Kotlin List groupingBy()
Syntax & Examples

Syntax of List.groupingBy()

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

fun <T, K> Iterable<T>.groupingBy( keySelector: (T) -> K ): Grouping<T, K>

This groupingBy() extension function of List creates a Grouping source from a collection to be used later with one of group-and-fold operations using the specified keySelector function to extract a key from each element.

✐ Examples

1 Example

In this example,

We create a list named list containing strings representing various fruits.
We use the groupingBy function to create a Grouping source by grouping the fruits based on the first letter of each fruit, converted to uppercase.
The resulting Grouping source can be used later with group-and-fold operations.
We print the resulting Grouping source to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry", "orange", "grape");
    val result = list.groupingBy { it.first().toUpperCase() }
    println(result);
}

Output

MainKt$main$$inlined$groupingBy$1@3498ed

2 Example

In this example,

We define a data class Person with properties name and age.
We create a list named list containing instances of Person with different names and ages.
We use the groupingBy function to create a Grouping source by grouping the persons by their names.
The resulting Grouping source can be used later with group-and-fold operations.
We print the resulting Grouping source to standard output.

Kotlin Program

fun main(args: Array<String>) {
    data class Person(val name: String, val age: Int)
    val list = listOf(
        Person("Alice", 20),
        Person("Bob", 25),
        Person("Alice", 30)
    )
    val result = list.groupingBy { it.name }
    println(result);
}

Output

MainKt$main$$inlined$groupingBy$1@1a407d53

3 Example

In this example,

We create a list named list containing strings representing various fruits.
We use the groupingBy function to create a Grouping source by grouping the fruits based on their lengths.
The resulting Grouping source can be used later with group-and-fold operations.
We print the resulting Grouping source to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry", "orange", "grape")
    val result = list.groupingBy { it.length }
    println(result);
}

Output

MainKt$main$$inlined$groupingBy$1@3498ed

Summary

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