为什么我不能投一个混合类型字典的值并更新值？ [英] Why can't I cast a value of a mixed type dictionary and update the value?

问题描述

我正在尝试回答另一个StackOverflow问题，并发现我对Swift的了解不足。

I was attempting to answer another StackOverflow question and found my knowledge of Swift lacking.

以下是演示我遇到的问题的游乐场代码。

Below is playground code that demonstrates the issue I'm having.

import Foundation

// How do I access the value of dictionary and update the value, without replacing it?
// How do I make the inner dictionary mutable?

var sampleDict = [
    "dictionary" : [String: String](),
    "array" : [String]()
]


(sampleDict["dictionary"] as! [String:String]).updateValue("avalue", forKey: "akey")
// ERROR: Swift.playground:12:27: error: immutable value of type '[String : String]' only has mutating members named 'updateValue'
//(sampleDict["dictionary"] as! [String:String]).updateValue("avalue", forKey: "akey")

(sampleDict["dictionary"] as! [String:String])["akey"] = "avalue"
// ERROR: Swift.playground:14:56: error: cannot assign to the result of this expression
// (sampleDict["dictionary"] as! [String:String])["akey"] = "avalue"

我的问题是

• 我是否访问mixedType字典的值并更新该值，而不替换它？


• 如何使内部字典变得可变？


• 是这是不可能的？

推荐答案

我认为这是不可能的。当查看 Dictionary 声明时，似乎没有办法更改值，只能替换它。这是有道理的，因为当你获得一个值时，它会返回字典中的真实结构的副本（结构总是被复制）。当然，它会在使用课程时起作用。这是我的理论..

I think it's impossible. When looking at the Dictionary declaration, there seems to be no method to changing a value, only replacing it. It kind of makes sense because when you get a value, it returns a copy of the real struct in the dictionary (structs are always copied). Of course it would work when using a class. That's my theory..

编辑：我刚刚发现，它的工作没有转换为[String：String]和一个只有正确类型的字典： p>

I just found out that it does work without the cast to [String: String] and a dictionary with just the right type:

var sampleDict = ["dictionary" : [String: String]()]
sampleDict["dictionary"]?.updateValue("avalue", forKey: "akey")
sampleDict["dictionary"]?["akey"] = "avalue"

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