输入与分数相关的错误

Question

我想弄清楚什么是错的.似乎类型有问题,但单独使用相同的表达式可以正常工作.

这是代码:

a = [9, 4, 12, 0, -6, 16] :: [Int]

qsort:: [Int] -> [Int] 
qsort [] = []
qsort [x] = [x]
qsort xs = (qsort l)++(qsort r)
    where   m = (realToFrac(sum xs)) / (realToFrac(length xs))
            l = filter (<=m) xs
            r = filter (>m) xs

main::IO()
main = do
        print (show (qsort a))

它抛出:

main.hs:7:36:
    No instance for (Fractional Int)
      arising from a use of `/'
    Possible fix: add an instance declaration for (Fractional Int)
    In the expression: (realToFrac (sum xs)) / (realToFrac (length xs))
    In an equation for `m':
        m = realToFrac (sum xs) / (realToFrac (length xs))
    In an equation for `qsort':
        qsort xs
          = (qsort l) ++ (qsort r)
          where
              m = realToFrac (sum xs) / (realToFrac (length xs))
              l = filter (<= m) xs
              r = filter (> m) xs

但这很有效:

a = [9, 4, 12, 0, -6, 16] :: [Int]
main::IO()
main = do
    print (show (   (realToFrac(sum a)) / (realToFrac(length a))   ))

Answer 1

你的m定义给出了一些类型Fractional.但是你比较m于Int在定义l和r其迷惑编译器.以下将做到这一点:

a = [9, 4, 12, 0, -6, 16] :: [Int]

qsort:: [Int] -> [Int]
qsort [] = []
qsort [x] = [x]
qsort xs = (qsort l)++(qsort r)
    where   m = realToFrac(sum xs) / (realToFrac(length xs))
            l = filter ((<=m) . fromIntegral) xs
            r = filter ((>m) . fromIntegral) xs

main::IO()
main = do
        print (show (qsort a))

Answer 2

realToFrac(sum xs) / (realToFrac(length xs))确实有效,但其结果是一个小数.所以在你的代码中m是一个小数.但是你要比较m列表中的项目,即Ints.由于您只能比较相同类型的值而Ints不是小数,因此会导致类型错误.

要解决您的问题,您可以使用整数除法(div)或将列表中的整数转换为小数,然后再与它们进行比较m.