inline fun <S, T : S> Sequence<T>.reduceIndexed(
    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.

Throws an exception if this sequence is empty. If the 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 an element, current accumulator value and the element itself, and calculates the next accumulator value.

The operation is terminal.