# 99 questions/Solutions/11

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Revision as of 19:31, 18 January 2014 by Henk-Jan van Tuyl (talk | contribs) (Added Category:Programming exercise spoilers)

(*) 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
where
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.