groupingBy
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.
inline fun <T, K> Array<out T>.groupingBy(crossinline keySelector: (T) -> K): Grouping<T, K>(source)
fun main() {
val words = arrayOf("apple", "apricot", "banana", "avocado")
val counts = words.groupingBy { it[0] }.eachCount()
println(counts) // Output: {a=3, b=1}
}