Github: FilterPositionsInList.hs We will now manipulate Lists by filtering through and returning only the elements that occur in the odd positions.

We declare our function type which takes in a List as an argument and returns a List.

type FilterPos = [Int] -> [Int]

Using List Comprehension

f :: FilterPos
f lst = [lst!!n | n<-[0..length lst - 1], odd n]

f [2,4,6,8,10,12,14]

[4,8,12]

Using Recursion

f2 :: FilterPos
f2 (_:x:xs) = x : f2 xs
f2 _ = []

f2 [2,4,6,8,10,12,14]

[4,8,12]

Using Higher Order Functions (map, filter, zip)

f3 :: FilterPos
f3 = map snd . filter (odd . fst) . zip [0..]

f3 [2,4,6,8,10,12,14]

[4,8,12]