Hi everyone, I have this simple code and it doesn't run as expect. I want to add an element to an array of int and then I expect it will have 1 element. The current code print 0

val res : Array<Int>  = emptyArray()
 res.plus(1)
 println(res.size)

Kotlin arrays are fixed-size length so you shouldn’t be able to add anything to an empty array.

Thanks Paula. What is the mutable data structure I should use here.

Calling the

plus()

function seems to create a new empty array (hence why it’s not throwing an error).

Got it. Using listOf works with plus , whereby using mutableListOf works with add. I should use

add

in my use case.

Ah sorry, I wasn’t quite right: arrays are fixed-length, but mutable. So if you create an emptyArray, it can only ever have 0 elements. Similarly, an array of size 3 can only have 3 elements. But you can change the elements in the array, e.g.

myArray[1] = 2

(as long as you obey the array type).

Lists

are immutable but you can create a new list by using

plus

, which would generally be safer / preferable as you aren’t modifying the original list.

val newList = oldList.plus(1)

ImmutableLists

are less safe (something else could be modifying your list at the same time) so it’s generally preferable to use an immutable list.

Yeah, I guess array is something from Java. So it is mutable.