Question

void print_combinations(const std::string &str)
{
        int i, j, k;
        int len = str.length();

        for (i = 0; i < len - 2; i++)
        {
                for (j = i + 1; j < len - 1; j++)
                {
                        for (k = j + 1; k < len; k++)
                                // show combination
                                cout << str.at(i) << str.at(j) << str.at(k) << endl;

                }
        }
}

This function does what I want, but I want to make it recursive, so that it can create combinations of arbitrary length.

Answer 1

Here is a try:

#include <iostream>
#include <string.h> // for strlen
#include <stdlib.h> // for atoi
#include <sstream>
void expand_combinations(const char *remaining_string, std::ostringstream& i, int remain_depth)
{
    if(remain_depth==0)
    {
        std::cout << i.str() << std::endl;
        return;
    }

    for(int k=0; k < strlen(remaining_string); ++k)
    {
        std::ostringstream l;
        l << i.str();
        l << remaining_string[k];
        expand_combinations(remaining_string+k+1, l, remain_depth - 1);
    }
    return;
}
int main(int argc, char **argv)
{
    std::ostringstream i;
    if(argc<3) return 1;
    expand_combinations(argv[1], i, atoi(argv[2]));
    return 0;
}

Output of ./test fjord 3:

fjo
fjr
fjd
for
fod
frd
jor
jod
jrd
ord