Kotlin函数参数:无法重新分配Val

Question

我在科特林写过红黑树。insertFixup在插入新元素后恢复平衡(z:节点？)是新元素)。树平衡算法取自这里 (第2-3页).problem是Kotlin 不允许我将z重新分配到z.parent和z.parent.parent。我希望z是一个指针。问题是如何让科特林明白我想从他那里得到什么？

class Node(key: Int) {...}

class BinarySearchTree {
    var root: Node? = null

    fun insert(newNode: Node) {...}

    fun RotateLeft(x: Node?) {...}

    fun RotateRight(x: Node?) {...}

    fun insertFixup(z: Node?) {
        var y: Node?
        while (z?.parent?.color == "RED") {
            if (z?.parent == z?.parent?.parent?.left) {
                y = z?.parent?.parent?.right
                if (y?.color == "RED") {
                    z?.parent?.color = "BLACK"
                    y?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    z = z?.parent?.parent
                }
                if (z == z?.parent?.right) {
                    z = z?.parent
                    RotateLeft(z)
                    z?.parent?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    RotateRight(z?.parent?.parent)
                }
            } else {
                y = z?.parent?.parent?.left
                if (y?.color == "RED") {
                    z?.parent?.color = "BLACK"
                    y?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    z = z?.parent?.parent
                }
                if (z != z?.parent?.left) {
                    z = z?.parent
                    RotateLeft(z)
                    z?.parent?.color = "BLACK"
                    z?.parent?.parent?.color = "RED"
                    RotateRight(z?.parent?.parent)
                }
            }
        }
        root?.color = "BLACK"
    }
}

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

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

UPD:感谢大家！所有的答案都很有帮助，我在你的答复中找到了解决办法。

Answer 1

Kotlin中的函数参数基本上是函数内部的只读val，所以这里的z总是引用传入的原始对象。

如果在运行函数时需要修改它指向的内容，则必须在函数开始时创建它的本地副本，然后可以将其设置为var。

例如，您可以像这样启动您的函数，它允许您稍后重新分配这个本地var：

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

Answer 2

Kotlin函数参数是只读值，不可分配.

但是，您可以创建一个ReadWriteProperty对象，以传递给insertFixup以获取/设置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)
    }
}

如果您愿意使用属性而不是变量，那么可以使用属性参考从insertFixup获取/设置newNode：

...
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 3

我也遇到了这个问题。我所做的就是创建一个数据类，并将数据类作为参数传递，然后我可以使用它来修改它的属性。

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
}