99 questions/Solutions/17

(*) Split a list into two parts; the length of the first part is given.

Do not use any predefined predicates.

Solution using take and drop:

split xs n = (take n xs, drop n xs)

Or even simpler using splitAt:

split = flip splitAt

But these should clearly be considered "predefined predicates". Alternatively, we have the following recursive solution:

split :: [a] -> Int -> ([a], [a])
split []         _             = ([], [])
split l@(x : xs) n | n > 0     = (x : ys, zs)
                   | otherwise = ([], l)
    where (ys,zs) = split xs (n - 1)

The same solution as above written more cleanly:

split :: [a] -> Int -> ([a], [a])
split (x:xs) n | n > 0 = let (f,l) = split xs (n-1) in (x : f, l)
split xs _             = ([], xs)

Or (ab)using the "&&&" arrow operator for tuples:

split :: [a] -> Int -> ([a], [a])
split (x:xs) n | n > 0 = (:) x . fst &&& snd $ split xs (n - 1)
split xs _             = ([], xs)

A similar solution using foldl:

split :: [a] -> Int -> ([a], [a])
split [] _ = ([], [])
split list n
  | n < 0 = (list, [])
  | otherwise  = (first output, second output)
    where output = foldl (\acc e -> if third acc > 0 then (first acc ++ [e], second acc, third acc - 1) else (first acc, second acc ++ [e], third acc)) ([], [], n) list

Note that for the above code to work you must define your own first, second, and third functions for tuples containing three elements like so:

first :: (a, b, c) -> a  
first (x, _, _) = x  
  
second :: (a, b, c) -> b  
second (_, y, _) = y  
  
third :: (a, b, c) -> c  
third (_, _, z) = z

Another foldl solution without defining tuple extractors:

split :: [a] -> Int -> ([a],[a])
split lst n = snd $ foldl helper (0,([],[])) lst
    where helper (i,(left,right)) x = if i >= n then (i+1,(left,right++[x])) else (i+1,(left++[x],right))