Kotlin Tutorials

Kotlin Set runningFoldIndexed()
Syntax & Examples

Set.runningFoldIndexed() extension function

The runningFoldIndexed() extension function in Kotlin returns a list containing successive accumulation values generated by applying an operation from left to right to each element, its index in the original collection, and the current accumulator value that starts with an initial value.

Syntax of Set.runningFoldIndexed()

The syntax of Set.runningFoldIndexed() extension function is:

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

This runningFoldIndexed() extension function of Set returns a list containing successive accumulation values generated by applying operation from left to right to each element, its index in the original collection and current accumulator value that starts with initial value.

Parameters

ParameterOptional/RequiredDescription
initialrequiredThe initial value for the accumulator.
operationrequiredA function that takes the current index, the current accumulator value, and an element, and returns the new accumulator value.

Return Type

Set.runningFoldIndexed() returns value of type List.


✐ Examples

1 Running sum of elements in a set of integers with index

Using runningFoldIndexed() to calculate the running sum of elements in a set of integers while considering the index.

For example,

  1. Create a set of integers.
  2. Use runningFoldIndexed() with an initial value of 0 and an operation that adds each element to the accumulator while considering the index.
  3. Print the resulting list of running sums.

Kotlin Program

fun main() {
    val numbers = setOf(1, 2, 3, 4, 5)
    val runningSums = numbers.runningFoldIndexed(0) { index, acc, num -> acc + num + index }
    println(runningSums)
}

Output

[0, 1, 4, 9, 16, 25]

2 Running concatenation of strings in a set with index

Using runningFoldIndexed() to calculate the running concatenation of strings in a set while considering the index.

For example,

  1. Create a set of strings.
  2. Use runningFoldIndexed() with an initial value of an empty string and an operation that concatenates each string to the accumulator while considering the index.
  3. Print the resulting list of running concatenations.

Kotlin Program

fun main() {
    val strings = setOf("Kotlin", "is", "fun")
    val runningConcat = strings.runningFoldIndexed("") { index, acc, str -> "$acc $index:$str".trim() }
    println(runningConcat)
}

Output

[, 0:Kotlin, 0:Kotlin 1:is, 0:Kotlin 1:is 2:fun]

3 Running product of elements in a set of integers with index

Using runningFoldIndexed() to calculate the running product of elements in a set of integers while considering the index.

For example,

  1. Create a set of integers.
  2. Use runningFoldIndexed() with an initial value of 1 and an operation that multiplies each element to the accumulator while considering the index.
  3. Print the resulting list of running products.

Kotlin Program

fun main() {
    val numbers = setOf(1, 2, 3, 4)
    val runningProducts = numbers.runningFoldIndexed(1) { index, acc, num -> acc * (num + index) }
    println(runningProducts)
}

Output

[1, 1, 4, 18, 90]

Summary

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