# 99 questions/Solutions/16

(**) Drop every N'th element from a list.

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery (x:xs) n = dropEvery' (x:xs) n 1 where
dropEvery' (x:xs) n i = (if (n `divides` i) then
[] else
[x])
++ (dropEvery' xs n (i+1))
dropEvery' [] _ _ = []
divides x y = y `mod` x == 0

An alternative iterative solution:

dropEvery :: [a] -> Int -> [a]
dropEvery list count = helper list count count
where helper [] _ _ = []
helper (x:xs) count 1 = helper xs count count
helper (x:xs) count n = x : (helper xs count (n - 1))

A similar iterative solution but using a closure:

dropEvery :: [a] -> Int -> [a]
dropEvery xs n = helper xs n
where helper [] _ = []
helper (x:xs) 1 = helper xs n
helper (x:xs) k = x : helper xs (k-1)

Yet another iterative solution which divides lists using Prelude:

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery list count = (take (count-1) list) ++ dropEvery (drop count list) count

A similar approach using guards:

dropEvery :: [a] -> Int -> [a]
dropEvery xs n
| length xs < n = xs
| otherwise     = take (n-1) xs ++ dropEvery (drop n xs) n

Using zip:

dropEvery = flip \$ \n -> map snd . filter ((n/=) . fst) . zip (cycle [1..n])

Using zip and list comprehensions

dropEvery :: [a] -> Int -> [a]
dropEvery xs n = [ i | (i,c) <- ( zip xs [1,2..]), (mod c n) /= 0]

A more complicated approach which first divides the input list into sublists that do not contain the nth element, and then concatenates the sublists to a result list (if not apparent: the author's a novice):

dropEvery :: [a] -> Int -> [a]
dropEvery [] _ = []
dropEvery xs n = concat (split n xs)
where
split _ [] = []
split n xs = fst splitted : split n ((safetail . snd) splitted)
where
splitted = splitAt (n-1) xs
safetail xs | null xs = []
| otherwise = tail xs

First thing that came to mind:

dropEvery xs n = map fst \$ filter (\(x,i) -> i `mod` n /= 0) \$ zip xs [1..]

The filter function can be simplified as seen above:

dropEvery xs n = map fst \$ filter ((n/=) . snd) \$ zip xs [1..]