Select random element from a set, faster than linear time (Haskell)

Question

I\'d like to create this function, which selects a random element from a Set:

randElem :: (RandomGen g) => Set a -> g -> (a, g)

Si

Answer 1

Another way to achieve this might be to use Data.Sequence instead of Data.Set. This would allow you to add elements to the end in O(1) time and index elements in O(log n) time. If you also need to be able to do membership tests or deletions, you would have to use the more general fingertree package and use something like FingerTree (Sum 1, Max a) a. To insert an element, use the Max a annotation to find the right place to insert; this basically takes O(log n) time (for some usage patterns it might be a bit less). To do a membership test, do basically the same thing, so it's O(log n) time (again, for some usage patterns this might be a bit less). To pick a random element, use the Sum 1 annotation to do your indexing, taking O(log n) time (this will be the average case for uniformly random indices).