Pick a unique random subset from a set of unique values

Question

C++. Visual Studio 2010.

I have a std::vector V of N unique elements (heavy structs). How can efficiently pick M random, unique, elemen

Answer 1

Create a random permutation of the range 0, 1, ..., N - 1 and pick the first M of them; use those as indices into your original vector.

A random permutation is easily made with the standard library by using std::iota together with std::random_shuffle:

std::vector v; // given

std::vector indices(V.size());
std::iota(indices.begin(), indices.end(), 0);
std::random_shuffle(indices.begin(), indices.end());

// use V[indices[0]], V[indices[1]], ..., V[indices[M-1]]

You can supply random_shuffle with a random number generator of your choice; check the docu­men­tation for details.