Lotto: Draw N different random numbers from the set 1..M.
import System.Random diff_select :: Int -> Int -> IO [Int] diff_select n to = diff_select' n [1..to] diff_select' 0 _ = return  diff_select' _  = error "too few elements to choose from" diff_select' n xs = do r <- randomRIO (0,(length xs)-1) let remaining = take r xs ++ drop (r+1) xs rest <- diff_select' (n-1) remaining return ((xs!!r) : rest)
The random numbers have to be distinct!
In order to use randomRIO here, we need import module System.Random.
As can be seen, having implemented problem 23, rnd_select, the solution is trivial.
diff_select n to = rnd_select [1..to] n
diffSelect :: Int -> Int -> IO [Int] diffSelect n m = do gen <- getStdGen return . take n $ randomRs (1, m) gen
(Note that this doesn't really solve the problem, since it doesn't generate distinct numbers).
Using nub from Data.List:
diff_select :: Int -> Int -> StdGen -> [Int] diff_select n m = take n . nub . randomRs (1, m)
Or without giving StdGen as argument and returning IO [Int]:
import System.Random import Data.List import Control.Applicative diff_select :: Int -> Int -> IO [Int] diff_select n m = take n . nub . randomRs (1, m) <$> getStdGen
Note: With the above solutions using nub, the function will not terminate if m < n. I suggest adding a guard to protect against this.