Kotlin List mapIndexedNotNullTo()
Syntax & Examples

Syntax of List.mapIndexedNotNullTo()

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

fun <T, R : Any, C : MutableCollection<in R>> Iterable<T>.mapIndexedNotNullTo( destination: C, transform: (index: Int, T) -> R? ): C

This mapIndexedNotNullTo() extension function of List applies the given transform function to each element and its index in the original collection and appends only the non-null results to the given destination.


✐ Examples

1 Example

In this example,

  • We create a list named list1 containing the integers 1, 2, 3.
  • We create an empty mutable list named destination to store non-null results.
  • We then apply the mapIndexedNotNullTo function to list1, which takes a lambda with two parameters: index and value.
  • Within the lambda, we check if the value is even. If it is, we concatenate the index and value into a string format; otherwise, we return null.
  • The mapIndexedNotNullTo function appends only the non-null results to the destination list.
  • The resulting list, containing strings representing each even number along with its index, is stored in result.
  • Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3);
    val destination = mutableListOf<String>();
    val result = list1.mapIndexedNotNullTo(destination) { index, value -> if (value % 2 == 0) "\$index: \$value" else null }
    print(result);
}

Output

[1: 2]

2 Example

In this example,

  • We create a list named list1 containing the characters 'a', 'b', 'c'.
  • We create an empty mutable list named destination to store non-null results.
  • We then apply the mapIndexedNotNullTo function to list1, which takes a lambda with two parameters: index and value.
  • Within the lambda, we check if the value is equal to 'b'. If it is, we concatenate the index and value into a string format; otherwise, we return null.
  • The mapIndexedNotNullTo function appends only the non-null results to the destination list.
  • The resulting list, containing strings representing the index and value of 'b', is stored in result.
  • Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(&quot;a&quot;, &quot;b&quot;, &quot;c&quot;);
    val destination = mutableListOf<String>();
    val result = list1.mapIndexedNotNullTo(destination) { index, value -> if (value == "b") "\$index: \$value" else null }
    print(result);
}

Output

[1: b]

3 Example

In this example,

  • We create a list named list1 containing the strings 'apple', 'banana', 'cherry'.
  • We create an empty mutable list named destination to store non-null results.
  • We then apply the mapIndexedNotNullTo function to list1, which takes a lambda with two parameters: index and value.
  • Within the lambda, we check if the length of the string is greater than 5. If it is, we concatenate the index and value into a string format; otherwise, we return null.
  • The mapIndexedNotNullTo function appends only the non-null results to the destination list.
  • The resulting list, containing strings representing each string with a length greater than 5 along with its index, is stored in result.
  • Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf("apple", "banana", "cherry");
    val destination = mutableListOf<String>();
    val result = list1.mapIndexedNotNullTo(destination) { index, value -> if (value.length > 5) "\$index: \$value" else null }
    print(result);
}

Output

[0: banana, 1: cherry]

Summary

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