Downcast element when reading from Dictionary in Swift

Question

I'm trying to access an element a dictionary element and downcast it to a type other than AnyObject but keep getting the same compiler error: Could not find an overload for 'subscript' that accepts the supplied arguments.

I know I can just do this using two if statements like so:

if let x = dict["key"] {
    if let y = x as? String {
        // ...
    }
}

But I feel there has to be a more elegant solution than this. The format that makes the most sense to me is:

if let x = dict["key"] as? String {
    // ...
}

But this just results in the error, mentioned above. I've tried dozens of variations of this, but none of it seems to make any difference. Is this something that just can't be done in Swift?

Have you tried casting the dictionary before you dereference it? Not sure if it will work if the dictionary isn't homogenous, but it's worth a try. — David Berry, Jun 20 '14 at 15:41
I have, the error I get then says `'String' is not identical to AnyObject` — Matt Donnelly, Jun 20 '14 at 15:52

score 7 · Accepted Answer · answered Jun 20 '14 at 17:19

The reason your desired formulation isn't working is that you're trying to unwrap two Optionals with a single as?. There are two Optionals because both the subscripting of your dictionary and the attempted cast to String return optional values. There isn't a way to do it in one if statement that will be runtime-safe, but there is a way:

if let x = dict["key"]! as? String {
    println(x)
}

The problem is that if dict["key"] ends up being nil, the forced unwrapping will crash your app. Better to do it this way (you can skip the first if), even if it is an extra step:

let x: AnyObject? = dict["key"]
if let y = x as? String {
    println(y)
}

The extra step is just the cost of working with a Dictionary<String, AnyObject> - if you can get your dictionary type more specific, you won't have to do it any more.

Thank you, that makes sense. Didn't occur to me that I'd be creating two optionals — Matt Donnelly, Jun 20 '14 at 17:24

score 0 · Answer 2 · answered Sep 02 '14 at 10:21

0

I was also struggling with this thing but then i fount out that optional chaining is the solution. Just use

if let x = dict["key"]? as? String {
      println(x)

}

It gives you both the safety and compactness.

answered Sep 02 '14 at 10:21

salman140

1,510
10
11

This does not work for me although it seems as a logical solution – Matej Ukmar Oct 06 '14 at 15:44

score 0 · Answer 3 · answered Oct 06 '14 at 15:37

0

In the end I had to use this one-line solution:

    if let name = jsonDict["name"] as AnyObject? as? String {
        println("name is \(name)")
    } else {
        println("property was nil")
    }

answered Oct 06 '14 at 15:37

Matej Ukmar

2,157
22
27

Downcast element when reading from Dictionary in Swift

3 Answers3