# 99 questions/Solutions/9

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Line 8: | Line 8: | ||

pack [] = [] | pack [] = [] | ||

</haskell> | </haskell> | ||

+ | |||

+ | This is implemented as <hask>group</hask> in <hask>Data.List</hask>. | ||

A more verbose solution is | A more verbose solution is | ||

Line 23: | Line 25: | ||

</haskell> | </haskell> | ||

− | + | Similarly, using <hask>splitAt</hask> and <hask>findIndex</hask>: | |

− | Another solution using <hask>takeWhile</hask> and <hask>dropWhile</hask>: | + | <haskell> |

+ | pack :: Eq a => [a] -> [[a]] | ||

+ | pack [] = [] | ||

+ | pack (x:xs) = (x:reps) : (pack rest) | ||

+ | where | ||

+ | (reps, rest) = maybe (xs,[]) (\i -> splitAt i xs) | ||

+ | (findIndex (/=x) xs) | ||

+ | </haskell> | ||

+ | |||

+ | Another solution using <hask>takeWhile</hask> and <hask>dropWhile</hask>: | ||

<haskell> | <haskell> | ||

Line 31: | Line 42: | ||

pack [] = [] | pack [] = [] | ||

pack (x:xs) = (x : takeWhile (==x) xs) : pack (dropWhile (==x) xs) | pack (x:xs) = (x : takeWhile (==x) xs) : pack (dropWhile (==x) xs) | ||

+ | </haskell> | ||

+ | |||

+ | Or we can use <hask>foldr</hask> to implement this: | ||

+ | |||

+ | <haskell> | ||

+ | pack :: (Eq a) => [a] -> [[a]] | ||

+ | pack = foldr func [] | ||

+ | where func x [] = [[x]] | ||

+ | func x (y:xs) = | ||

+ | if x == (head y) then ((x:y):xs) else ([x]:y:xs) | ||

+ | </haskell> | ||

+ | |||

+ | A simple solution: | ||

+ | <haskell> | ||

+ | pack :: (Eq a) => [a] -> [[a]] | ||

+ | pack [] = [] | ||

+ | pack [x] = [[x]] | ||

+ | pack (x:xs) = if x `elem` (head (pack xs)) | ||

+ | then (x:(head (pack xs))):(tail (pack xs)) | ||

+ | else [x]:(pack xs) | ||

+ | |||

+ | pack' [] = [] | ||

+ | pack' [x] = [[x]] | ||

+ | pack' (x:xs) | ||

+ | | x == head h_p_xs = (x:h_p_xs):t_p_hs | ||

+ | | otherwise = [x]:p_xs | ||

+ | where p_xs@(h_p_xs:t_p_hs) = pack' xs | ||

+ | </haskell> | ||

+ | |||

+ | <haskell> | ||

+ | myPack [] = [] | ||

+ | myPack (y:ys) = impl ys [[y]] | ||

+ | where | ||

+ | impl [] packed = packed | ||

+ | impl (x:xs) packed | ||

+ | | x == (head (last packed)) = impl xs ((init packed) ++ [x:(last packed)]) | ||

+ | | otherwise = impl xs (packed ++ [[x]]) | ||

+ | |||

+ | myPack' [] = [] | ||

+ | myPack' (y:ys) = reverse $ impl ys [[y]] | ||

+ | where | ||

+ | impl [] packed = packed | ||

+ | impl (x:xs) p@(z:zs) | ||

+ | | x == (head z) = impl xs ((x:z):zs) | ||

+ | | otherwise = impl xs ([x]:p) | ||

</haskell> | </haskell> |

## Revision as of 13:33, 24 June 2012

(**) 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 [] = []

group

Data.List

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

splitAt

findIndex

pack :: Eq a => [a] -> [[a]] pack [] = [] pack (x:xs) = (x:reps) : (pack rest) where (reps, rest) = maybe (xs,[]) (\i -> splitAt i xs) (findIndex (/=x) xs)

takeWhile

dropWhile

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

foldr

pack :: (Eq a) => [a] -> [[a]] pack = foldr func [] where func x [] = [[x]] func x (y:xs) = if x == (head y) then ((x:y):xs) else ([x]:y:xs)

A simple solution:

pack :: (Eq a) => [a] -> [[a]] pack [] = [] pack [x] = [[x]] pack (x:xs) = if x `elem` (head (pack xs)) then (x:(head (pack xs))):(tail (pack xs)) else [x]:(pack xs) pack' [] = [] pack' [x] = [[x]] pack' (x:xs) | x == head h_p_xs = (x:h_p_xs):t_p_hs | otherwise = [x]:p_xs where p_xs@(h_p_xs:t_p_hs) = pack' xs

myPack [] = [] myPack (y:ys) = impl ys [[y]] where impl [] packed = packed impl (x:xs) packed | x == (head (last packed)) = impl xs ((init packed) ++ [x:(last packed)]) | otherwise = impl xs (packed ++ [[x]]) myPack' [] = [] myPack' (y:ys) = reverse $ impl ys [[y]] where impl [] packed = packed impl (x:xs) p@(z:zs) | x == (head z) = impl xs ((x:z):zs) | otherwise = impl xs ([x]:p)