Kotlin List tail function

user_4759312 0
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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?

kotlin-extension kotlin
4 Answers

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())
user_9511550

If you need both head and tail (which is a basic sequence operation) and don't eagerly consumed stream, you might want to have a sequence.

fun <T> Sequence<T>.headTail() =
    HeadTailSequence(this)
        .let { it.head to it.iterator.asSequence() }

private class HeadTailSequence<T>(parent: Sequence<T>) : Sequence<T> {
    val iterator = parent.iterator()
    val head: T? = iterator.next()

    override fun iterator(): Iterator<T> {
        return object : Iterator<T> {
            override fun hasNext(): Boolean = iterator.hasNext()
            override fun next(): T = if (iterator.hasNext()) throw NoSuchElementException() else iterator.next()
        }
    }
}

An extention for Iterable can be done in a similar manner.

user_7369220
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