Question

I have a list of list . All elements will be inserted at run-time. i wish to try all combinations through all list. So the simple solution comes to mind as

for ( l1 in L1)   
{   for ( l2 in L2)   
    { for ( l3 in L3)  
          { 
               ... // use the pair (l1,l2,l3) 
          }
    }
}

But i dont know the number of fors required at compile time. So how do i iterate through all pairs within c++?

Answer 1

Use recursion.

typedef std::list<std::list<int>> ListOfList;

void actOnHelper(ListOfList::const_iterator c, ListOfList::const_iterator end, std::list<int> v)
{
   if (c == e) {
      // do something on v
   } else {
      ListOfList::const_iterator nextc(c);
      ++nextc;
      for (std::list<int>::const_iterator i = c->begin(), e = c->end(); i != e; ++i) {
        v.push_back(*i);
        actOnHelper(nextc, end, v);
        v.pop_back();
     }
   }
}
void actOn(std::list<std::list<int>> const& l)
{
   actOnHelper(l.begin(), l.end(), std::list<int>());
}