On Jul 17, 2007, at 22:26 , James Hunt wrote:
Hi,
As a struggling newbie, I've started to try various exercises in order to improve. I decided to try the latest Ruby Quiz (http:// www.rubyquiz.com/quiz131.html) in Haskell. Would someone be kind enough to cast their eye over my code? I get the feeling there's a better way of doing it!
subarrays :: [a] -> [[a]] subarrays [] = [[]] subarrays xs = (sa xs) ++ subarrays (tail xs) where sa xs = [ys | n <- [1..length xs], ys <- [(take n xs)]]
maxsubarrays :: [Integer] -> [Integer] maxsubarrays xs = msa [] (subarrays xs) where msa m [] = m msa m (x:xs) | sum x > sum m = msa x xs | otherwise = msa m xs
--for testing: should return [2, 5, -1, 3] main = maxsubarrays [-1, 2, 5, -1, 3, -2, 1]
I've read tutorials about the syntax of Haskell, but I can't seem to find any that teach you how to really "think" in a Haskell way. Is there anything (books, online tutorials, exercises) that anyone could recommend?
Thanks, James
Hi james, here's one solution: import Data.List maxsubarrays xs = maximumBy (\x y -> sum x `compare` sum y) [zs | ys <- inits xs, zs <- tails ys] This can be made somewhat nicer with 'on': import Data.List maxsubarrays xs = maximumBy (compare `on` sum) [zs | ys <- inits xs, zs <- tails ys] on, which will appear in Data.Function in the next release of base, is defined thusly: on :: (b -> b -> c) -> (a -> b) -> a -> a -> c (*) `on` f = \x y -> f x * f y /Björn