Question

I have some code that looks like this:

std::set<int> s1, s2, out;

// ... s1 and s2 are populated ...

std::set_intersection(s1.begin(), s1.end(),
                      s2.begin(), s2.end(),
                      std::inserter(out, out.end()));

I’ve read inserts can be done in amortized constant time if the value being inserted to the set immediately follows the iterator given as a “hint”. This would obviously be beneficial when running the set intersection, especially since everything being written to out is already in sorted order.

How do I guarantee this optimal performance? When creating the std::inserter, out is empty so out.begin() == out.end() so I can’t see it makes any difference whether I specify out.begin() or out.end() as the hint. However, if this is interpreted at inserting every element at begin(), it doesn’t seem that I would get the optimum algorithmic performance. Can this be done better?

Answer 1

You could use a custom functor instead of std::inserter and re-call out.end() every time a new element is inserted.

Alternatively, if your values are sorted descendingly, out.begin() will be fine.