Personal tools

Haskell Quiz/Maximum Sub-Array/Solution Jkramar

From HaskellWiki

Jump to: navigation, search

This includes a solution to the "extra credit" problem of finding the maximum subrectangle.

import Data.List
 
maxSubArray' :: (Ord a, Num a) => [a] -> (a, (Int, Int))
maxSubArray' xs = maximum$zipWith diff sumswithpos$scanl1 min sumswithpos where
  sumswithpos = zip (scanl (+) 0 xs) [0..]
  diff (a,ai) (b,bi) = (a-b,(ai,bi))
 
maxSubArray :: (Ord a, Num a) => [a] -> [a]
maxSubArray xs = drop from$take to xs where (_, (to, from)) = maxSubArray' xs
 
maxSubRect' :: (Ord a, Num a) => [[a]] -> ((a, (Int, Int)), (Int, Int))
maxSubRect' as = maximum rectsums where 
  sums ((c,b):rs) = [(maxSubArray'$zipWith (-) b' b, (c',c))|(c',b') <- rs]
  rectsums = concatMap sums$init$tails$zip [0..]$transpose$map (scanl (+) 0) as
 
maxSubRect :: (Ord a, Num a) => [[a]] -> [[a]]
maxSubRect as = map (drop y1.take y2)$drop x1$take x2 as where
  ((_,(x2,x1)),(y2,y1)) = maxSubRect' as