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 String
s 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.
fun main() {
//sampleStart
val numbers = listOf("one", "two", "three", "four", "five")
println(numbers.groupBy { it.first().toUpperCase() })
println(numbers.groupBy(keySelector = { it.first() }, valueTransform = { it.toUpperCase() }))
//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()
andreduce()
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 aGrouping
. Use it to implement custom operations when fold or reduce are not enough.
fun main() {
//sampleStart
val numbers = listOf("one", "two", "three", "four", "five", "six")
println(numbers.groupingBy { it.first() }.eachCount())
//sampleEnd
}