Haskell Quiz/Sampling/Solution Dolio

< Haskell Quiz | Sampling

Categories: Haskell Quiz solutions

This is a somewhat naive algorithm, but it manages to run the challenge problem in a reasonable amount of time. It simply uses the IntSet data structure, testing and adding random numbers until enough are found.

module Main where
import qualified Data.IntSet as I
import System
import System.Random
 
build n k s (r:rs)
    | k `seq` s `seq` False = undefined -- strictness
    | k >= n             = s
    | not $ I.member r s = build n (k+1) (I.insert r s) rs
    | otherwise          = build n k s rs
 
main = do [n, l] <- fmap (map read) getArgs
          g <- getStdGen
          if n > l
           then putStrLn "Your request is impossible."
           else putStr . unlines . map show . I.elems
                       $ build n 0 I.empty (randomRs (0, l-1) g)

A run of the sample problem on my machine took about 1 minute.

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