Kotlin Map mapTo()
Syntax & Examples

Syntax of Map.mapTo()

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

fun <K, V, R, C : MutableCollection<in R>> Map<out K, V>.mapTo( destination: C, transform: (Entry<K, V>) -> R ): C

This mapTo() extension function of Map applies the given transform function to each entry of the original map and appends the results to the given destination.

✐ Examples

1 Map key-value pairs to strings

In this example,

We create a map named map1 with integer keys and string values.
We initialize an empty mutable list named destination.
We use the mapTo() function to apply a transformation to each entry in map1, concatenating the key and value into a string.
The results of the transformation are appended to the destination list.
We print the destination list to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf(1 to "apple", 2 to "banana", 3 to "cherry")
    val destination = mutableListOf<String>()
    map1.mapTo(destination) { entry -> "Key: ${entry.key}, Value: ${entry.value}" }
    println(destination)
}

Output

[Key: 1, Value: apple, Key: 2, Value: banana, Key: 3, Value: cherry]

2 Double values or assign 0 if null

In this example,

We create a map named map2 with string keys and integer values.
We initialize an empty mutable list named destination.
We use the mapTo() function to apply a transformation to each entry in map2, doubling the value if it's not null, otherwise assigning 0.
The results of the transformation are appended to the destination list.
We print the destination list to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val destination = mutableListOf<Int>()
    map2.mapTo(destination) { entry -> entry.value?.let { it * 2 } ?: 0 }
    println(destination)
}

Output

[2, 4, 6]

3 Map keys to strings

In this example,

We create a map named map3 with string keys and integer values.
We initialize an empty mutable list named destination.
We use the mapTo() function to apply a transformation to each entry in map3, mapping each key to its string representation.
The results of the transformation are appended to the destination list.
We print the destination list to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val destination = mutableListOf<String>()
    map3.mapTo(destination) { entry -> entry.key }
    println(destination)
}

Output

[apple, banana, cherry]

Summary

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