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

If you don't need to modify your set or need to modify it infrequently you can use arrays as lookup table with O(1) access time.

import qualified Data.Vector 
import qualified Data.Set

newtype RandSet a = RandSet (V.Vector a)

randElem :: RandSet a -> RandomGen -> (a, RandomGen)
randElem (RandSet v) g
  | V.empty v = error "Cannot select from empty set" 
  | otherwise = 
    let (i,g') = randomR (0, V.length v - 1) g
    in (v ! i, g')

-- Of course you have to rebuild array on insertion/deletion which is O(n)
insert :: a -> RandSet a -> RandSet a
insert x = V.fromList . Set.toList . Set.insert x . Set.fromList . V.toList`