Kotlin function parameter: Val cannot be reassigned

Question

I have written Red–black tree in Kotlin. Fun insertFixup restores balance after inserting new element (z: Node? is new element). Algorithm of tree balancin

Answer 1

Kotlin function parameters are read-only values and are not assignable.

You can however create a ReadWriteProperty object to pass to insertFixup for getting/setting newNode:

...
class BinarySearchTree {
...
    fun insertFixup(zProperty: ReadWriteProperty<Any?, Node?>) {
        var z by zProperty
...

fun main(args: Array<String>) {
    val bst = BinarySearchTree()

    var newNode: Node? = null
    val newNodeProperty = object : ReadWriteProperty<Any?, Node?> {
        override operator fun getValue(thisRef: Any?, property: KProperty<*>): Node? {
            return newNode
        }

        override operator fun setValue(thisRef: Any?, property: KProperty<*>,
                                       value: Node?) {
            newNode = value
        }
    }

    while (true) {
        newNode = Node(readLine()!!.toInt())
        bst.insert(newNode!!)
        bst.insertFixup(newNodeProperty)
    }
}

And if you are willing to use a property instead of a variable then you can use a property reference for getting/setting newNode from insertFixup instead:

...
class BinarySearchTree {
...
    fun insertFixup(zProperty: KMutableProperty0<Node?>) {
        var z by zProperty
...

var newNode: Node? = null

fun main(args: Array<String>) {
    val bst = BinarySearchTree()

    while (true) {
        newNode = Node(readLine()!!.toInt())
        bst.insert(newNode!!)
        bst.insertFixup(::newNode)
    }
}

// the following allow `KMutableProperty0` to be used as a read/write delegate
operator fun <T> KProperty0<T>.getValue(thisRef: Any?, property: KProperty<*>): T = get()
operator fun <T> KMutableProperty0<T>.setValue(thisRef: Any?, property: KProperty<*>, 
                                               value: T) = set(value)

Answer 2

I ran into this issue as well. What i did was create a data class and pass the data class as a parameter that i could then use to modify its properties.

data class SomeDataClass(
    val x: Int,
    val y: Int,
    val z: Int
)

fun someMethod(someDataClass: SomeDataClass) {
    someDataClass.z = 23 //whatever Int value you please
    // more computations...
    someDataClass.z = 67 // or whatever new value you need to assign.
}

fun parentMethod() {
    val someDataClass = SomeDataClass()
    someMethod(someDataClass)
    val newZValue = someDataClass.z // someDataClass holds modified data from above 
                                    // method
}

Answer 3

Function parameters in Kotlin are basically read-only val's inside the function, so z here will always refer to the original object that was passed in.

If you need to modify what it points to while your function is running, you'll have to make a local copy of it at the beginning of the function, and then you can make that a var.

For example, you could start your function like this, which lets you reassign this local var later:

fun insertFixup(_z: Node?) {
    var z = _z
    // ...
    z = z.parent
    // ...
}