Kotlin Array - reduce() Function

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The Kotlin array reduce() function is used to accumulate or combine all elements of an array to a single value, starting with the first element, using a binary operation, specified by a lambda function.

This function applies operation from left to right to current accumulator value and each element.

Exception

If an array is empty this function will throw an "UnsupportedOperationException".

Syntax

Following is the syntax of Kotlin array reduce() function −

inline fun <S, T : S> Array<out T>.reduce(
   operation: (acc: S, T) -> S
): S

Parameters

This function accepts an operation represent a current accumulator value and an element, and calculates the next accumulator value.

Return value

This function returns a list containing successive accumulation values generated by applied operation.

Example 1

The following is a basic example to demonstrate the use of reduce() function −

fun main(args: Array<String>) {
   val numbers = arrayOf(1, 2, 3, 4, 5)
   val sum = numbers.reduce { acc, num -> acc + num }
   println("Sum of elements: $sum")
}

Output

The above code generate following output −

Sum of elements: 15

Example 2

Let's see another example. What will be the output if we use reduce() function and an array is empty −

fun main(args: Array<String>) {
   val numbers = arrayOf<Int>()
   val exec = numbers.reduce { acc, num -> acc }
   println("Single value: $exec")
}

Output

The above code throw an exception if an array is empty −

Exception in thread "main" java.lang.UnsupportedOperationException: Empty array can't be reduced.

Example 3

The below example, creates an array and accumulating all elements into a single value using reduce() function −

fun main(args: Array<String>) {
   val strings = arrayOf<String>("a", "b", "c", "d", "e")
   val res = strings.reduce { acc, string -> acc + string }
   println("Single value: $res")
}

Output

Following is the output −

Single value: abcde