CODEWITHSUNDEEP
haskell
user11574067
user11574067

Reputation:

Returning number of occurences with condition from list in Haskell

I want to filter number from list by condition that they have to be less than 3 and then how many elements like this were in starting list

filterLove :: (Num a, Ord a) => [a] -> Int
filterLove [42, 66, 15, 2] ~>* 1

I tried this filterLove filter (< 3) a = length a but it doesn't work. I can't figure out how would i do it. Thanks for help.

Upvotes: 0

Views: 45

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477641

filterLove is a function that maps a list of items so that means you define it as:

filterLove :: (Num a, Ord a) => [a] -> Int
filterLove xs = …

where xs is the list of numbers, and the … part is an expression that should have Int as type.

We thus create a new list with only the elements of xs that are smaller than three with filter (<3) xs where filter :: (a -> Bool) -> [a] -> [a] takes a predicate and a list and produces a list with all the elements that satisfy the predicate. Then we can determine the length of that list with length (filter (<3) xs) where length :: [a] -> Int will determine the length of the list:

filterLove :: (Num a, Ord a) => [a] -> Int
filterLove xs = length (filter (<3) xs)

we can remove the parameter xs with an η-reduction to:

filterLove :: (Num a, Ord a) => [a] -> Int
filterLove = length . filter (<3)

Upvotes: 1

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