# 99 questions/Solutions/16

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< 99 questions | Solutions(Difference between revisions)

(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> |
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dropEvery n = map snd . filter ((n/=) . fst) . zip (cycle [1..n]) |
dropEvery n = map snd . filter ((n/=) . fst) . zip (cycle [1..n]) |
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+ | </haskell> |
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+ | |||

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