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encode xs = map (length &&& head) $ group xs
 
encode xs = map (length &&& head) $ group xs
 
</haskell>
 
</haskell>
  +
  +
Or using the slightly more verbose (w.r.t. <hask>(&&&)</hask>) Applicative combinators:
  +
  +
<haskell>
  +
encode :: Eq a => [a] -> [(Int, a)]
  +
encode = map ((,) <$> length <*> head) . pack
  +
</haskell>
  +
  +
Or with the help of foldr (''pack'' is the resulting function from P09):
  +
  +
<haskell>
  +
encode xs = (enc . pack) xs
  +
where enc = foldr (\x acc -> (length x, head x) : acc) []
  +
</haskell>
  +
  +
Or using takeWhile and dropWhile:
  +
  +
<haskell>
  +
encode [] = []
  +
encode (x:xs) = (length $ x : takeWhile (==x) xs, x)
  +
: encode (dropWhile (==x) xs)
  +
</haskell>
  +
  +
Or without higher order functions:
  +
  +
<haskell>
  +
encode [] = []
  +
encode (x:xs) = encode' 1 x xs where
  +
encode' n x [] = [(n, x)]
  +
encode' n x (y:ys)
  +
| x == y = encode' (n + 1) x ys
  +
| otherwise = (n, x) : encode' 1 y ys
  +
</haskell>
  +
  +
Or we can make use of zip and group:
  +
  +
<haskell>
  +
import List
  +
encode :: Eq a => [a] -> [(Int, a)]
  +
encode xs=zip (map length l) h where
  +
l = (group xs)
  +
h = map head l
  +
</haskell>
  +
  +
[[Category:Programming exercise spoilers]]

Latest revision as of 19:31, 18 January 2014

(*) Run-length encoding of a list.

Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists (N E) where N is the number of duplicates of the element E.

encode xs = map (\x -> (length x,head x)) (group xs)

which can also be expressed as a list comprehension:

[(length x, head x) | x <- group xs]

Or writing it Pointfree (Note that the type signature is essential here to avoid hitting the Monomorphism Restriction):

encode :: Eq a => [a] -> [(Int, a)]
encode = map (\x -> (length x, head x)) . group

Or (ab)using the "&&&" arrow operator for tuples:

encode :: Eq a => [a] -> [(Int, a)]
encode xs = map (length &&& head) $ group xs
Or using the slightly more verbose (w.r.t.
(&&&)
) Applicative combinators:
encode :: Eq a => [a] -> [(Int, a)]
encode = map ((,) <$> length <*> head) . pack

Or with the help of foldr (pack is the resulting function from P09):

encode xs = (enc . pack) xs
	where enc = foldr (\x acc -> (length x, head x) : acc) []

Or using takeWhile and dropWhile:

encode [] = []
encode (x:xs) = (length $ x : takeWhile (==x) xs, x)
                 : encode (dropWhile (==x) xs)

Or without higher order functions:

encode []     = []
encode (x:xs) = encode' 1 x xs where
    encode' n x [] = [(n, x)]
    encode' n x (y:ys)
        | x == y    = encode' (n + 1) x ys
        | otherwise = (n, x) : encode' 1 y ys

Or we can make use of zip and group:

 
import List
encode :: Eq a => [a] -> [(Int, a)]
encode xs=zip (map length l) h where 
    l = (group xs)
    h = map head l