在列表中查找元素并返回字符串 - haskell

Question

Hi I want to display the names of the dogs that have 0 mins walked for the their second day

type Dogs = (String, Float, Float, [Int]) -- Dog,Age,Weight,Time walked daily


testData :: [Dogs]
testData = [("Morris", 5, 8, [0,0,5,8,8,0,0]), 
        ("Tinks", 3, 10, [12,8,15,0,0,0,2]), 
        ("Cash", 6, 11, [0,6,5,0,0,0,3]), 
        ("Lou", 5, 17, [0,2,10,7,8,2,2]),
        ("Kobi", 10, 12, [0,0,8,3,6,7,5]) 
        ("Nala", 7, 8, [8,16,20,3,4,9,2])] 

So the output would be Morris and Kobi as they have 0 second in the list but I don't know how to go about this, I'm quite new to haskell. I understand that I should use !! 1 !! 1 to get the second element out of a list but I don't know how to do this for multiple lists within tuples within a list and then output as string for the names. Any help would be appreciated thanks

Answer 1

First off, you can create a separate function for determining whether the condition you are looking for is met:

isDay2Walked :: Dogs -> Bool
isDay2Walked (_, _, _, (_:x:_)) = x /= 0 -- pattern matching would be more elegant than (!! 1)
isDay2Walked _ = False -- return something if there is no information about the second day

Then, use this predicate in a filter over your list, and extract the first element - the name:

getDogName :: Dogs -> String
getDogName (name, _, _, _) = name

getDogsNotWalkedOnDay2 :: [Dogs] -> [String]
getDogsNotWalkedOnDay2 = map getDogName . filter (not . isDay2Walked)