Kotlin List foldIndexed()
Syntax & Examples

Syntax of List.foldIndexed()

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

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

This foldIndexed() extension function of List accumulates value starting with initial value 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 integers named list1.
  • We use the foldIndexed() function on list1 starting with an initial value of 0.
  • The operation in foldIndexed() adds the value of each element with its index to the accumulator.
  • 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.foldIndexed(0) { index, acc, value -> acc + value + index }
    println(result)
}

Output

25

2 Example

In this example,

  • We create a list of characters named list2.
  • We use the foldIndexed() function on list2 starting with an initial value of an empty string.
  • The operation in foldIndexed() concatenates each character with its index to the accumulator.
  • 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.foldIndexed("", { index, acc, value -> acc + value + index })
    println(result)
}

Output

a0b1c2

3 Example

In this example,

  • We create a list of strings named list3.
  • We use the foldIndexed() function on list3 starting with an initial value of an empty string.
  • The operation in foldIndexed() concatenates each string with its index to the accumulator.
  • 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.foldIndexed("", { index, acc, value -> acc + value + index })
    println(result)
}

Output

apple0banana1cherry2

Summary

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