combinations

Print all combinations of M members of a set of N elements in a sequence such that each set can be obtained from the previous set by deleting one member and adding one member.

or Rather print ⁿC_r

Answer:

#include "stdio.h"
#include "iostream"
#include "algorithm"
#include "iterator"
#include "assert.h"
using namespace std;

int main(int argc, char *argv[])
{
    const size_t n = atoi(argv[1]);
    const size_t k = atoi(argv[2]);
    cout << "N = " << n << " k = " << k << endl;
    assert ( (k <= n) && (k > 0));
    size_t *array = new size_t[n];
    size_t up = 0;
    size_t down = 0;
    int i = 0;
    // Initialize the arrray
    for (i = 1; i <= n; ++i) array[i-1] = i;

    for ( up = 0 , down = up+k-1; down < n; ++up , ++down)
    {
        printf(" \n up %d down %d \n", up, down);
        for( i = down; i < n; ++i )
        {
            swap( array[down], array[i] );
            //--- Print the combination ---
            for(int k = up; k<=down; ++k)
                cout << array[k] << " ";
            cout << endl;
            //--- End Print the combination ---
            swap( array[i], array[down] );
        }

    }
    return 0;
}