Kotlin Map filter()
Syntax & Examples

Syntax of Map.filter()

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

fun <K, V> Map<out K, V>.filter( predicate: (Entry<K, V>) -> Boolean ): Map<K, V>

This filter() extension function of Map returns a new map containing all key-value pairs matching the given predicate.

✐ Examples

1 Filter map entries by value ('b')

In this example,

We create a map named map1 containing pairs of numbers and characters.
We apply the filter() function on map1 to retain only the entries where the value is equal to 'b'.
The resulting filtered map contains the entry with the key 2 and value 'b'.
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 filteredMap1 = map1.filter { (_, value) -> value == 'b' }
    println(filteredMap1)
}

Output

{2=b}

2 Filter map entries by key (exclude 'a')

In this example,

We create a map named map2 containing pairs of characters and numbers.
We apply the filter() function on map2 to exclude the entry with the key 'a'.
The resulting filtered map contains entries excluding the key 'a'.
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 filteredMap2 = map2.filter { (key, _) -> key != 'a' }
    println(filteredMap2)
}

Output

{b=2, c=3}

3 Filter map entries by value (even numbers)

In this example,

We create a map named map3 containing pairs of strings and numbers.
We apply the filter() function on map3 to retain only the entries where the value is an even number.
The resulting filtered map contains the entries with even-numbered values.
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 filteredMap3 = map3.filter { (_, value) -> value % 2 == 0 }
    println(filteredMap3)
}

Output

{banana=6}

Summary

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