inline fun <K> CharSequence.groupingBy( crossinline keySelector: (Char) -> K ): Grouping<Char, K>
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.
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
}
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https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/grouping-by.html