inline fun CharSequence.reduceIndexedOrNull(
    operation: (index: Int, acc: Char, Char) -> Char
): Char?

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

Returns null if the char 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 a character, current accumulator value and the character itself, and calculates the next accumulator value.