Kotlin Map mapValues()
Syntax & Examples

Syntax of Map.mapValues()

The syntax of Map.mapValues() extension function is:

fun <K, V, R> Map<out K, V>.mapValues( transform: (Entry<K, V>) -> R ): Map<K, R>

This mapValues() extension function of Map returns a new map with entries having the keys of this map and the values obtained by applying the transform function to each entry in this Map.

✐ Examples

1 Double the values in the map

In this example,

We create a map named map1 with key-value pairs.
We use the mapValues function on map1, applying a transform function that doubles each value.
The resulting map contains the same keys but with doubled values.
We print the resulting map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf("key1" to 1, "key2" to 2, "key3" to 3)
    val result = map1.mapValues { (key, value) -> value * 2 }
    println(result)
}

Output

{key1=2, key2=4, key3=6}

2 Concatenate keys and values in the map

In this example,

We create a map named map2 with key-value pairs.
We use the mapValues function on map2, applying a transform function that concatenates each key with its corresponding value.
The resulting map contains keys concatenated with their values.
We print the resulting map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val result = map2.mapValues { (key, value) -> key + value }
    println(result)
}

Output

{apple=apple1, banana=banana2, cherry=cherry3}

3 Calculate sum of key lengths and values in the map

In this example,

We create a map named map3 with key-value pairs.
We use the mapValues function on map3, applying a transform function that calculates the sum of the key's length and its corresponding value.
The resulting map contains keys mapped to the sum of their lengths and values.
We print the resulting map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf("one" to 1, "two" to 2, "three" to 3)
    val result = map3.mapValues { (key, value) -> key.length + value }
    println(result)
}

Output

{one=4, two=5, three=8}

Summary

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