Kotlin List filterIndexed()
Syntax & Examples

Syntax of List.filterIndexed()

The syntax of List.filterIndexed() extension function is:

fun <T> Iterable<T>.filterIndexed( predicate: (index: Int, T) -> Boolean ): List<T>

This filterIndexed() extension function of List returns a list containing only elements matching the given predicate.


✐ Examples

1 Example

In this example,

  • We create a list named list1 containing the integers 1, 2, 3, 4, 5.
  • We use the filterIndexed function on list1 with a predicate that filters elements based on their index.
  • The predicate { index, value -> index % 2 == 0 } keeps elements where the index is even.
  • The filtered list, containing elements at even indices, is stored in result.
  • Finally, we print the value of result to standard output using the print statement.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3, 4, 5);
    val result = list1.filterIndexed { index, value -> index % 2 == 0 }
    print(result);
}

Output

[1, 3, 5]

2 Example

In this example,

  • We create a list named list2 containing the characters 'a', 'b', 'c', 'd', 'e'.
  • We use the filterIndexed function on list2 with a predicate that filters elements based on their index.
  • The predicate { index, value -> index % 2 != 0 } keeps elements where the index is odd.
  • The filtered list, containing elements at odd indices, is stored in result.
  • Finally, we print the value of result to standard output using the print statement.

Kotlin Program

fun main(args: Array<String>) {
    val list2 = listOf('a', 'b', 'c', 'd', 'e');
    val result = list2.filterIndexed { index, value -> index % 2 != 0 }
    print(result);
}

Output

[b, d]

3 Example

In this example,

  • We create a list named list3 containing the strings "apple", "banana", "cherry", "date", "elderberry".
  • We use the filterIndexed function on list3 with a predicate that filters elements based on their index.
  • The predicate { index, value -> index > 1 } keeps elements where the index is greater than 1 (i.e., skips the first two elements).
  • The filtered list, containing elements starting from index 2, is stored in result.
  • Finally, we print the value of result to standard output using the print statement.

Kotlin Program

fun main(args: Array<String>) {
    val list3 = listOf("apple", "banana", "cherry", "date", "elderberry");
    val result = list3.filterIndexed { index, value -> index > 1 }
    print(result);
}

Output

[cherry, date, elderberry]

Summary

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