How to add a list in list of list in Kotlin?-CodePudding

val myList = mutableListOf<Int>()
    val listOfList = mutableListOf<List<Int>>()
    for (i in 0..2) {
        for (j in 0..2) {
            myList.add(j)
        }
        listOfList.add(myList)
    }
    println(listOfList)

output is

[[0, 1, 2, 0, 1, 2, 0, 1, 2], [0, 1, 2, 0, 1, 2, 0, 1, 2], [0, 1, 2, 0, 1, 2, 0, 1, 2]]

instead of

[[0, 1, 2],[0, 1, 2, 0, 1, 2],[0, 1, 2, 0, 1, 2, 0, 1, 2]]

the final result populates the list with last inner list

CodePudding user response：

Note that in your listOfList.add(myList), myList is the same object for every i. Your myList.add(j) statement will affect the same list every time.
One way around it, is to add a clone (instead of the same object) to listOfList.

fun main() {
    val myList = mutableListOf<Int>()
    val listOfList = mutableListOf<List<Int>>()
    for (i in 0..2) {
        for (j in 0..2) {
            myList.add(j)
        }
        println("mylist -> "   myList)
        listOfList.add(myList.toMutableList()) //add a clone instead of using the same myList object every time
        println("list of lists-> "   listOfList)
    }
    println("final list of lists -> "   listOfList)

}

Output:

mylist -> [0, 1, 2]
list of lists-> [[0, 1, 2]]
mylist -> [0, 1, 2, 0, 1, 2]
list of lists-> [[0, 1, 2], [0, 1, 2, 0, 1, 2]]
mylist -> [0, 1, 2, 0, 1, 2, 0, 1, 2]
list of lists-> [[0, 1, 2], [0, 1, 2, 0, 1, 2], [0, 1, 2, 0, 1, 2, 0, 1, 2]]
final list of lists -> [[0, 1, 2], [0, 1, 2, 0, 1, 2], [0, 1, 2, 0, 1, 2, 0, 1, 2]]