# 99 questions/Solutions/9

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(**) Pack consecutive duplicates of list elements into sublists. |
(**) Pack consecutive duplicates of list elements into sublists. |
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+ | |||

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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pack [] = [] |
pack [] = [] |
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</haskell> |
</haskell> |
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+ | |||

+ | This is implemented as <hask>group</hask> in <hask>Data.List</hask>. |
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A more verbose solution is |
A more verbose solution is |
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+ | |||

<haskell> |
<haskell> |
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pack :: Eq a => [a] -> [[a]] |
pack :: Eq a => [a] -> [[a]] |
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</haskell> |
</haskell> |
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− | This is implemented as <hask>group</hask> in <hask>Data.List</hask>. |
+ | Similarly, using <hask>splitAt</hask> and <hask>findIndex</hask>: |

+ | |||

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

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

+ | <haskell> |
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+ | pack :: (Eq a) => [a] -> [[a]] |
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+ | pack [] = [] |
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+ | pack (x:xs) = (x : takeWhile (==x) xs) : pack (dropWhile (==x) xs) |
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+ | </haskell> |
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+ | |||

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

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

+ | A simple solution: |
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+ | <haskell> |
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+ | pack :: (Eq a) => [a] -> [[a]] |
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+ | pack [] = [] |
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+ | pack [x] = [[x]] |
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+ | pack (x:xs) = if x `elem` (head (pack xs)) |
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+ | then (x:(head (pack xs))):(tail (pack xs)) |
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+ | else [x]:(pack xs) |
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+ | |||

+ | pack' [] = [] |
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+ | pack' [x] = [[x]] |
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+ | pack' (x:xs) |
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+ | | x == head h_p_xs = (x:h_p_xs):t_p_hs |
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+ | | otherwise = [x]:p_xs |
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+ | where p_xs@(h_p_xs:t_p_hs) = pack' xs |
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+ | </haskell> |
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+ | |||

+ | <haskell> |
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+ | myPack [] = [] |
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+ | myPack (y:ys) = impl ys [[y]] |
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+ | where |
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+ | impl [] packed = packed |
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+ | impl (x:xs) packed |
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+ | | x == (head (last packed)) = impl xs ((init packed) ++ [x:(last packed)]) |
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+ | | otherwise = impl xs (packed ++ [[x]]) |
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+ | |||

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

## Latest 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)