Grouping

The Kotlin standard library provides extension functions for grouping collection elements. The basic function groupBy() takes a lambda function and returns a Map. In this map, each key is the lambda result, and the corresponding value is the List of elements on which this result is returned. This function can be used, for example, to group a list of Strings by their first letter.

You can also call groupBy() with a second lambda argument – a value transformation function. In the result map of groupBy() with two lambdas, the keys produced by keySelector function are mapped to the results of the value transformation function instead of the original elements.

This example illustrates using the groupBy() function to group the strings by their first letter, iterating through the groups on the resulting Map with the for operator, and then transforming the values to uppercase using the keySelector function:

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five")

    // Groups the strings by their first letter using groupBy()
    val groupedByFirstLetter = numbers.groupBy { it.first().uppercase() }
    println(groupedByFirstLetter)
    // {O=[one], T=[two, three], F=[four, five]}

    // Iterates through each group and prints the key and its associated values
    for ((key, value) in groupedByFirstLetter) {
        println("Key: $key, Values: $value")
    }
    // Key: O, Values: [one]
    // Key: T, Values: [two, three]
    // Key: F, Values: [four, five]

    // Groups the strings by their first letter and transforms the values to uppercase
    val groupedAndTransformed = numbers.groupBy(keySelector = { it.first() }, valueTransform = { it.uppercase() })
    println(groupedAndTransformed)
    // {o=[ONE], t=[TWO, THREE], f=[FOUR, FIVE]}
//sampleEnd
}

If you want to group elements and then apply an operation to all groups at one time, use the function groupingBy(). It returns an instance of the Grouping type. The Grouping instance lets you apply operations to all groups in a lazy manner: the groups are actually built right before the operation execution.

Namely, Grouping supports the following operations:

eachCount() counts the elements in each group.
fold() and reduce() perform fold and reduce operations on each group as a separate collection and return the results.
aggregate() applies a given operation subsequently to all the elements in each group and returns the result. This is the generic way to perform any operations on a Grouping. Use it to implement custom operations when fold or reduce are not enough.

You can use the for operator on the resulting Map to iterate through the groups created by the groupingBy() function. This allows you to access each key and the count of elements associated with that key.

The following example demonstrates how to group strings by their first letter using the groupingBy() function, count the elements in each group, and then iterate through each group to print the key and count of elements:

fun main() {
//sampleStart
    val numbers = listOf("one", "two", "three", "four", "five")

    // Groups the strings by their first letter using groupingBy() and counts the elements in each group
    val grouped = numbers.groupingBy { it.first() }.eachCount()

    // Iterates through each group and prints the key and its associated values
    for ((key, count) in grouped) {
        println("Key: $key, Count: $count")
        // Key: o, Count: 1
        // Key: t, Count: 2
        // Key: f, Count: 2
    }
//sampleEnd
}

Last modified: 17 June 2024