Question

let’s say , a course with 8 participans, i must output the first 3 places in all possible ways.
ex :

123
124
125
126
127
128
213
so on..

I know there is next_permutation algorithm but it returns all the possible permuations with all the numbers (from 1 through 8), but i need first 3 places with all the participans
ex:

1  2  3  4  5  6  7  8  
1  2  3  4  5  6  8  7

Answer 1

This program produces the output you are looking for, not necessarily in the order you expect. If you want it in a particular order, you may need to capture the output and sort it. To see it run, look here.

#include <algorithm>
#include <iostream>

template <typename Iterator>
inline bool next_combination(Iterator first,
  Iterator k,
  Iterator last);

int main () {
  int array[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
  do {
    do {
       std::cout << array[0] << array[1] << array[2] << "\n";
    } while(std::next_permutation(array, array+3));
  } while(next_combination(array,array+3,array+8));
}

template <typename Iterator>
inline bool next_combination(const Iterator first, Iterator k, const Iterator last)
{
   /* Credits: Thomas Draper */
   // http://stackoverflow.com/a/5097100/8747
   if ((first == last) || (first == k) || (last == k))
      return false;
   Iterator itr1 = first;
   Iterator itr2 = last;
   ++itr1;
   if (last == itr1)
      return false;
   itr1 = last;
   --itr1;
   itr1 = k;
   --itr2;
   while (first != itr1)
   {
      if (*--itr1 < *itr2)
      {
         Iterator j = k;
         while (!(*itr1 < *j)) ++j;
         std::iter_swap(itr1,j);
         ++itr1;
         ++j;
         itr2 = k;
         std::rotate(itr1,j,last);
         while (last != j)
         {
            ++j;
            ++itr2;
         }
         std::rotate(k,itr2,last);
         return true;
      }
   }
   std::rotate(first,k,last);
   return false;
}