Question

I was trying to learn about swift basics and I encountered this problem:

Assume we have a dic:

var presidentialPetsDict = ["Barack Obama":"Bo", "Bill Clinton": "Socks", "George Bush": "Miss Beazley", "Ronald Reagan": "Lucky"]

And to Remove the entry for "George Bush" and replace it with an entry for "George W. Bush":

What I did:

var oldvalue = presidentialPetsDict.removeValueForKey("George Bush")
if let value = oldvalue
{
    presidentialPetsDict["George W. Bush"] = value
}else
{
    print("no matching found")
}

Because I believe removeValueForKey method will return an optional value in case key "George Bush" will not return a value but nil so we need to safely unwrap it by using if let. However, the solution code looks like this:

var oldValue = presidentialPetsDict.removeValueForKey("Georgee Bush")
presidentialPetsDict["George W. Bush"] = oldValue

The part I don't understand is that if we want to assign nil to a var we usually do this:

var value:String?
value = nil

But the solution code above works even though the method returns nil, could somebody explain why it worked because I think in solution we didn't declare oldValue as optional at all.

Answer 1

非常好的问题！

由于您的数组定义为[String:String]，您想知道为什么编译器允许您为值指定可选的String（String?}。

编译器如何冒险将nil置于应该是非可选字符串的内容中？

这段代码怎么编译？

var oldValue: String? = presidentialPetsDict.removeValueForKey("Georgee Bush")
presidentialPetsDict["George W. Bush"] = oldValue

这是答案

下标具有以下逻辑，即使Dictionary的值为String，您也可以使用下标来指定nil。在这种情况下，密钥将从阵列中删除。

看这里

var numbers: [String:Int] = ["one": 1, "two": 2, "three": 3]
numbers["two"] = nil // it looks like I'm putting nil into a Int right?
numbers // ["one": 1, "three": 3]

Answer 2

var oldValue = presidentialPetsDict.removeValueForKey("George Bush")

由于返回类型的类型推断，

oldValue获取可选字符串。

请参阅removeValueForKey的函数定义 public mutating func removeValueForKey（key：Key） - ＆gt;值？

Swift: About assign nil to a var

2 个答案:

非常好的问题！

这是答案