Kotlin List.associate() – Examples

Kotlin List.associate()

The Kotlin List.associate() function is used to create a map where the keys and values are derived from the elements of a collection.

The function takes a lambda expression that defines how to extract the key-value pairs from the elements of the collection.

Syntax

List.associate(transform: (T) -> Pair<K, V>)

The function returns a Map containing key-value pairs provided by transform function applied to elements of the given collection.

Example 1: Create a Map from List of user defined objects

In this example, we take a list of user defined objects, and create a Map from this list.

1. Take a list of objects in persons, where each object is of type Person(id, name, age).
2. We have to convert this list of Person objects into a map, where the id property is used for the key, and the name property is used for value.
3. Call associate() function on persons list, and pass a transform (lambda expression) that maps the id property to the name property.
4. You may print the returned map idToNameMap to output.

Program

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

fun main() {
    val persons = listOf(
        Person(1, "Alice", 24),
        Person(2, "Bob", 27),
        Person(3, "Charlie", 31)
    )

    val idToNameMap: Map<Int, String> = persons.associate { person -> person.id to person.name }

    println(idToNameMap)
}

Output

{1=Alice, 2=Bob, 3=Charlie}

The List.associate() function is a convenient way to transform elements of a collection into a map, and it’s particularly useful when you have a collection of objects and want to create a mapping based on some properties of those objects.

Example 2: Create a Map from List of integers

In this example, we take a list of integer values, and associate these integer values with their respective ASCII character values. The integer value shall be the key, and the ASCII character shall be the value.

1. Take a list of integers in charCodes. This is an integer list containing ASCII integer values of some of the alphabets.
2. Call associate() function on charCodes list, and pass a transform that maps the integer value in the list to respective Char. Store the returned map in byCharCode.
3. You may print the returned map byCharCode to output.

Program

fun main() {
    val charCodes = intArrayOf(72, 69, 76, 76, 79)
    val byCharCode = charCodes.associate { it to Char(it) }
    println(byCharCode)
}

Output

{72=H, 69=E, 76=L, 79=O}

If you observe the output, and relate it to the input, we can make the following points.

• The iteration order of the original list is preserved in the map.
• Since the map contains only unique keys, only one entry with key value of 76 is present in the returned map.