问题描述
我正在尝试制作一个函数来计算(然后作为 String
输出)列表中两个元素之间的差异 - 第 1 个和第 2 个,然后是第 2 个和第 3 个,依此类推 - 我想我试一试,但我目前一直遇到错误 whack-a-mole,我把当前的错误放在下面,但首先,强制性的代码转储:
type Name = String
type Coordinates = (Int,Int)
type Pop = Int
type TotalPop = [Pop]
type City = (Name,(Coordinates,TotalPop))
testData :: [City]
testData = [("New York City",((1,1),[5,4,3,2])),("Washingotn DC",((3,3),[3,2,1,1])),("Los Angeles",((2,2),[7,7,5]))]
getPopGrowth :: [City] -> Name -> String
getPopGrowth cs name = concat
[getPercentages z ++ "\n" | (x,z) <- maybeToList (lookup name cs)] where
getPercentages z = unwords (map show z1) ++ "% " where
z1 = percentageIncrease z
percentageIncrease :: [Int] -> [Float]
percentageIncrease (x:xs)
| length (x:xs) > 2 = percentageIncrease (tail xs)
| otherwise = (a / b - 1) * 100.0 where
a = fromIntegral x :: Float
b = fromIntegral (head xs) :: Float
我现在得到的错误是:
error:
• Couldn't match expected type ‘[Float]’ with actual type ‘Float’
• In the expression: (a / b - 1) * 100.0
In an equation for ‘percentageIncrease’:
percentageIncrease (x : xs)
| length (x : xs) > 2 = percentageIncrease (tail xs)
| otherwise = (a / b - 1) * 100.0
where
a = fromIntegral x :: Float
b = fromIntegral (head xs) :: Float
|
92 | | otherwise = (a / b - 1) * 100.0 where
| ^^^^^^^^^^^^^^^^^^^
我想强调,我理解错误,但我不知道如何解决它以获得所需的函数结果。
只是为了澄清我正在尝试做的事情。
输入:getPopGrowth testData "New York City"
应该输出:25% 33.333% 50%
解决方法
到目前为止,您只计算列表仅剩两个元素时的百分比。较少的元素没有被覆盖,对于较长的列表,在之前的所有步骤中,元素被删除而无需进一步操作。但是,在最后一步中,您返回的是一个 Float 而不是列表。
以下示例在每个步骤中创建一个增加百分比,将其与将函数应用于列表尾部所产生的列表连接起来。此外,所有基本情况都已涵盖:
percentageIncrease :: [Int] -> [Float]
percentageIncrease [] = []
percentageIncrease (x:[]) = []
percentageIncrease (x:y:xs) = ((a / b - 1) * 100.0) : percentageIncrease (y:xs) where
a = fromIntegral x :: Float
b = fromIntegral y :: Float
控制台输出:
*Main> getPopGrowth testData "New York City"
"25.0 33.333336 50.0% \n"