c++ - 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<int>& s) {
  double r = rand() % s.size();
  return *(s.begin() + r); // compile error
}

But the operator+ is not allowed in this way.

Answer 1

You could use the std::advance method.

#include <set>
#include <algorithm>

int main() {
  using namespace std;
  // generate a set...
  set<int> s;
  for( int i = 0; i != 10; ++i ) s.insert(i);
  auto r = rand() % s.size(); // not _really_ random
  auto n = *select_random(s, r);
}

Where

template<typename S>
auto select_random(const S &s, size_t n) {
  auto it = std::begin(s);
  // 'advance' the iterator n times
  std::advance(it,n);
  return it;
}