Kotlin List.distinctBy() – Examples

Kotlin List.distinctBy()

The List.distinctBy() function in Kotlin is used to return a list containing only the elements from the original list having distinct keys produced by the given selector function.

Syntax

fun <T, K> Iterable<T>.distinctBy(selector: (T) -> K): List<T>

The function takes a selector function that is applied to each element, and the distinct elements are determined based on the keys produced by this function.

Parameter	Description
T	The type of elements in the list.
K	The type of keys produced by the selector function.
selector	The function that produces keys used to determine distinct elements.

T is the type of elements in the list.

Example 1: Basic Usage of List.distinctBy()

Filter a list of names based on the first character to get distinct initials.

Kotlin Program

fun main() {
    val names = listOf("Alice", "Bob", "Charlie", "David", "Eva", "Eleanor")
    val distinctInitials = names.distinctBy { it.first() }

    println(distinctInitials)
}

Output

[Alice, Bob, Charlie]

This example uses the distinctBy function to filter the list of names based on the first character, resulting in a list of distinct initials.

Example 2: Using List.distinctBy() with Custom Objects

Filter a list of custom objects based on a property to get distinct values.

Kotlin Program

data class Person(val id: Int, val name: String)

fun main() {
    val people = listOf(
        Person(1, "Alice"),
        Person(2, "Bob"),
        Person(3, "Alice"),
        Person(4, "Charlie")
    )

    val distinctNames = people.distinctBy { it.name }

    println(distinctNames)
}

Output

[Person(id=1, name=Alice), Person(id=2, name=Bob), Person(id=4, name=Charlie)]

This example uses the distinctBy function to filter a list of custom objects based on the name property, resulting in a list of distinct names.

Summary

The List.distinctBy() function in Kotlin is a convenient way to obtain a list of distinct elements based on a key extracted by a selector function. It is useful when you want to filter a list and retain only the elements with distinct properties.