Kotlin List.count() Function

The Kotlin List.count() function finds the number of elements matching the given predicate and returns that value.

Syntax of List.count()

The syntax to call List.count() function is

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list.count(predicate)

Parameter	Description
predicate	A function which returns a boolean value for a given element.

Return Value

Value	Scenario
An integer representing the number of matches for predicate	When there are matches in the list for given predicate
0	When there are no matches in the list.

Example 1: Count Matches in List

In this example, we will take a list of integers, and count the number of even numbers in this list. The predicate is such that, it returns true if the number is even.

Kotlin Program

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fun main(args: Array<String>) {
    val list1 = listOf(2, 8, 5, 7, 9, 6, 10)
    val predicate: (Int) -> Boolean = {it % 2 == 0}
    val result = list1.count(predicate)
    print("Number of matches in the list: $result")
}

Output

Number of matches in the list: 4

Example 2: Count Matches in List of Strings

In this example, we will take list of strings, and a predicate which returns true if the string length is 2. We will use list.count() and predicate, to count the number of elements in the list with length 2.

Kotlin Program

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fun main(args: Array<String>) {
    val list1 = listOf("co", "com", "iso", "dev", "io", "in")
    val predicate: (String) -> Boolean = {it.length == 2}
    val result = list1.count(predicate)
    print("Number of matches in the list: $result")
}

Output

Number of matches in the list: 3

Conclusion

In this Kotlin Tutorial, we learned how to count the number of matches for a predicate in the given list, using Kotlin List.count() function.

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Kotlin List.count() Function – Syntax & Examples