Fast intersection of sets: C++ vs C#

Question

On my machine (Quad core, 8gb ram), running Vista x64 Business, with Visual Studio 2008 SP1, I am trying to intersect two sets of numbers very quickly.

I\'ve impleme

Answer 1

One problem I see right away is that you're passing the sets in C++ by value and not by const reference. So you're copying them every time you pass them around!

Also, I would not use a set for the target of set_intersection. I would use something like

int runSetIntersection(const set& set1, const set& set2)
{   
    vector intersection;
    intersection.reserve(10000) // or whatever the max is

    set_intersection(set1.begin(),set1.end(), set2.begin(), set2.end(), back_inserter(intersection));

    return intersection.size(); 
}

This code, however, still allocates inside the function. Even faster would be

int runSetIntersection(const set& set1, const set& set2, vector& scratch)
{   
    scratch.reserve(10000) // or whatever the max is

    set_intersection(set1.begin(),set1.end(), set2.begin(), set2.end(), back_inserter(scratch));

    return scratch.size(); 
}

And then allocate scratch before you start the timer.

Though, if you're just looking for the size, a hand-written for loop, combined with set::find might give even better results.