inline fun CharSequence.runningReduceIndexed(
    operation: (index: Int, acc: Char, Char) -> Char
): List<Char>

Returns a list containing successive accumulation values generated by applying operation from left to right to each character, its index in the original char sequence and current accumulator value that starts with the first character of this char sequence.

Note that acc value passed to operation function should not be mutated; otherwise it would affect the previous value in resulting list.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val strings = listOf("a", "b", "c", "d")
println(strings.runningReduce { acc, string -> acc + string }) // [a, ab, abc, abcd]
println(strings.runningReduceIndexed { index, acc, string -> acc + string + index }) // [a, ab1, ab1c2, ab1c2d3]

println(emptyList<String>().runningReduce { _, _ -> "X" }) // []
//sampleEnd
}

Parameters

operation - function that takes the index of a character, current accumulator value and the character itself, and calculates the next accumulator value.