Choose random array element satisfying certain property

Question

Suppose I have a list, called elements, each of which does or does not satisfy some boolean property p. I want to choose one of the elements that

Answer 1

For clarity's sake, I would try:

pickRandElement(elements, p)
     OrderedCollection coll = new OrderedCollection
     foreach element in elements
          if (p(element))
               coll.add(element)
     if (coll.size == 0) return null
     else return coll.get(randInt(coll.size))

To me, that makes it MUCH clearer what you're trying to do and is self-documenting. On top of that, it's simpler and more elegant, and it's now obvious that each will be picked with an even distribution.