Kotlin List associateByTo()
Syntax & Examples

Syntax of List.associateByTo()

There are 2 variations for the syntax of List.associateByTo() extension function. They are:

fun <T, K, M : MutableMap<in K, in T>> Iterable<T>.associateByTo( destination: M, keySelector: (T) -> K ): M

This extension function populates and returns the destination mutable map with key-value pairs, where key is provided by the keySelector function applied to each element of the given collection and value is the element itself.

fun <T, K, V, M : MutableMap<in K, in V>> Iterable<T>.associateByTo( destination: M, keySelector: (T) -> K, valueTransform: (T) -> V ): M

This extension function populates and returns the destination mutable map with key-value pairs, where key is provided by the keySelector function and and value is provided by the valueTransform function applied to elements of the given collection.

✐ Examples

1 Example

In this example,

We create a list named list1 containing the integers 1, 2, 3, 4.
We create an empty mutable map named map1.
We use the associateByTo function on list1 to associate each element with itself as the key in map1.
The resulting map map1 contains key-value pairs where each element of list1 is both the key and value.
We print map1 to standard output using println.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3, 4)
    val map1 = mutableMapOf<Int, Int>()
    list1.associateByTo(map1, { it })
    println(map1)
}

Output

{1=1, 2=2, 3=3, 4=4}

2 Example

In this example,

We define a data class Person with properties name and age.
We create a list named people containing instances of Person.
We create an empty mutable map named map2.
We use the associateByTo function on people to associate each person's name with their age in map2.
The resulting map map2 contains key-value pairs where each person's name is the key and their age is the value.
We print map2 to standard output using println.

Kotlin Program

fun main(args: Array<String>) {
    data class Person(val name: String, val age: Int)
    val people = listOf(Person("Alice", 30), Person("Bob", 25), Person("Charlie", 35))
    val map2 = mutableMapOf<String, Int>()
    people.associateByTo(map2, { it.name }, { it.age })
    println(map2)
}

Output

{Alice=30, Bob=25, Charlie=35}

Summary

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