generate all subsets of size k from a set

Question

I want to generate all the subsets of size k from a set.

eg:-say I have a set of 6 elements, I have to list all the subsets in which the cardinality of elements is 3

Answer 1

My old code gives the following result:

111000
110100
110010
110001
101100
101010
101001
100110
100101
100011
011100
011010
011001
010110
010101
010011
001110
001101
001011
000111

Enough optimized:

#include 

int firstPermutation(int n, int k) {
    return ((1 << k) - 1) << (n - k);
}

int shiftLast1(int a) {
    return (a - 1) ^ ((a^(a - 1)) >> 2);
}

int add1AfterLast1(int a) {
    return a | (((a^(a - 1)) + 1) >> 2);
}

int nextPermutation(int a) {
    if ((a & (a + 1)) == 0) {
        return 0;
    }

    if (a & 1) {
        return add1AfterLast1(nextPermutation(a >> 1) << 1);
    }
    else {
        return shiftLast1(a);
    }
}

int main() {
    int n = 6;
    int k = 3;
    int a = firstPermutation(n, k);
    do {
        for (int i = 0; i < n; i++) {
            std::cout << ((a >> (n - 1 - i)) & 1);
        }
        std::cout << std::endl;
    } while ((a = nextPermutation(a)));
}