How to select a random element in std::set?

Question

How can I select a random element in an std::set?

I naively tried this:

int GetSample(const std::set& s) {
  double r =          


        

Answer 1

C++17 std::sample

This will be a convenient, although not very efficient (O(n)) method:

#include 
#include 
#include 
#include 
#include 

int main() {
    std::set in{1, 2, 3, 5, 7};
    std::vector out;
    std::sample(in.begin(), in.end(), std::back_inserter(out),
                3, std::mt19937{std::random_device{}()});
    for (auto i : out)
        std::cout << i << std::endl;
}

But I think that for efficiency you just need to copy to another type of structure: How to select a random element in std::set in less than O(n) time?