99 questions/Solutions/9: Difference between revisions
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(**) Pack consecutive duplicates of list elements into sublists. | (**) Pack consecutive duplicates of list elements into sublists. | ||
If a list contains repeated elements they should be placed in separate sublists. | If a list contains repeated elements they should be placed in separate sublists. | ||
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A more verbose solution is | A more verbose solution is | ||
<haskell> | <haskell> | ||
pack :: Eq a => [a] -> [[a]] | pack :: Eq a => [a] -> [[a]] | ||
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This is implemented as <hask>group</hask> in <hask>Data.List</hask>. | This is implemented as <hask>group</hask> in <hask>Data.List</hask>. | ||
Another solution using <hask>takeWhile</hask> and <hask>dropWhile</hask>: | |||
<haskell> | |||
pack :: (Eq a) => [a] -> [[a]] | |||
pack [] = [] | |||
pack (x:xs) = (x : takeWhile (==x) xs) : pack (dropWhile (==x) xs) | |||
</haskell> | |||
Revision as of 03:00, 14 July 2010
(**) Pack consecutive duplicates of list elements into sublists.
If a list contains repeated elements they should be placed in separate sublists.
pack (x:xs) = let (first,rest) = span (==x) xs
in (x:first) : pack rest
pack [] = []A more verbose solution is
pack :: Eq a => [a] -> [[a]]
pack [] = []
pack (x:xs) = (x:first) : pack rest
where
getReps [] = ([], [])
getReps (y:ys)
| y == x = let (f,r) = getReps ys in (y:f, r)
| otherwise = ([], (y:ys))
(first,rest) = getReps xsThis is implemented as group in Data.List.
Another solution using takeWhile and dropWhile:
pack :: (Eq a) => [a] -> [[a]]
pack [] = []
pack (x:xs) = (x : takeWhile (==x) xs) : pack (dropWhile (==x) xs)