KC:
An interesting related problem is if you are only allowed one pass through the data how would you randomly choose one word.
Let's choose n items. You must know the length of the sequence, of course, otherwise the 'probability' loses its sense. So, for lists it might not be "just one pass"... Suppose the length of the sequence be m. Suppose you have a random generator called rg, just a simple function which transforms: seed -> seed' , between 0 and 1 Make n and m real to make the typechecker happy. Then the most straightforward solution for lists is: nran l n = nr l m n seed where m = fromIntegral(length(l)) nr [] _ _ _ = [] nr (x:q) m n seed = let seed'=rg seed in if seed' < n/m then x:nr q (m-1) (n-1) seed' else nr q (m-1) n seed' -- ========= Now, you may make it tail-recursive, use a different random generation protocol, or whatever. I believe that this solution is known for years... Jerzy Karczmarczuk