Difference between revisions of "99 questions/Solutions/24"
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</haskell> |
</haskell> |
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(Note that this doesn't really solve the problem, since it doesn't generate ''distinct'' numbers). |
(Note that this doesn't really solve the problem, since it doesn't generate ''distinct'' numbers). |
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| − | Alternative solution producing ''distinct'' numbers: |
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| − | <haskell> |
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| − | import System.Random |
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| − | import Control.Monad (replicateM) |
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| − | |||
| − | diff_select :: Int -> Int -> IO [Int] |
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| − | diff_select n m |
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| − | | n <= 0 = return [] |
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| − | | otherwise = replicateM n $ getStdRandom $ randomR(1,m) |
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| − | </haskell> |
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Revision as of 12:48, 20 November 2011
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).