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

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

The operation is intermediate and stateless.

Since Kotlin



fun main() { 
   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}