Question

C++. Visual Studio 2010.

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

E.g. V contains 10 elements: { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } and I pick three…

• 4, 0, 9
• 0, 7, 8
• But NOT this: 0, 5, 5 <— not unique!

STL is preferred. So, something like this?

std::minstd_rand gen; // linear congruential engine??
std::uniform_int<int> unif(0, v.size() - 1);
gen.seed((unsigned int)time(NULL));

// ...?

// Or is there a good solution using std::random_shuffle for heavy objects?

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<Heavy> v; // given

std::vector<unsigned int> 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.