kotlin-stdlib/kotlin.sequences/reduceIndexedOrNull

reduceIndexedOrNull

inline fun <S, T : S> Sequence<T>.reduceIndexedOrNull(operation: (index: Int, 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.

Since Kotlin

1.4

Samples

import samples.*
import kotlin.test.*
fun main() { 
   //sampleStart 
   val strings = listOf("a", "b", "c", "d")
assertPrints(strings.reduceOrNull { acc, string -> acc + string }, "abcd")
assertPrints(strings.reduceIndexedOrNull { index, acc, string -> acc + string + index }, "ab1c2d3")

assertPrints(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.