inline fun <K> CharSequence.groupingBy(
    crossinline keySelector: (Char) -> K
): Grouping<Char, K>

Platform and version requirements: Kotlin 1.1

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

import kotlin.test.*

fun main(args: Array<String>) {
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}