Kotlin List groupBy()

Kotlin List groupBy() Function

The List groupBy() function in Kotlin is used to group elements of a list based on a key returned by the provided selector function.

In this tutorial, we shall learn the syntax of List groupBy() function, and its usage, with the help of example programs.

Syntax

The syntax of List groupBy() function is

fun <T, K> Iterable<T>.groupBy(
    keySelector: (T) -> K
): Map<K, List<T>>

where

Parameter	Description
keySelector	A function that maps elements to keys based on which the grouping is done.

The List groupBy() function returns a map where each key is associated with a list of elements having that key.

The other variation of the groupBy() function is

fun <T, K> Iterable<T>.groupBy(
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): Map<K, List<V>>

where

Parameter	Description
keySelector	A function that maps elements to keys based on which the grouping is done.
valueTransform	This function is applied to each element of the original collection, and then grouped.

Example 1: Grouping Integers by Even or Odd

In this example, we’ll use List groupBy() function to group a list of integers into two groups: even and odd numbers.

Kotlin Program

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9)

    val groupedByEvenOdd = numbers.groupBy { it % 2 == 0 }

    println("Grouped by Even/Odd: $groupedByEvenOdd")
}

Output

Grouped by Even/Odd: {false=[1, 3, 5, 7, 9], true=[2, 4, 6, 8]}

The output shows the list of integers grouped into two categories: even and odd numbers.

Example 2: Grouping Strings by Length

In this example, we shall use List groupBy() function to categorize strings based on their length.

Kotlin Program

fun main() {
    val fruits = listOf("apple", "banana", "orange", "kiwi", "grape")

    val groupedByLength = fruits.groupBy { it.length }

    println("Grouped by Length: $groupedByLength")
}

Output

Grouped by Length: {5=[apple, grape], 6=[banana, orange], 4=[kiwi]}

The output shows the list of strings grouped based on their length.

Summary

In this tutorial, we have seen about List groupBy() function, its syntax, and how to use it for grouping elements in a list based on a specified criteria, with the help of example programs.