Kotlin List reduceRightIndexedOrNull()
Syntax & Examples

Syntax of List.reduceRightIndexedOrNull()

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

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

This reduceRightIndexedOrNull() extension function of List accumulates value starting with the last element and applying operation from right to left to each element with its index in the original list and current accumulator value.

✐ Examples

1 Example

In this example,

We create a list named list containing the integers [1, 2, 3, 4, 5].
We use the reduceRightIndexedOrNull function to accumulate values starting with the last element.
In the operation, index represents the index of the current element, element represents the current element, and acc represents the accumulator value.
If the index is even, we add the element to the accumulator; otherwise, we keep the accumulator unchanged.
Finally, we print the result to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(1, 2, 3, 4, 5);
    val result = list.reduceRightIndexedOrNull { index, element, acc -> if (index % 2 == 0) element + acc else acc };
    println(result);
}

Output

2 Example

In this example,

We create a list named list containing the strings ['apple', 'banana', 'cherry', 'date', 'elderberry'].
We use the reduceRightIndexedOrNull function to accumulate values starting with the last element.
In the operation, index represents the index of the current element, element represents the current element, and acc represents the accumulator value.
If the index is greater than 0, we add the length of the element to the accumulator; otherwise, we keep the accumulator unchanged.
Finally, we print the result to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry", "date", "elderberry");
    val result = list.reduceRightIndexedOrNull { index, element, acc -> if (index > 0) acc + element.length else acc };
    println(result);
}

Output

elderberry466

Summary

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