On Fri, 10 Apr 2009, Chad Scherrer wrote:
A nice generalization of this that can be really useful is
movingWindow :: Int -> [a] -> [[a]] movingWindow 1 xs = map (:[]) xs movingWindow n xs = zipWith (:) xs . tail $ movingWindow (n-1) xs
So for example,
movingWindow 3 [1..10] [[1,2,3],[2,3,4],[3,4,5],[4,5,6],[5,6,7],[6,7,8],[7,8,9],[8,9,10]]
movingWindow n xs = take (length xs - n +1) $ map (take n) $ tails xs or more efficient using utility-ht package: movingWindow n xs = Data.List.Match.take (drop (n-1) xs) $ map (take n) $ tails xs
Then you can write
diff :: (Num a) => [a] -> [a] diff = map (\[x,y] -> y - x) . movingWindow 2
Hopefully the intermediate lists are optimized away, but I haven't done any performance testing.
I'm not sure. You are safer and more efficient when you restrict to pairs. Since I often need the function, I defined: http://hackage.haskell.org/packages/archive/utility-ht/0.0.4/doc/html/Data-L... Then diff = mapAdjacent subtract