Kotlin List fold()
Syntax & Examples

Syntax of List.fold()

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

fun <T, R> Iterable<T>.fold( initial: R, operation: (acc: R, T) -> R ): R

This fold() extension function of List accumulates value starting with initial value and applying operation from left to right to current accumulator value and each element.

✐ Examples

1 Example

In this example,

We create a list named list containing strings representing fruits.
We apply the fold function on list starting with the initial value "Fruits:".
The operation concatenates each fruit with the accumulator string acc.
The resulting string is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry");
    val result = list.fold("Fruits:") { acc, fruit -> "$acc $fruit" };
    println(result);
}

Output

Fruits: apple banana cherry

2 Example

In this example,

We create a list named list containing integers.
We apply the fold function on list starting with the initial value 0.
The operation accumulates the sum of all numbers.
The resulting sum is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(10, 20, 30);
    val result = list.fold(0) { acc, number -> acc + number };
    println(result);
}

Output

3 Example

In this example,

We create a list named list containing characters representing alphabets.
We apply the fold function on list starting with the initial value "Alphabets:".
The operation concatenates each character with the accumulator string acc.
The resulting string is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf('a', 'b', 'c');
    val result = list.fold("Alphabets:") { acc, char -> "$acc $char" };
    println(result);
}

Output

Alphabets: a b c

Summary

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