Fastest way to obtain the largest X numbers from a very large unsorted list?

Question

I\'m trying to obtain the top say, 100 scores from a list of scores being generated by my program. Unfortuatly the list is huge (on the order of millions to billions) so sorting

Answer 1

Here's the 'natural' C++ way to do this:

std::vector v;
// fill in v
std::partial_sort(v.begin(), v.begin() + 100, v.end(), std::greater());
std::sort(v.begin(), v.begin() + 100);

This is linear in the number of scores.

The algorithm used by std::sort isn't specified by the standard, but libstdc++ (used by g++) uses an "adaptive introsort", which is essentially a median-of-3 quicksort down to a certain level, followed by an insertion sort.