Kotlin List associateBy()
Syntax & Examples

Syntax of List.associateBy()

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

fun <T, K> Iterable<T>.associateBy( keySelector: (T) -> K ): Map<K, T>

This extension function returns a Map containing the elements from the given collection indexed by the key returned from keySelector function applied to each element.

fun <T, K, V> Iterable<T>.associateBy( keySelector: (T) -> K, valueTransform: (T) -> V ): Map<K, V>

This extension function returns a Map containing the values provided by valueTransform and indexed by keySelector functions applied to elements of the given collection.

✐ Examples

1 Example

In this example,

We create a list named list1 containing strings "apple", "banana", and "cherry".
We use the associateBy function with keySelector as { it.first() } to index elements by their first character.
The resulting map map indexes elements like {'a': 'apple', 'b': 'banana', 'c': 'cherry'}.
We print the map using println(map).

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf("apple", "banana", "cherry");
    val map = list1.associateBy { it.first() };
    println(map);
}

Output

{a=apple, b=banana, c=cherry}

2 Example

In this example,

We define a data class Person with name and age properties.
We create a list named people containing instances of Person.
We use the associateBy function with keySelector as Person::name to index elements by their names.
The resulting map map indexes elements by names, e.g., {'Alice': Person('Alice', 25), 'Bob': Person('Bob', 30), 'Charlie': Person('Charlie', 35)}.
We print the map using println(map).

Kotlin Program

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

Output

{Alice=Person(name=Alice, age=25), Bob=Person(name=Bob, age=30), Charlie=Person(name=Charlie, age=35)}

3 Example

In this example,

We create a list named list1 containing integers from 1 to 5.
We use the associateBy function with keySelector as { it * 2 } to index elements by their double values.
We also provide a valueTransform function to prepend the string "Number " to each element.
The resulting map map indexes elements like {2: 'Number 1', 4: 'Number 2', 6: 'Number 3', 8: 'Number 4', 10: 'Number 5'}.
We print the map using println(map).

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3, 4, 5);
    val map = list1.associateBy({ it * 2 }, { "Number " + it.toString() });
    println(map);
}

Output

{2=Number 1, 4=Number 2, 6=Number 3, 8=Number 4, 10=Number 5}

Summary

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