Keneth Adrian Keneth Adrian - 1 month ago 17
C++ Question

Generating combinations in c++

I have been searching a source code for generating combination using c++. I found some advanced codes for this but that is good for only specific number predefined data. Can anyone give me some hints, or perhaps, some idea to generate combination. As an example, suppose the set S = { 1, 2, 3, ...., n} and we pick r= 2 out of it. The input would be

n
and
r
.In this case, the program will generate arrays of length two, like 5 2 outputs 1 2, 1 3, etc.. I had difficulty in constructing the algorithm. It took me a month thinking about this.

Answer

A simple way using std::next_permutation:

#include <iostream>
#include <algorithm>
#include <vector>

int main() {
    int n, r;
    std::cin >> n;
    std::cin >> r;

    std::vector<bool> v(n);
    std::fill(v.end() - r, v.end(), true);

    do {
        for (int i = 0; i < n; ++i) {
            if (v[i]) {
                std::cout << (i + 1) << " ";
            }
        }
        std::cout << "\n";
    } while (std::next_permutation(v.begin(), v.end()));
    return 0;
}

or a slight variation that outputs the results in an easier to follow order:

#include <iostream>
#include <algorithm>
#include <vector>

int main() {
   int n, r;
   std::cin >> n;
   std::cin >> r;

   std::vector<bool> v(n);
   std::fill(v.begin(), v.begin() + r, true);

   do {
       for (int i = 0; i < n; ++i) {
           if (v[i]) {
               std::cout << (i + 1) << " ";
           }
       }
       std::cout << "\n";
   } while (std::prev_permutation(v.begin(), v.end()));
   return 0;
}

A bit of explanation:

It works by creating a "selection array" (v), where we place r selectors, then we create all permutations of these selectors, and print the corresponding set member if it is selected in in the current permutation of v. Hope this helps.

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