Question

我是Haskell的新手，正在尝试计算浮点列表的标准偏差。 Ubuntu 18.04，ghc 8.0.2。我遇到了以下错误，但我一直在谷歌上搜索，但仍然不明白“ Integral Float”到底是什么。

*Main> let z = stdDev 0 1 y x
<interactive>:250:9: error:
• Could not deduce (Integral Float) arising from a use of ‘stdDev’
  from the context: Floating a
    bound by the inferred type of z :: Floating a => [a]
    at <interactive>:250:5-38
• In the expression: stdDev 0 (length (head (x))) y x
  In an equation for ‘z’: z = stdDev 0 (length (head (x))) y x

代码：

-- i is start index, l is length of each list, ms is list of means, 
--    xs is Matrix
stdDev i l ms xs
     | i < l     = sqrt(fromIntegral(sumOfMinusMeans i (ms!!i) xs) / 
                             fromIntegral(l)):(stdDev (i+1) l ms xs)
     | otherwise = []

--i is index, m is mean for the index
sumOfMinusMeans i m (x:xs)
     | xs == []     = (x!!i - m)**2
     | i < length x = (x!!i - m)**2 + (sumOfMinusMeans i m xs)
     | otherwise    = 0

类型：

*Main> :t stdDev
stdDev
  :: (Floating a1, Floating a, Integral a1) =>
     Int -> Int -> [a1] -> [[a1]] -> [a]

*Main> :t sumOfMinusMeans
sumOfMinusMeans :: (Eq t, Floating t) => Int -> t -> [[t]] -> t

变量：

*Main> y
[380.0,1.0]
*Main> x
[[600.0,1.0],[400.0,1.0],[170.0,1.0],[430.0,1.0],[300.0,1.0]]

Answer 1

我找到了解决方案。我需要指定函数类型并添加一个“ fromIntegral”，而我的某些算法逻辑是错误的。

stdDev :: Int -> Float -> [Float] -> [[Float]] -> [Float]
stdDev i l ms xs
    | fromIntegral(i) < length(head(xs)) = sqrt((sumOfMinusMeans i (ms!!i) xs) 
                                               / l): (stdDev (i+1) l ms xs)
    | otherwise                          = []


sumOfMinusMeans :: Int -> Float -> [[Float]] -> Float
sumOfMinusMeans i m (x:xs)
    | xs == []  = (x!!i - m)**2
    | otherwise = (x!!i - m)**2 + (sumOfMinusMeans i m xs)

Haskell错误“无法推断出由于使用..而引起的（整数浮点数）。”

1 个答案: