Question

I would like to get 2 random different elements from an std::vector. How can I do this so that:

• It is fast (it is done thousands of times in my algorithm)
• It is elegant
• The elements selection is really uniformly distributed

Answer 1

For elegance and simplicty:

void Choose (const int size, int &first, int &second)
{
  // pick a random element
  first = rand () * size / MAX_RAND;
  // pick a random element from what's left (there is one fewer to choose from)...
  second = rand () * (size - 1) / MAX_RAND;
  // ...and adjust second choice to take into account the first choice
  if (second >= first)
  {
     ++second;
  }
}

using first and second to index the vector.

For uniformness, this is very tricky since as size approaches RAND_MAX there will be a bias towards the lower values and if size exceeds RAND_MAX then there will be elements that are never chosen. One solution to overcome this is to use a binary search:

int GetRand (int size)
{
  int lower = 0, upper = size;
  do
  {
    int mid = (lower + upper) / 2;

    if (rand () > RAND_MAX / 2) // not a great test, perhaps use parity of rand ()?
    {
       lower = mid;
    }
    else
    {
       upper = mid;
    }
  } while (upper != lower); // this is just to show the idea,
                            // need to cope with lower == mid and lower != upper
                            // and all the other edge conditions

  return lower;
}