Kotlin List reduceIndexedOrNull()
Syntax & Examples

Syntax of List.reduceIndexedOrNull()

The syntax of List.reduceIndexedOrNull() extension function is:

fun <S, T : S> Iterable<T>.reduceIndexedOrNull( operation: (index: Int, acc: S, T) -> S ): S?

This reduceIndexedOrNull() extension function of List 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 collection.


✐ Examples

1 Example

In this example,

  • We create a list of strings named list containing elements "apple", "banana", "cherry", "date", "elderberry".
  • We use the reduceIndexedOrNull function to accumulate the strings by concatenating them with their indices and a colon from left to right.
  • The final accumulated value, which is a string combining strings, their indices, and colons, is stored in result.
  • We print result using println().

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry", "date", "elderberry")
    val result = list.reduceIndexedOrNull { index, acc, element -> "$acc[$index]: $element" }
    println(result)
}

Output

apple[1]: banana[2]: cherry[3]: date[4]: elderberry

2 Example

In this example,

  • We create a list of integers named list containing elements 1, 2, 3, 4, 5.
  • We use the reduceIndexedOrNull function to accumulate the elements by adding them together with their indices from left to right.
  • The final accumulated value, which is a string combining elements and their indices, is stored in result.
  • We print result using println().

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(1, 2, 3, 4, 5)
    val result = list.reduceIndexedOrNull { index, acc, element -> acc + element + index }
    println(result)
}

Output

25

Summary

In this Kotlin tutorial, we learned about reduceIndexedOrNull() extension function of List: the syntax and few working examples with output and detailed explanation for each example.