Count items in a List matching a predicate in Kotlin

This article explores different ways to count items in a list matching a predicate in Kotlin.

1. Using `size` property

The standard solution to find the number of elements in a list in Kotlin is calling its size property.

fun main() {

val l = (0.. 100).toList()

val count = l.size

println(count) // 100

}

To count only items that match a predicate, use the filter() method:

data class Product(val name: String, val price: Int)

fun main() {

val items = listOf(Product("1", 10), Product("2", 20), Product("3", 30))

val count = items.filter { it.price > 10 }.size

println(count) // 2

}

Another possibility is using the count() function, which returns the count of elements in the list.

fun main() {

val l = (0.. 100).toList()

val count = l.count()

println(count) // 100

}

To count the number of elements matching a predicate, you can do like below:

data class Product(val name: String, val price: Int)

fun main() {

val items = listOf(Product("1", 10), Product("2", 20), Product("3", 30))

val count = items.count { it.price > 10 }

println(count) // 2

}

That’s all about counting items in a list matching a predicate in Kotlin.