How to get a random element from a C++ container?

Question

What is a good way to get a [pseudo-]random element from an STL range?

The best I can come up with is to do std::random_shuffle(c.begin(), c.end()) an

Answer 1

If you can't access the size, I think you would want to do the following. It returns the iterator to the random element.

#include 
#include 

template  InputIterator 
random_n(InputIterator first, InputIterator last) {
   typename std::iterator_traits::difference_type distance = 
        std::distance(first, last);
   InputIterator result = first;
   if (distance > 1) {
      // Uses std::rand() naively.  Should replace with more uniform solution. 
      std::advance( result, std::rand() % distance );
   }
   return result;
}
// Added in case you want to specify the RNG.  RNG uses same 
// definition as std::random_shuffle
template  InputIterator 
random_n(InputIterator first, InputIterator last, RandomGenerator& rand) {
   typename std::iterator_traits::difference_type distance = 
       std::distance(first, last);
   InputIterator result = first;
   if (distance > 1) {
      std::advance( result, rand(distance) );
   }
   return result;
}