Kotlin List reduceOrNull()
Syntax & Examples

Syntax of List.reduceOrNull()

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

fun <S, T : S> Iterable<T>.reduceOrNull( operation: (acc: S, T) -> S ): S?

This reduceOrNull() 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.


✐ Examples

1 Example

In this example,

  • We create a list of integers named list containing elements 1, 2, 3, 4, 5.
  • We use the reduceOrNull function to accumulate the elements by adding them together from left to right.
  • The final accumulated value, which is an integer sum, 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.reduceOrNull { acc, element -> acc + element }
    println(result)
}

Output

15

2 Example

In this example,

  • We create a list of strings named list containing elements "apple", "banana", "cherry", "date", "elderberry".
  • We use the reduceOrNull function to accumulate the strings by concatenating them with a hyphen from left to right.
  • The final accumulated value, which is a string concatenation with 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.reduceOrNull { acc, element -> "$acc - $element" }
    println(result)
}

Output

apple - banana - cherry - date - elderberry

Summary

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