Question

我还在学习Haskell，并编写了以下基数排序函数。它似乎工作正常，但问题是它相当于内存效率低下。如果用ghc编译，则内存大小超过500MB，输入列表大小为10000个元素。

所以我想问你如何改进以下算法/代码，使其在速度和内存方面更有效率。什么是最好的起点？

import System.Random

-- radixsort for positive integers. uses 10 buckets
radixsort :: [Int] -> [Int]
radixsort [] = []
radixsort xs =
    -- given the data, get the number of passes that are required for sorting
    -- the largest integer
    let maxPos = floor ((log (fromIntegral (foldl max 0 xs)) / log 10) + 1)

        -- start sorting from digit on position 0 (lowest position) to position 'maxPos'
        radixsort' ys pos
         | pos < 0   = ys
         | otherwise = let sortedYs   = radixsort' ys (pos - 1)
                           newBuckets = radixsort'' sortedYs [[] | i <- [1..10]] pos
                       in  [element | bucket <- newBuckets, element <- bucket]

        -- given empty buckets, digit position and list, sort the values into
        -- buckets
        radixsort'' []     buckets _   = buckets
        radixsort'' (y:ys) buckets pos =
            let digit = div (mod y (10 ^ (pos + 1))) (10 ^ pos)
                (bucketsBegin, bucketsEnd) = splitAt digit buckets
                bucket = head bucketsEnd
                newBucket = bucket ++ [y]
            in radixsort'' ys (bucketsBegin ++ [newBucket] ++ (tail bucketsEnd)) pos
    in radixsort' xs maxPos

-- get an random array given an seed
getRandIntArray :: Int -> [Int] 
getRandIntArray seed = (randomRs (0, div (maxBound :: Int) 2) (mkStdGen seed))

main = do
        value <- (\x -> return x ) (length (radixsort (take 10000 (getRandIntArray 0))))
        print value

Answer 1

主要问题是你的函数radixsort''，因为++是O（n），并且每次作为第一个参数给出的列表都会复制。

pack (-1) r' _ = r'
pack n  r' relems =
    let getn = (map snd) . (filter ((n==) . fst))
    in pack (n - 1) ((getn relems):r') relems
radixsort'' elems pos = 
    let digit = \y -> div (mod y (10 ^ (pos + 1))) (10 ^ pos)
        relems = zip (map digit elems) elems
    in pack 9 [] relems

优化Haskell中的基数排序

1 个答案: