Kotlin Map filterTo()
Syntax & Examples

Syntax of Map.filterTo()

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

fun <K, V, M : MutableMap<in K, in V>> Map<out K, V>.filterTo( destination: M, predicate: (Entry<K, V>) -> Boolean ): M

This filterTo() extension function of Map appends all entries matching the given predicate into the mutable map given as destination parameter.

✐ Examples

1 Filter and append entries to the mutable map (exclude 'b')

In this example,

We create a map named map1 containing pairs of numbers and characters.
We create a mutable map named destination1.
We apply the filterTo() function on map1, appending entries to destination1, excluding the entry where the value is equal to 'b'.
The resulting map contains the filtered entries.
We print the filtered map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf(1 to 'a', 2 to 'b', 3 to 'c')
    val destination1 = mutableMapOf<Int, Char>()
    map1.filterTo(destination1) { (_, value) -> value != 'b' }
    println(destination1)
}

Output

{1=a, 3=c}

2 Filter and append entries to the mutable map (include 'a')

In this example,

We create a map named map2 containing pairs of characters and numbers.
We create a mutable map named destination2.
We apply the filterTo() function on map2, appending entries to destination2, including the entry with the key 'a'.
The resulting map contains the filtered entries.
We print the filtered map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf('a' to 1, 'b' to 2, 'c' to 3)
    val destination2 = mutableMapOf<Char, Int>()
    map2.filterTo(destination2) { (key, _) -> key != 'a' }
    println(destination2)
}

Output

{b=2, c=3}

3 Filter and append entries to the mutable map (even numbers)

In this example,

We create a map named map3 containing pairs of strings and numbers.
We create a mutable map named destination3.
We apply the filterTo() function on map3, appending entries to destination3, including only the entries with even-numbered values.
The resulting map contains the filtered entries.
We print the filtered map to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf("apple" to 5, "banana" to 6, "cherry" to 7)
    val destination3 = mutableMapOf<String, Int>()
    map3.filterTo(destination3) { (_, value) -> value % 2 == 0 }
    println(destination3)
}

Output

{banana=6}

Summary

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