inline fun <S, T : S> Sequence<T>.reduceIndexedOrNull(
    operation: (index: Int, acc: S, T) -> S
): S?

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element with its index in the original sequence.

Returns null if the sequence is empty.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val strings = listOf("a", "b", "c", "d")
println(strings.reduceOrNull { acc, string -> acc + string }) // abcd
println(strings.reduceIndexedOrNull { index, acc, string -> acc + string + index }) // ab1c2d3

println(emptyList<String>().reduceOrNull { _, _ -> "" }) // null
//sampleEnd
}

Parameters

operation -

function that takes the index of an element, current accumulator value and the element itself, and calculates the next accumulator value.

The operation is terminal.