Problem 21 Solutions

Solution 1

Use split from Problem 17.

split : List a -> Int -> ( List a, List a )
split list count =
    ( List.take count list, List.drop count list )


insertAt : Int -> a -> List a -> List a
insertAt n v xs =
    let
        ( front, back ) =
            split xs (n - 1)
    in
        front ++ (v :: back)

Solution 2

Do the split internally with List.drop and List.take

insertAt : Int -> a -> List a -> List a
insertAt n v xs =
    List.take (n - 1) xs ++ (v :: List.drop (n - 1) xs)

Solution 3

Is there anything you can't do with recursion?

insertAt : Int -> a -> List a -> List a
insertAt n v xs =
    if n < 2 then
        v :: xs
    else
        case xs of
            [] ->
                [ v ]

            y :: ys ->
                y :: insertAt (n - 1) v ys

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