99 questions/Solutions/24

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

Alternative solution:

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.