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.