all combinations of k elements out of n

Question

Can somebody provide me a link or pseudocode of a function for finding all combinations of k elements out of n? possibly in STL. I don\'t need to compute n choose k, I need

Answer 1

You could use std::next_permutation but it is n! and not n choose k. You could filter them after you created them. But this solution is O(n!), not really perfect. Here is the trial and error solution:

int factorial(int value)
{
    int result = 1;

    for(int i = 1; i <= value; i++)
    {
        result *= i;
    }

    return result;
}

std::set> binomial_coefficient(std::vector input, int k)
{
    std::set> solutions;

    for(unsigned int i = 0; i < factorial(input.size()); i++)
    {
        std::next_permutation(input.begin(), input.end());

        solutions.insert(std::set(input.begin(), input.begin() + k));
    }

    return solutions;
}