99 questions/Solutions/11

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(*) Modified run-length encoding.

Modify the result of problem 10 in such a way that if an element has no duplicates it is simply copied into the result list. Only elements with duplicates are transferred as (N E) lists.

data ListItem a = Single a | Multiple Int a
    deriving (Show)

encodeModified :: Eq a => [a] -> [ListItem a]
encodeModified = map encodeHelper . encode
      encodeHelper (1,x) = Single x
      encodeHelper (n,x) = Multiple n x

Again, like in problem 7, we need a utility type because lists in haskell are homogeneous. Afterwards we use the encode function from problem 10 and map single instances of a list item to Single and multiple ones to Multiple.

The ListItem definition contains 'deriving (Show)' so that we can get interactive output.

This problem could also be solved using a list comprehension like so:

encodeModified xs = [y | x <- group xs, let y = if (length x) == 1 then Single (head x) else Multiple (length x) (head x)]

In this case, ListItem type can be used from the above solution and group can be found in Data.List module.