Kotlin Set filterTo()
Syntax & Examples

Set.filterTo() extension function

The filterTo() extension function in Kotlin filters elements in a set, appending all elements that match the given predicate to the specified destination collection.

Syntax of Set.filterTo()

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

fun <T, C : MutableCollection<in T>> Set<T>.filterTo(destination: C, predicate: (T) -> Boolean): C

This filterTo() extension function of Set appends all elements matching the given predicate to the given destination.

Parameters

ParameterOptional/RequiredDescription
destinationrequiredThe collection to which the matching elements will be appended.
predicaterequiredA function that takes an element and returns true if the element should be included in the result.

Return Type

Set.filterTo() returns value of type C.


✐ Examples

1 Appending even numbers to a list

Using filterTo() to filter elements in a set, appending even numbers to a list.

For example,

  1. Create a set of integers.
  2. Create an empty list to hold the even elements.
  3. Define a predicate function that returns true for even numbers.
  4. Use filterTo() to append even numbers from the set to the list.
  5. Print the resulting list.

Kotlin Program

fun main() {
    val numbers = setOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    val evenList = mutableListOf<Int>()
    numbers.filterTo(evenList) { it % 2 == 0 }
    println(evenList)
}

Output

[2, 4, 6, 8, 10]

2 Appending non-null values to a set

Using filterTo() to filter elements in a set, appending non-null values to another set.

For example,

  1. Create a set containing integers and null values.
  2. Create an empty set to hold the non-null elements.
  3. Define a predicate function that returns true for non-null values.
  4. Use filterTo() to append non-null values from the set to the new set.
  5. Print the resulting set.

Kotlin Program

fun main() {
    val mixedSet: Set<Int?> = setOf(1, 2, null, 4, null, 6)
    val nonNullSet = mutableSetOf<Int?>()
    mixedSet.filterTo(nonNullSet) { it != null }
    println(nonNullSet)
}

Output

[1, 2, 4, 6]

3 Appending strings with length greater than 2 to a list

Using filterTo() to filter elements in a set, appending strings with length greater than 2 to a list.

For example,

  1. Create a set of strings.
  2. Create an empty list to hold the strings with length greater than 2.
  3. Define a predicate function that returns true for strings with length greater than 2.
  4. Use filterTo() to append strings with length greater than 2 from the set to the list.
  5. Print the resulting list.

Kotlin Program

fun main() {
    val strings = setOf("a", "ab", "abc", "abcd")
    val longStrings = mutableListOf<String>()
    strings.filterTo(longStrings) { it.length > 2 }
    println(longStrings)
}

Output

["abc", "abcd"]

Summary

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