Adding a list to the end of another list - kotlin

Question

I am writing a piece of code in which I need to add a list to another list.
fun a(x:Int):List<List<Int>>{
var a = 1.rangeTo(x);
var b: List<List<Int>> = emptyList();
for (i in a){
var num1 = if(i<=3) i else 4;
var list_a= 0.rangeTo(num1 - 1);
b.add(list_a);
}
return b
}
I problem is b.add(list_a) is giving error(unresolved reference).
In this function if I input println(a(2))
The output should be [[0],[0,1]]
Please help.

Answer 1

The reason why you're getting the unresolved reference for b.add() is because you have initialized b as a List instead of MutableList.
Please note that once initialized, you can add elements to a MutableList but not a List in Kotlin. Hence change your initialization of b as var b: MutableList<List<Int>> = mutableListOf()
fun a(x: Int): List<List<Int>> {
var a = 1.rangeTo(x)
var b: MutableList<List<Int>> = mutableListOf()
for (i in a) {
var num1 = if (i <= 3) i else 4
var list_a = 0.rangeTo(num1 - 1).toList()
b.add(list_a)
}
return b
}
Refactoring it a bit further, you can use 1..x for the range in the for loop and (0 until num1) instead of 0.rangeTo(num1 - 1) as below
fun a(x: Int): List<List<Int>> {
val listB: MutableList<List<Int>> = mutableListOf()
for (i in 1..x) {
val num1 = if (i <= 3) i else 4
val listA = (0 until num1).toList()
listB.add(listA)
}
return listB
}