groupingBy

inline fun <T, K> Array<out T>.groupingBy(
    crossinline keySelector: (T) -> K
): Grouping<T, K>
(source)

Platform and version requirements: Kotlin 1.1

Creates a Grouping source from an array to be used later with one of group-and-fold operations using the specified keySelector function to extract a key from each element.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val words = "one two three four five six seven eight nine ten".split(' ')
val frequenciesByFirstChar = words.groupingBy { it.first() }.eachCount()
println("Counting first letters:")
println(frequenciesByFirstChar) // {o=1, t=3, f=2, s=2, e=1, n=1}

val moreWords = "eleven twelve".split(' ')
val moreFrequencies = moreWords.groupingBy { it.first() }.eachCountTo(frequenciesByFirstChar.toMutableMap())
println(moreFrequencies) // {o=1, t=4, f=2, s=2, e=2, n=1}
//sampleEnd
}
inline fun <T, K> Iterable<T>.groupingBy(
    crossinline keySelector: (T) -> K
): Grouping<T, K>
(source)

Platform and version requirements: Kotlin 1.1

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.

import kotlin.test.*

fun main(args: Array<String>) {
//sampleStart
val words = "one two three four five six seven eight nine ten".split(' ')
val frequenciesByFirstChar = words.groupingBy { it.first() }.eachCount()
println("Counting first letters:")
println(frequenciesByFirstChar) // {o=1, t=3, f=2, s=2, e=1, n=1}

val moreWords = "eleven twelve".split(' ')
val moreFrequencies = moreWords.groupingBy { it.first() }.eachCountTo(frequenciesByFirstChar.toMutableMap())
println(moreFrequencies) // {o=1, t=4, f=2, s=2, e=2, n=1}
//sampleEnd
}