Haskell 数组条件混乱

Question

如何输出第 4 天（第 4 个索引）温度高于 5 的城市名称？我想不通条件。我尝试了几件事，但没有任何效果。我可以获得项目 5 的索引，但我不知道如何将其应用于项目数组。我不能像在 Java.

中那样做循环

data City = City { cityName :: String
                 , temperature :: [Double] 
                 }

city1 = City {cityName = "city1", temperature = [4.50,6.50,5.0,6.48,8.54]}
city2 = City {cityName = "city2", temperature = [6.35,5.12,3.21,3.25,4.56]}
city3 = City {cityName = "city3", temperature = [7.3,5.32,4.4,4.6]}

cities :: [City]
cities = [city1,city2,city3]

getTemperatures (City _ _ temperature) = temperature

test :: City -> Double 
test x = (getTemperatures x)!!5

Answer 1

您可以使用 filter 和 map（我在这里使用第三个索引（0-base））：

main = print $ map cityName $ filter ((>5) . (!!3) . temperature) cities -- ["city1"]

data City = City { cityName :: String, temperature :: [Double] }

city1 = City {cityName = "city1", temperature = [4.50,6.50,5.0,6.48,8.54]}
city2 = City {cityName = "city2", temperature = [6.35,5.12,3.21,3.25,4.56]}
city3 = City {cityName = "city3", temperature = [7.3,5.32,4.4,4.6]}

cities :: [City]
cities = [city1,city2,city3]

filter通过取温度部分（temperature）过滤（来自列表cities）元素，取第三个元素（!!3），然后将其与 5 (>5).

进行比较

map 给定 filter 的输出，然后采用名称部分 (cityName)。最后，print 打印城市名称列表。

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