groupBy

Groups elements of the original array by the key returned by the given keySelector function applied to each element and returns a map where each group key is associated with a list of corresponding elements.

inline fun <T, K> Array<out T>.groupBy(keySelector: (T) -> K): Map<K, List<T>>(source)

fun main() {
    val words = arrayOf("apple", "apricot", "banana", "blueberry", "cherry")
    val grouped = words.groupBy { it.first() }   // group by first letter
    println(grouped)   // {a=[apple, apricot], b=[banana, blueberry], c=[cherry]}
}

Source