Kotlin List tail function

Question

I am trying to find a tail function in List<T> but I couldn't find any. I ended up doing this.

fun <T> List<T>.tail() = this.takeLast(this.size -1)

Is there a better way to do this?

Answer 1

Yours and @Vladimir Mironov 's solutions will work, but they automatically create eager copies of the original list (sans the first element), which can take a very long time for larger lists. I would define it with a wrapper List class that delegates its methods to the wrapped one, ignoring the first element using index adjustments:

private class TailList<T> (private val list: List<T>) : List<T> {
    override val size: Int
        get() = list.size -1

    override fun isEmpty(): Boolean = size == 0

    override fun iterator(): Iterator<T> = listIterator()
    override fun listIterator(): ListIterator<T> = list.listIterator(1)
    override fun listIterator(index: Int): ListIterator<T> = list.listIterator(index + 1)
    override fun subList(fromIndex: Int, toIndex: Int): List<T> = list.subList(fromIndex + 1, toIndex + 1)
    override fun lastIndexOf(element: T): Int = list.lastIndexOf(element) - 1
    override operator fun get(index: Int): T = list[index + 1]

    // The following member functions require the copy of a new list
    override fun containsAll(elements: Collection<T>): Boolean = tailList.containsAll(elements)
    override fun contains(element: T): Boolean = tailList.contains(element)
    override fun indexOf(element: T): Int = tailList.indexOf(element)

    private val tailList by lazy { ArrayList(this) }  // makes a proper copy the elements this list represents
}

You may notice the functions in the section after the comment still end up making an eager copy. I only did this for simplicity's sake. For memory's sake, I made a lazy tailList property

They could all be implemented by iterating over the collection manually, rather than doing some sort of delegating. If that's what you'd prefer, I'm sure you can figure it out.

With that, the head and tail properties become this:

val <T> List<T>.tail: List<T> 
    get() =
        if(this.isEmpty())
            throw IllegalStateException("Cannot get the tail of an empty List")
        else
            TailList(this)

val <T> List<T>.head: T
    get() = this[0]  // or first()

If you really need it, I can add an update to make the last three member functions so that they don't make eager copies.

EDIT: Note: If you followed the conventions that Kotlin has followed so far, you wouldn't make the List's tail be lazy, like this, since all of their functions on List make eager copies. Instead, especially if you're using head and tail to recursively iterate over a list, I would see if you could attempt this wrapper idea on Sequence somehow. Sequence's whole point of existence is for lazy work on collections.

EDIT 2: Apparently sublist() creates a view, and is therefore already lazy. Essentially, I've just taught you how to create the implementation for sublist, except I narrowed it to only the tail.

So, in that case just use sublist() for your tail function.

Answer 2

If working with non mutable lists, it is perfectly safe, and less memory consuming, to use simply :

fun <T> List<T>.tail(): List<T> =
    if (isEmpty()) throw IllegalArgumentException("tail called on empty list")
    else subList(1, count())

Answer 3

Kotlin doesn't have a built-in List<T>.tail() function, so implementing your own extension function is the only way. Although your implementation is perfectly fine, it can be simplified a bit:

fun <T> List<T>.tail() = drop(1)

Or, instead of extension function, you can define an extension property:

val <T> List<T>.tail: List<T>
  get() = drop(1)

val <T> List<T>.head: T
  get() = first()

And then use it like:

val list = listOf("1", "2", "3")
val head = list.head
val tail = list.tail