PhyrexTsai/Function composition.md

## Function composition.md

      
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              Function composition.md
            
          
    Function composition

Use the functions, remdup, elemOcc, and map to define a function, occurrences, that receives a list of characters and returns a list of pairs of an element and the number of its occurrences in the input list. For example:
occurrences [‘a’, ‘c’, ‘d’, ‘a’, ‘c’] = [(‘a’, 2), (‘c’, 2), (‘d’, 1)]
occurrences :: [Char] -> [(Char,Int)]
occurrences xs = zip (??) (??)
remdup :: [Char]->[Char]
remdup [] = []
remdup (x:xs) = x : remdup (…)
elemOcc :: Char-> [Char]->Int
elemOcc x = length . (filter ??)


## occurrences.hs
remdup :: [Char]->[Char]
remdup [] = []
remdup (x:xs) = x : remdup (filter (/=x) (x:xs))

elemOcc :: Char->[Char]->Int
elemOcc k = length . filter (==k)

occurrences :: [Char] -> [(Char, Int)]
occurrences xs = zip [x | x <- remdup xs] [elemOcc x xs | x <- remdup xs]

main =  do
    print(occurrences ['a', 'b', 'c', 'a', 'c'])
	remdup :: [Char]->[Char]
	remdup [] = []
	remdup (x:xs) = x : remdup (filter (/=x) (x:xs))

	elemOcc :: Char->[Char]->Int
	elemOcc k = length . filter (==k)

	occurrences :: [Char] -> [(Char, Int)]
	occurrences xs = zip [x \| x <- remdup xs] [elemOcc x xs \| x <- remdup xs]

	main = do
	print(occurrences ['a', 'b', 'c', 'a', 'c'])