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(a slight variation using guards)
(Added a new (complicated, maybe ugly) solution. This was my first approach, but after getting to know take and drop, I came up with the same solution as the 3rd one. I thought I may post it anyway)
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<haskell>
 
<haskell>
 
dropEvery n = map snd . filter ((n/=) . fst) . zip (cycle [1..n])
 
dropEvery n = map snd . filter ((n/=) . fst) . zip (cycle [1..n])
  +
</haskell>
  +
  +
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):
  +
<haskell>
  +
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
 
</haskell>
 
</haskell>

Revision as of 05:43, 18 August 2010

(**) 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))

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 n = map snd . filter ((n/=) . fst) . zip (cycle [1..n])

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