Haskell array conditions confusion-CodePudding

How do I output city names that had temps higher than 5 on the 4th day (4th index)? I can't figure out the conditions. I've tried a couple of things, but nothing worked. I can get the index of item 5, but I don't know how to apply that to the array of items. I can't just do loops like I would in Java.

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

CodePudding user response：

You can use filter and map (I am using 3rd index (0-base) here):

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]

The filter filters (from the list cities) elements by taking the temperature part (temperature), take the third element (!!3), and then compare it with 5 (>5).

The map, given the output from filter, then takes the name part (cityName). Finally, print prints the list of city names.