# 99 questions/Solutions/16

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< 99 questions | Solutions(Difference between revisions)

(a slight variation using guards) |
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− | or an alternative iterative solution: |
+ | An alternative iterative solution: |

+ | |||

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dropEvery :: [a] -> Int -> [a] |
dropEvery :: [a] -> Int -> [a] |
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− | or yet another iterative solution which divides lists using Prelude: |
+ | Yet another iterative solution which divides lists using Prelude: |

+ | |||

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dropEvery :: [a] -> Int -> [a] |
dropEvery :: [a] -> Int -> [a] |
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− | or using zip: |
+ | A similar approach using guards: |

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

+ | Using zip: |
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+ | |||

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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]) |

## Revision as of 20:01, 15 July 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])