Kotlin List groupByTo()
Syntax & Examples

Syntax of List.groupByTo()

There are 2 variations for the syntax of List.groupByTo() extension function. They are:

fun <T, K, M : MutableMap<in K, MutableList<T>>> Iterable<T>.groupByTo( destination: M, keySelector: (T) -> K ): M

This extension function groups elements of the original collection by the key returned by the given keySelector function applied to each element and puts to the destination map each group key associated with a list of corresponding elements.

fun <T, K, V, M : MutableMap<in K, MutableList<V>>> Iterable<T>.groupByTo( destination: M, keySelector: (T) -> K, valueTransform: (T) -> V ): M

This extension function groups values returned by the valueTransform function applied to each element of the original collection by the key returned by the given keySelector function applied to the element and puts to the destination map each group key associated with a list of corresponding values.

✐ Examples

1 Example

In this example,

We create a list of strings named list.
We also create an empty mutable map named resultMap.
We use the groupByTo() function on list with a keySelector that extracts the first character of each string.
The result is grouped elements in list by the first character, and the groups are stored in resultMap.
Finally, we print the resultMap to standard output using println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry", "blueberry", "apricot")
    val resultMap = mutableMapOf<Char, MutableList<String>>()
    list.groupByTo(resultMap) { it.first() }
    println(resultMap)
}

Output

{a=[apple, apricot], b=[banana, blueberry], c=[cherry]}

2 Example

In this example,

We define a data class Person with name and age properties.
We create a list of Person objects named people.
We also create an empty mutable map named ageMap.
We use the groupByTo() function on people with a keySelector that extracts the age and a valueTransform that gets the name of each person.
The result is groups of people in people by age, and their names are stored in ageMap.
Finally, we print the ageMap to standard output using println statement.

Kotlin Program

fun main(args: Array<String>) {
    data class Person(val name: String, val age: Int)
    val people = listOf(
        Person("Alice", 30),
        Person("Bob", 25),
        Person("Charlie", 35),
        Person("David", 40)
    )
    val ageMap = mutableMapOf<Int, MutableList<String>>()
    people.groupByTo(ageMap, { it.age }, { it.name })
    println(ageMap)
}

Output

{30=[Alice], 25=[Bob], 35=[Charlie], 40=[David]}

3 Example

In this example,

We create a list of integers named numbers.
We also create an empty mutable map named isEvenMap.
We use the groupByTo() function on numbers with a keySelector that checks if each number is even.
The result is groups of numbers in numbers by whether they are even or not, and the groups are stored in isEvenMap.
Finally, we print the isEvenMap to standard output using println statement.

Kotlin Program

fun main(args: Array<String>) {
    val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9)
    val isEvenMap = mutableMapOf<Boolean, MutableList<Int>>()
    numbers.groupByTo(isEvenMap) { it % 2 == 0 }
    println(isEvenMap)
}

Output

{false=[1, 3, 5, 7, 9], true=[2, 4, 6, 8]}

Summary

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