inline fun <T, K, R> Grouping<T, K>.aggregate(
    operation: (key: K, accumulator: R?, element: T, first: Boolean) -> R
): Map<K, R>

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in a new map.

The key for each element is provided by the Grouping.keyOf function.

fun main(args: Array<String>) {
//sampleStart
val numbers = listOf(3, 4, 5, 6, 7, 8, 9)

val aggregated = numbers.groupingBy { it % 3 }.aggregate { key, accumulator: StringBuilder?, element, first ->
    if (first) // first element
        StringBuilder().append(key).append(":").append(element)
    else
        accumulator!!.append("-").append(element)
}

println(aggregated.values) // [0:3-6-9, 1:4-7, 2:5-8]
//sampleEnd
}

Parameters

operation - function is invoked on each element with the following parameters:

Return a Map associating the key of each group with the result of aggregation of the group elements.