ++i) {
		position.push_back(i);
	}
	for (;;) {
		assert(position.size() == K);
		for (size_t i = 0; i < K; ++i) {
			assert(position[i] >= 1);
			assert(position[i] <= N);
		}
		f(position);
		// Update to next position.
		while (!position.empty()) {
			size_t limit = N + position.size() - K + 1;
			if (++position.back() < limit) {
				break;
			}
			position.pop_back();
		}
		if (position.empty())
			break;
		while (position.size() < K) {
			position.push_back(position.back() + 1);
		}
	}
}