Create list of lists in kotlin? - kotlin

Question

How do I create a list of lists?
I have a mutable list of ints:
val rsrpList = mutableListOf<Int>()
Now I am trying to create a list of lists, like this:
val rsrpList = mutableListOf<Int>()
val rsrqList = mutableListOf<Int>()
val levelList = mutableListOf<Int>()
val listoflists = List<List<Int>>
listoflists.add(rsrpList)
listoflists.add(rsrqList)
listoflists.add(levelList)
but I know this is wrong, because I'm adding a list one at a time, instead of a list of lists. How would I do this?

Answer 1

You can do this with the Kotlin Standard Library. Both List and MutableList can be created to a specific size (3 in this case) and specify a lambda that will initialize each value.
val listOfList = MutableList(3) { mutableListOf<Int>() }
Or:
val listOfList = List(3) { mutableListOf<Int>() }
Update: To initialize a List with precreated lists:
val listOfList = listOf(list1, list2, list3)
Or in your specific case:
val listOfList = listOf(rsrpList, rsrqList, levelList)
And in both cases you can replace listOf with mutableListOf if you want a mutable list as the main type.

Answer 2

Your example is fine (you have an empty list, you're adding lists to that list, you end up with a list of lists!) but if you're trying to avoid mutability by declaring everything at once:
val listOfLists = listOf(rsrpList, rsrqList, levelList)
and you can declare those lists at the same time if you want
val listOfLists = listOf(
listOf(1, 2, 3),
listOf(9, 8, 7),
listOf(7, 7, 7)
)
or you can use mutableListOf if you need any of them to be mutable! The formatting there isn't necessary, I just think it looks clearer how they're nested