Kotlin List foldRightIndexed()
Syntax & Examples

Syntax of List.foldRightIndexed()

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

fun <T, R> List<T>.foldRightIndexed( initial: R, operation: (index: Int, T, acc: R) -> R ): R

This foldRightIndexed() extension function of List accumulates value starting with initial value 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 of integers named list1.
  • We use the foldRightIndexed() function on list1 starting with an initial value of 0.
  • The operation in foldRightIndexed() adds the value of each element with its index to the accumulator from right to left.
  • The final result is printed to standard output using the println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3, 4, 5)
    val result = list1.foldRightIndexed(0) { index, value, acc -> acc + value + index }
    println(result)
}

Output

25

2 Example

In this example,

  • We create a list of characters named list2.
  • We use the foldRightIndexed() function on list2 starting with an initial value of an empty string.
  • The operation in foldRightIndexed() concatenates each character with its index to the accumulator from right to left.
  • The final result is printed to standard output using the println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list2 = listOf('a', 'b', 'c')
    val result = list2.foldRightIndexed("", { index, value, acc -> acc + value + index })
    println(result)
}

Output

c2b1a0

3 Example

In this example,

  • We create a list of strings named list3.
  • We use the foldRightIndexed() function on list3 starting with an initial value of an empty string.
  • The operation in foldRightIndexed() concatenates each string with its index to the accumulator from right to left.
  • The final result is printed to standard output using the println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list3 = listOf("apple", "banana", "cherry")
    val result = list3.foldRightIndexed("", { index, value, acc -> acc + value + index })
    println(result)
}

Output

cherry2banana1apple0

Summary

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