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Haskell Quiz/Weird Numbers/Solution Ninju

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< Haskell Quiz | Weird Numbers
Revision as of 23:54, 10 August 2008 by Ninju (Talk | contribs)

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module Main where
import System.Environment
 
main :: IO ()
main = do args <- getArgs
          let max = read (head args)
          if length args == 1
            then do putStrLn $ "The weird numbers up to " ++ (head args) ++ ":"
                    putStrLn $ show $ takeWhile (<= max) weirdNumbers
            else putStrLn "Usage: weird_numbers LIMIT"
          return ()
 
weirdNumbers :: [Integer]
weirdNumbers = filter weird [70..]
               where
               weird n = sum (divisors n) > n && (not . any (==n) . map sum . subsets . divisors) n
 
divisors :: Integer -> [Integer]
divisors n = filter ((==0) . (mod n)) [1..(n `div` 2)]
 
subsets :: [a] -> [[a]]
subsets [] = [[]]
subsets (x:xs) = subsets xs ++ map (x:) (subsets xs)