Frank Guo Frank Guo - 2 years ago 96
Swift Question

Swift: About assign nil to a var

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
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 Source

Very good question!

Since your array is defined as [String:String] you wonder why the compiler let you assign an optional String (so String?) to a value.

How can the compiler risk you to put a nil inside something that should be a non optional String ?

How can this code compile?

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

Here's the answer

Subscript has the following logic, even if the value of the Dictionary is String, you can use subscript to assign nil. In that case the key is removed from the array.

Look here

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]
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