99 questions/Solutions/16
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(Difference between revisions)
(a slight variation using guards) |
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</haskell> | </haskell> | ||
| - | + | An alternative iterative solution: | |
| + | |||
<haskell> | <haskell> | ||
dropEvery :: [a] -> Int -> [a] | dropEvery :: [a] -> Int -> [a] | ||
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</haskell> | </haskell> | ||
| - | + | Yet another iterative solution which divides lists using Prelude: | |
| + | |||
<haskell> | <haskell> | ||
dropEvery :: [a] -> Int -> [a] | dropEvery :: [a] -> Int -> [a] | ||
| Line 29: | Line 31: | ||
</haskell> | </haskell> | ||
| - | + | A similar approach using guards: | |
| + | |||
| + | <haskell> | ||
| + | dropEvery :: [a] -> Int -> [a] | ||
| + | dropEvery xs n | ||
| + | | length xs < n = xs | ||
| + | | otherwise = take (n-1) xs ++ dropEvery (drop n xs) n | ||
| + | </haskell> | ||
| + | |||
| + | Using zip: | ||
| + | |||
<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> | </haskell> | ||
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])
