Problem 10

Run-length encode a list of list to a list of tuples. Unlike lists, tuples can mix types. Use tuples (n, e) to encode a list where n is the number of duplicates of the element e.

Example

tuples = 
    [(3, 'a')
    , (1, 'b')
    , (3, 'c')
    , (4, 'd')
    , (5, 'e')
    , (6,'f')]

lists =
    [ ['a','a','a']
    , ['b']
    , ['c', 'c', 'c']
    , ['d','d','d','d']
    , ['e']
    , ['f', 'f']
    ]

runLengths lists == tuples

Unit Test

import Html
import List
import Maybe


runLengths : List (List a) -> List ( Int, a )
runLengths xss =
    -- your implementation here
    []


main : Html.Html a
main =
    Html.text
        <| case test of
            0 ->
                "Your implementation passed all tests."

            1 ->
                "Your implementation failed one test."

            x ->
                "Your implementation failed " ++ (toString x) ++ " tests."


test : Int
test =
    List.length
        <| List.filter ((==) False)
            [ runLengths [ [ 1, 1, 1, 1 ], [ 2 ], [ 5, 5 ], [ 2 ], [ 1 ] ]
                == [ ( 4, 1 ), ( 1, 2 ), ( 2, 5 ), ( 1, 2 ), ( 1, 1 ) ]
            , runLengths [ [ 2 ], [ 5, 5 ], [ 2 ], [ 1 ] ]
                == [ ( 1, 2 ), ( 2, 5 ), ( 1, 2 ), ( 1, 1 ) ]
            , runLengths [ [ 1, 1, 1, 1 ], [ 2 ], [ 5, 5 ] ]
                == [ ( 4, 1 ), ( 1, 2 ), ( 2, 5 ) ]
            , runLengths [ [ 1, 1, 1, 1 ] ]
                == [ ( 4, 1 ) ]
            , runLengths [ [ "a", "a", "a", "a" ], [ "b" ], [ "c", "c" ], [ "b" ], [ "a" ] ]
                == [ ( 4, "a" ), ( 1, "b" ), ( 2, "c" ), ( 1, "b" ), ( 1, "a" ) ]
            , runLengths [ [] ] == []
            , runLengths [ [], [ "a", "a" ] ] == [ ( 2, "a") ]
            , runLengths [] == []
            ]

Hints

1. What's a function to convert a list of one type to a list of another type?

Solutions

Solutions