Advice on cleaning up this termination condition

Question

I've been playing with C++11 functional in order to do the same as python's itertools.combinations(input, 2), so far this is what I have:

EDIT removed outer lambda as suggested by @DavidRodríguez-dribeas

#include <iostream>
#include <functional>
#include <vector>

using namespace std;

template <class T>
function<pair<T*, T*>()> combinations(vector<T> & input) {
  auto it1 = input.begin();
  auto end = input.end();
  auto it2 = next(it1);
  return [=]() mutable {
      if (it2 == end) {
        it1++;
        it2 = next(it1);
      }   
      if (it2 != end)
        return pair<T*,T*>(&(*it1), &(*it2++));
      return pair<T*,T*>(&*end, &*end);
    };  
};

int main (void) {
  vector<int> numbers{1,2,3,4,5,6};
  auto func = combinations(numbers);
  while ( true ) { 
    auto i = func();
    if (i.first == &*(numbers.end())) break;
    cout << *(i.first) << ',' << *(i.second) << endl;
  }

  return 0;
};

I'm not happy with the method used to iterate over the combinations any advice on cleaning it up?

Answer 1

I found out that Oliver Kowalke's coroutine library has been accepted by Boosts peer review and should be included hopefully in the next version. Jumping the gun a bit I gave it a go by using the coroutine branch of the boost-dev repo (https://gitorious.org/boost-dev/boost-dev).

g++ -I path/to/boost-dev -std=c++11 test_code.cpp -o run_test_code -static -L path/to/boost-dev/stage/lib/ -lboost_context

#include <boost/coroutine/all.hpp>
#include <boost/bind.hpp>
#include <boost/range.hpp>
#include <iostream>
#include <vector>

using namespace std;
using namespace boost;

template <typename T>
using coro_pairT_void = coroutines::coroutine<pair<T&,T&>(void)>;

template <typename T>
void combinations(typename coro_pairT_void<T>::caller_type & self, vector<T> & input ) { 
  for (auto it1 = input.begin(), itend = input.end(); it1 != itend; it1++) {
    for (auto it2 = std::next(it1); it2 != itend; it2++) {
      self(pair<T&, T&>(*it1,*it2));
    }   
  }
};

int main( void ) { 
  vector<int> numbers{1,2,3,4,5,6};
  coro_pairT_void<int> func(bind(combinations<int>, _1, numbers));
  for (auto it(begin(func)), itend(end(func)); it != itend; ++it) {
    cout << it->first << ',' << it->second << endl;
  }
  return 0;
};

Answer 2

Here is documentation and code on my favorite way of doing this. And here is how that library would be used for your example:

#include <iostream>
#include <vector>
#include "combinations"

using namespace std;

int main (void) {
  vector<int> numbers{1,2,3,4,5,6};
  for_each_combination(numbers.begin(), numbers.begin()+2, numbers.end(),
           [](vector<int>::const_iterator b, vector<int>::const_iterator e)
           {
              if (b != e)
              {
                cout << *b;
                for (auto i = b+1; i != e; ++i)
                    cout << ',' << *i;
                cout << endl;
              }
              return false;
           });
}

1,2
1,3
1,4
1,5
1,6
2,3
2,4
2,5
2,6
3,4
3,5
3,6
4,5
4,6
5,6

Should the need arise, it is trivial to change the example use to consider 3 or 4 items at time instead of 2. One can also deal with various permutations k out of N at a time.

Update

Adding a level of indirection to illustrate how you would deal with a vector of items that were not efficient at moving/swapping around in the vector:

#include <iostream>
#include <vector>
#include "combinations"

using namespace std;

int main (void) {
  vector<int> numbers{1,2,3,4,5,6};
  vector<vector<int>::const_iterator> num_iters;
  num_iters.reserve(numbers.size());
  for (auto i = numbers.begin(); i != numbers.end(); ++i)
    num_iters.push_back(i);
  for_each_combination(num_iters.begin(), num_iters.begin()+2, num_iters.end(),
           [](vector<vector<int>::const_iterator>::const_iterator b,
              vector<vector<int>::const_iterator>::const_iterator e)
           {
              if (b != e)
              {
                cout << **b;
                for (auto i = b+1; i != e; ++i)
                    cout << ',' << **i;
                cout << endl;
              }
              return false;
           });
}