case and switch in swift

Question

在下面显示的代码中，蔬菜多次声明为常量。但是Xcode没有收到错误。为什么Xcode成功地编译了它，但没有得到和错误？设是一个常数。？

let​ ​vegetable​ = ​"red pepper"
​switch​ ​vegetable​ {
​case​ ​"celery"​:
​    ​let​ ​vegetableComment​ = ​"Add some raisins and make ants on a log."
​case​ ​"cucumber"​, ​"watercress"​:
​    ​let​ ​vegetableComment​ = ​"That would make a good tea sandwich."
​case​ ​let​ ​x​ ​where​ ​x​.​hasSuffix​(​"pepper"​):
​    ​let​ ​vegetableComment​ = ​"Is it a spicy ​\(​x​)​?"
​default​:
​    ​let​ ​vegetableComment​ = ​"Everything tastes good in soup."
​}

摘自:苹果公司。“Swift编程语言。”iBooks。

Answer 1

我假设您实际上是在谈论将vegetableComment定义为常量，这就是您所困惑的。

在Swift中，switch语句中的每个case:块都有自己的词法作用域。这意味着您可以在每个变量中为所有变量指定相同的名称，并且它们不会冲突。几乎就像它们在不同的功能中一样。

同样，您不能访问其他变量。下面是一些示例：

let​ ​vegetable​ = ​"red pepper"
var comment = ""
​switch​ ​vegetable​ {
​case​ ​"celery"​:
​    ​comment​ = ​"Add some raisins and make ants on a log."
    // This is only defined here
    var favoriteVegetable = "celery"

​case​ ​"cucumber"​, ​"watercress"​:
​    ​comment​ = ​"That would make a good tea sandwich."
    // This will be an error, because `favoriteVegetable` is only valid inside the celery case block
    // favoriteVegetable = "either cucumber or watercress"

​case​ ​let​ ​x​ ​where​ ​x​.​hasSuffix​(​"pepper"​):
​    ​comment​ = ​"Is it a spicy ​\(​x​)​?"

    // We can redefine favoriteVegetable here, because it has nothing to do with the one in the celery block
    let favoriteVegetable = "a pepper"

​default​:
​    ​comment​ = ​"Everything tastes good in soup."
​}

// Similarly, we can't access `favoriteVegetable` here
// println(favoriteVegetable)

// This was defined before the switch statement, so we can get the value that was calculated
println(comment)