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Haskell 99 Problems - Number 17

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problem 17, Split a list into two parts; the length of the first part is given.
Do not use any predefined predicates. (Meaning no splitAt or take or drop)
*Main> split "abcdefghik" 3
("abc", "defghik")

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  1. -- Introducing my convoluted implementation of splitAt
  2. split :: [a] -> Int -> ([a], [a])
  3. split xs n = (pre xs 0 n, post xs 0 (n-1))
  4.     where pre :: [a] -> Int -> Int -> [a]
  5.           pre [] _ _ = []
  6.           pre xs dex n
  7.                 | dex > length xs = xs
  8.                 | n > length xs = xs
  9.                 | otherwise = if (dex < n) 
  10.                                     then xs!!dex : pre xs (dex+1) n
  11.                                     else [] 
  12.           post :: [a] -> Int -> Int -> [a]
  13.           post [] _ _ = []
  14.           post (x:xs) dex n
  15.                 | n < 0 = (x:xs)
  16.                 | otherwise = if (dex < n)
  17.                                 then post xs (dex+1) n
  18.                                 else xs
  19.  
  20. -- Or, to borrow from the solutions on Haskell.org 
  21. -- (which is incorrectfor negative n, BTW)
  22.  
  23. split' :: [a] -> Int -> ([a], [a])
  24. split' xs 0 = ([], xs)
  25. split' (x:xs) n = let (f,l) = split xs (n-1) in (x : f, l)

URL: http://haskell.org/haskellwiki/99_questions/11_to_20

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