Difference between revisions of "99 questions/Solutions/12"
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(Added another alternative one-liner solution using foldl.) |
m (I don't think "ugly code ahead" was necessary.) |
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− | <hask>foldl</hask> can also be used to solve this problem |
+ | <hask>foldl</hask> can also be used to solve this problem: |
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Revision as of 04:35, 10 August 2011
(**) Decode a run-length encoded list.
Given a run-length code list generated as specified in problem 11. Construct its uncompressed version.
decodeModified :: [ListItem a] -> [a]
decodeModified = concatMap decodeHelper
where
decodeHelper (Single x) = [x]
decodeHelper (Multiple n x) = replicate n x
We only need to map single instances of an element to a list containing only one element and multiple ones to a list containing the specified number of elements and concatenate these lists.
A solution for the simpler encoding from problem 10 can be given as:
decode :: [(Int, a)] -> [a]
decode = concatMap (uncurry replicate)
This can be easily extended given a helper function:
toTuple :: ListItem a -> (Int, a)
toTuple (Single x) = (1, x)
toTuple (Multiple n x) = (n, x)
as:
decode :: [ListItem a] -> [a]
decode = concatMap (uncurry replicate . toTuple)
foldl
can also be used to solve this problem:
decode :: [ListItem a] -> [a]
decode = foldl (\acc e -> case e of Single x -> acc ++ [x]; Multiple n x -> acc ++ replicate n x) []