Kotlin List distinctBy()
Syntax & Examples

Syntax of List.distinctBy()

The syntax of List.distinctBy() extension function is:

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

This distinctBy() extension function of List returns a list containing only elements from the given collection having distinct keys returned by the given selector function.

✐ Examples

1 Example

In this example,

We define a data class Person with properties id and name.
We create a list of Person objects with some duplicate entries based on id and name.
We use the distinctBy function to get a new list distinctById containing only elements with distinct id.
The resulting list distinctById contains elements with id values 1, 2, and 3, with duplicates removed based on id.
Finally, we print the distinctById list to standard output using print statement.

Kotlin Program

fun main(args: Array<String>) {
    data class Person(val id: Int, val name: String)
    val list = listOf(
        Person(1, &quot;John&quot;),
        Person(2, &quot;Jane&quot;),
        Person(1, &quot;John&quot;),
        Person(3, &quot;Alex&quot;)
    )
    val distinctById = list.distinctBy { it.id }
    print(distinctById)
}

Output

[Person(id=1, name=John), Person(id=2, name=Jane), Person(id=3, name=Alex)]

2 Example

In this example,

We create a list of strings containing various fruits.
We apply the distinctBy function with a selector function that returns the length of each string.
The resulting list distinctByLength contains only elements with distinct lengths.
Finally, we print the distinctByLength list to standard output using print statement.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(&quot;apple&quot;, &quot;banana&quot;, &quot;mango&quot;, &quot;avocado&quot;, &quot;apricot&quot;)
    val distinctByLength = list.distinctBy { it.length }
    print(distinctByLength)
}

Output

[apple, banana, avocado]

3 Example

In this example,

We create a list of integers.
We apply the distinctBy function with a selector function that returns the remainder of each number when divided by 5.
The resulting list distinctByMod5 contains only elements with distinct remainders when divided by 5.
Finally, we print the distinctByMod5 list to standard output using print statement.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(10, 17, 20, 25, 30, 35)
    val distinctByMod5 = list.distinctBy { it % 5 }
    print(distinctByMod5)
}

Output

[10, 17]

Summary

In this Kotlin tutorial, we learned about distinctBy() extension function of List: the syntax and few working examples with output and detailed explanation for each example.