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);

  set<int>::const_iterator it(s.begin());

  // 'advance' the iterator 5 times
  advance(it,5);
}

Answer 2

int GetSample(const std::set<int>& s) {
  double r = rand() % s.size();
  std::set<int>::iterator it = s.begin();
  for (; r != 0; r--) it++;
  return *it;
}

would be one way of doing it, although not pretty;