2009/3/24 Peter Verswyvelen
But aren't these two definitions different algoritms? At first sight I think the second one is more efficient than the first one.
Some performance numbers: ---------------------------------------------------------------------- module Main where import System.Environment (getArgs) import Control.Monad.State (State(..), evalState) takeList1, takeList2, takeList3 :: [Int] -> [a] -> [[a]] takeList1 [] _ = [] takeList1 _ [] = [] takeList1 (n : ns) xs = head : takeList1 ns tail where (head, tail) = splitAt n xs takeList2 ns xs = zipWith take ns . init . scanl (flip drop) xs $ ns takeList3 = evalState . mapM (State . splitAt) test :: Int -> [[Int]] test n = takeList1 (take n [1..]) [1..] main :: IO () main = print . sum . map sum . test . read . head =<< getArgs ---------------------------------------------------------------------- compile with: ghc --make TakeList.hs -o takeList1 -O2 $ time ./takeList1 5000 739490938 real 0m6.229s user 0m5.787s sys 0m0.342s $ time ./takeList2 5000 739490938 real 0m5.089s user 0m4.455s sys 0m0.348s $ time ./takeList3 5000 739490938 real 0m6.224s user 0m5.750s sys 0m0.347s ---------------------------------------------------------------------- regards Bas