swift字典嵌套数组操作 - 不能在嵌套字典内突变嵌套数组 [英] swift dictionary nested array manipulation - cannot mutate nested array inside dictionary

问题描述

var dict = ["alpha": ["a", "b", "c", "d"]]
// output : ["alpha": ["a", "b", "c", "d"]]

var alphaList = dict["alpha"]
// output : {["a", "b", "c", "d"]

alphaList?.removeAtIndex(1)
// output : {Some "b"}

alphaList
// output : {["a", "c", "d"]}

dict
// output : ["alpha": ["a", "b", "c", "d"]]

为什么'dict'没有改变？是因为alphaList是数组的副本，而不是字典中的实际数组？任何人都可以指出我在Swift语言文档中可以找到这些信息？

Why is 'dict' not altered? Is it because 'alphaList' is a copy of the array and not the actual array inside the dictionary? Can anyone point me where in Swift language documentation I can find this information?

什么是正确/功能的方式来操纵字典的值（复杂类型）？ / p>

What is the correct/functional way to manipulate values (of complex type) of a dictionary?

推荐答案

好的问题是，它创建了您的案例中的值的副本，值为Array

Good question yes it creates the copy of the value in your case the value is Array

    var alphaList = dict["alpha"] 
/* which is the copy of original array 
changing it will change the local array alphaList as you can see by your output */
    output : {Some "b"}

要获得原始数组直接使用

In order to get the original array directly use

dict["alpha"]?.removeAtIndex(1)

或使用密钥更新它

alphaList?.removeAtIndex(1)
dict["alpha"] = alphaList 

Apple：字符串，数组和字典的分配和复制行为

Swift的String，Array和Dictionary类型实现为结构。这意味着当字符串，数组和字典分配给一个新的常量或变量，或者当它们被传递给一个函数或方法时，它们将被复制。

Swift’s String, Array, and Dictionary types are implemented as structures. This means that strings, arrays, and dictionaries are copied when they are assigned to a new constant or variable, or when they are passed to a function or method.

与NSString，NSArray和NSDictionary在Foundation中不同，它们被实现为类而不是结构。 NSString，NSArray和NSDictionary实例总是作为对现有实例的引用分配和传递，而不是作为副本。

This behavior is different from NSString, NSArray, and NSDictionary in Foundation, which are implemented as classes, not structures. NSString, NSArray, and NSDictionary instances are always assigned and passed around as a reference to an existing instance, rather than as a copy. "

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