# 99 questions/Solutions/9

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< 99 questions | Solutions(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 xs

group

Data.List

takeWhile

dropWhile

pack :: (Eq a) => [a] -> [[a]] pack [] = [] pack (x:xs) = (x : takeWhile (==x) xs) : pack (dropWhile (==x) xs)