Advice on cleaning up this termination condition - c++

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

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;
});
}

Answer 2

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;
};