Kotlin Tutorials

Kotlin Map filterKeys()
Syntax & Examples

Syntax of Map.filterKeys()

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

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

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


✐ Examples

1 Filter out key 'b' from the map

In this example,

  • We create a map named map1 with key-value pairs ('a' to 1), ('b' to 2), ('c' to 3).
  • We use the filterKeys() function on map1 with a predicate that filters out key 'b'.
  • The resulting map filteredMap1 contains key-value pairs ('a' to 1), ('c' to 3).
  • We print filteredMap1 to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf('a' to 1, 'b' to 2, 'c' to 3)
    val filteredMap1 = map1.filterKeys { it != 'b' }
    println(filteredMap1)
}

Output

{a=1, c=3}

2 Filter keys 'a' and 'c' from the map

In this example,

  • We create a map named map2 with key-value pairs ('a' to 1), ('b' to 2), ('c' to 3).
  • We use the filterKeys() function on map2 with a predicate that filters keys 'a' and 'c'.
  • The resulting map filteredMap2 contains key-value pairs ('a' to 1), ('c' to 3).
  • We print filteredMap2 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.filterKeys { it == 'a' || it == 'c' }
    println(filteredMap2)
}

Output

{a=1, c=3}

3 Filter keys with length greater than 5 from the map

In this example,

  • We create a map named map3 with key-value pairs ('apple' to 1), ('banana' to 2), ('cherry' to 3).
  • We use the filterKeys() function on map3 with a predicate that filters keys with length greater than 5.
  • The resulting map filteredMap3 contains key-value pairs ('banana' to 2), ('cherry' to 3).
  • We print filteredMap3 to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf("apple" to 1, "banana" to 2, "cherry" to 3)
    val filteredMap3 = map3.filterKeys { it.length > 5 }
    println(filteredMap3)
}

Output

{banana=2, cherry=3}

Summary

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