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
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