CODEWITHSUNDEEP
haskell
Chalupa
Chalupa

Reputation: 367

getting every 3rd element & looping back (Haskell)

I want to write a function that returns every nth element of a given list but then updates the head to the second element and does this again until it has gone through all elements in the list. I know the code for going through every nth element in a list is:

everyf n [] = []
everyf n as  = head as : everyf n (drop n as)

but this does not loop back. How can I get this to update head so that i get the following result:

everyf 3 [1,2,3,4,5,6,7]
returns [[1,4,7],[2,5],[3,6]]

Upvotes: 1

Views: 259

Answers (1)

zerkms
zerkms

Reputation: 255095

Disclaimer: I don't know haskell (at all) :-)

listOfNths :: Int -> [a] -> [[a]]
listOfNths n xs = map (\x -> everyf n (drop x xs)) [0..n-1]

and a slightly "improved" version:

listOfNths :: Int -> [a] -> [[a]]
listOfNths n xs = map pickEveryf [0..n-1]
    where pickEveryf = (everyf n) . (`drop` xs)

Upvotes: 2

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