How to pass a variable from main function to another function and modify it? (Kotlin)

Issue

I am aware that an argument passed to a function will be seen as "val", even if the variable was initialized as "var". But this has been a problem for me. In this example code below, I want to modify the value of variable "num" by using the function "changeNum". But of course, Kotlin won't allow me to modify it. How do I get around this? How can I use the function "changeNum" to modify the variable "num"?

fun main() {
    var num: Int = 5
    changeNum(num)
}

fun changeNum(num: Int){
    num += 2
    print(num)
}

Solution

Kotlin is pass-by-value, so you can't do that directly with primitive types like Int.

The usual approach is to just return a new value instead of modifying what you receive as argument (a functional approach). It makes it clearer from the calling code's perspective:

fun main() {
    var num: Int = 5
    num = changeNum(num)
}

fun changeNum(num: Int): Int {
    val newValue = num + 2
    print(newValue)
    return newValue
}

If you really want to mutate the variable instead of returning a new value, here are 2 approaches I'm thinking of to do this:

1. put all the code operating on this value into a class, and make the local variable a property of that class (this is the OO approach)
2. put this primitive variable into a class, and pass an instance of that class

Option 1 would look like this:

class SomeClass(var num: Int) {

    fun changeNum() {
        num += 2
        print(num)
    }
}

fun main() {
    val container = SomeClass(5)
    container.changeNum()
}

Option 2 would look like this:

class SomeMutableContainer(var num: Int)

fun main() {
    val container = SomeMutableContainer(5)
    changeNum(container)
}

fun changeNum(container: SomeMutableContainer) {
    container.num += 2
    print(container.num)
}

Answered By - Joffrey