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Kotlin predicate

last modified August 15, 2022

Kotlin predicate tutorial shows how to use predicates in Kotlin.

Predicate

Predicate in general meaning is a statement about something that is either true or false. In programming, predicates represent single argument functions that return a boolean value.

Kotlin predicate example

The following example creates a simple Kotlin predicate.

main.kt
fun main() {

    val vals = listOf(1, 2, 0, 5, -4, 5, -6, 7, 8, 3)

    val r = vals.find { e -> isNegative(e) }
    println(r)
}

fun isNegative(e: Int): Boolean {

    return e < 0
}

In the example, the predicate is used to filter out positive values.

val vals = listOf(1, 2, 0, 5, -4, 5, -6, 7, 8, 3)

We have a list of integer values.

val r = vals.find { e -> isNegative(e) }

The find function returns the first element matching the given predicate, or null if no such element was found. We pass the isNegative function.

fun isNegative(e: Int): Boolean {

    return e < 0
}

The isNegative function returns true for each value that is greater than zero.


The following is a small modification of the previous example.

main.kt
fun main() {

    val vals = listOf(1, 2, 0, 5, -4, 5, -6, 7, 8, 3)

    val r = vals.find { isNegative(it) }
    println(r)
}

fun isNegative(e: Int): Boolean {
    return e < 0
}

In this example, we use the it: the implicit name for a single argument parameter.

Kotlin anonymous predicate

In the next example, we use an anonymous predicate function.

main.kt
fun main() {

    val vals = listOf(1, 2, 0, 5, -4, 5, -6, 7, 8, 3)

    val r = vals.find { e -> e < 0 }
    println(r)

}

In this example, the predicate is defined as a anonymous lambda expression.


Finally, we have a lambda expression with it.

main.kt
fun main() {

    val vals = listOf(1, 2, 0, 5, -4, 5, -6, 7, 8, 3)

    val r = vals.find { it < 0 }
    println(r)
}

This is the shortest syntax.

Kotlin filter predicate example

The filter function returns a list containing only elements matching the given predicate.

main.kt
fun main() {

    val vals = listOf(1, 2, 0, 5, -4, 5, -6, 7, 8, 3)

    val r1 = vals.filter { it < 0 }
    println(r1)

    val r2 = vals.filter { it > 0 }
    println(r2)
}

In the example, we filter out negative and positive values.

Kotlin predicate multiple conditions

The next example uses a predicate with two conditions.

main.kt
data class Country(val name: String, val population: Int)

fun main() {

    val countries = listOf(
        Country("Iran", 80840713),
        Country("Hungary", 9845000),
        Country("Poland", 38485000),
        Country("India", 1342512000),
        Country("Latvia", 1978000),
        Country("Vietnam", 95261000),
        Country("Sweden", 9967000),
        Country("Iceland", 337600),
        Country("Israel", 8622000)
    )

    val r = countries.filter {
        it.name.startsWith("I")
                && it.population > 100_000
    }

    r.forEach { println(it) }
}

We create a list of countries. We find all countries that start with 'I' and have population over one million.

val r = countries.filter {
    it.name.startsWith("I")
            && it.population > 100_000
}

We combine two expression with the and operator.

Kotlin negating predicates

We have functions that negate the given predicates.

main.kt
fun main() {

    val words = listOf(
        "falcon", "cup", "wood", "tree", "top", "car",
        "rock", "cloud", "new", "rain", "forest"
    );

    val r = words.filterNot { it.length == 3 }
    println(r)
}

With filterNot, we find all elements that do not match the given predicate. We find all strings whose length is not three latin characters.

In this tutorial, we have covered predicates in Kotlin.

List all Kotlin tutorials.