inline fun CharSequence.reduceIndexed(
    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.

Throws an exception if this char sequence is empty. If the char sequence can be empty in an expected way, please use reduceIndexedOrNull instead. It returns null when its receiver is empty.

import kotlin.test.*

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

// emptyList<Int>().reduce { _, _ -> 0 } //  will fail
//sampleEnd
}

Parameters

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