Kotlin：将列表分组到地图中

Question

How to group list into map of maps using lambdas? Imperative solution looks following:

private class Data(val a: Int, val b: Int, val c: Int)

@JvmStatic
fun main(args: Array<String>) {
    val dataList = listOf(Data(1, 2, 3), Data(4, 5, 6), Data(7, 8, 9), Data(1, 10, 11))
    val result = mutableMapOf<Int, MutableMap<Int, Int>>()
    for (data in dataList) {
        val aMap = result.getOrPut(data.a) { mutableMapOf() }
        aMap[data.b] = data.c
    }
    println(result)
}

Answer 1

You can do this:

dataList.groupBy { it.a }
        .mapValues { (_, v) -> v.associate { it.b to it.c } }

The groupBy function creates a Map<Int, List<Data>> , where the List<Data> is all data-objects that shares the same value in a . Then I map the values of the Map , creating the structure you needed.