inline fun <T, K, R, M : MutableMap<in K, R>> Grouping<T, K>.aggregateTo(
    destination: M, 
    operation: (key: K, accumulator: R?, element: T, first: Boolean) -> R
): M

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 the given destination 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 }.aggregateTo(mutableMapOf()) { 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]

// aggregated is a mutable map
aggregated.clear()
//sampleEnd
}

Parameters

operation -

a function that is invoked on each element with the following parameters:

• key: the key of the group this element belongs to;
• accumulator: the current value of the accumulator of the group, can be null if it's the first element encountered in the group;
• element: the element from the source being aggregated;
• first: indicates whether it's the first element encountered in the group.

If the destination map already has a value corresponding to some key, then the elements being aggregated for that key are never considered as first.

Return the destination map associating the key of each group with the result of aggregation of the group elements.