idiomatic way to unwrap many optional NSDictionary elements?

[Jean-Denis Muys]

I have a dictionary coming from the parsing of a JSON (coming from an outside source). I want to store the values that are present in fixed properties, something similar to:

func feedJSONdata(jsonDict: NSDictionary) {

  var1 = jsonDict["var1"] as! String

  var2 = jsonDict["var2"] as! String

  var3 = jsonDict["var3"] as! String

  var4 = jsonDict["var4"] as! String

  var5 = jsonDict["var5"] as! String

 // and so on

}

Is there a way to guard against any of the entries in the dictionary to be missing?

Repeating:

if let var1Entry = jsonDict["var1"] as? String {

    var1 = var1Entry

}

seems heavy handed to me. Any lighter option?

Second, I may also want to guard against the dictionary entry being present but not a string.

Thanks,

Jean-Denis

[Jens Alfke]

On Jul 16, 2015, at 6:56 AM, Jean-Denis Muys <jdm...@gmail.com> wrote:

Is there a way to guard against any of the entries in the dictionary to be missing?

Depends what you want the fallback behavior to be. It’s easy to fall back to a default value:

var1 = jsonDict["var1"] as? String ?? ""

If you want to skip assigning it at all, you can write a little helper function, called like

assignIfPresent(jsonDict, “var1”, &var1)

But yeah, binding between JSON and statically-typed languages can be a pain. The best implementation I’ve seen is Go’s  which is really simple and convenient — what you’re asking for would be a trivial one-liner in Go. But it takes advantage of reflection capabilities that Swift doesn’t have.

—Jens

[Marco S Hyman]

> func feedJSONdata(jsonDict: NSDictionary) {
> var1 = jsonDict["var1"] as! String
> var2 = jsonDict["var2"] as! String
> var3 = jsonDict["var3"] as! String
> var4 = jsonDict["var4"] as! String
> var5 = jsonDict["var5"] as! String
> // and so on
> }

if let var1 = jsonDict["var1"] as? String where !var1.isEmpty,
let var2 = jsonDict["var2"] as? String where !var2.isEmpty,
let var3 = jsonDict["var3"] as? String where !var3.isEmpty,
let var4 = jsonDict["var4"] as? String where !var4.isEmpty,
let var5 = jsonDict["var5"] as? String where !var5.isEmpty {
// all vars are strings and not empty
// ...
} else {
// error
}

Marc

[Marco S Hyman]

I wrote....

This works too and might be closer to what you want.

func feedJSONdata(jsonDict: NSDictionary) {

{
guard let var1 = jsonDict["var1"] as? String where !var1.isEmpty,

let var2 = jsonDict["var2"] as? String where !var2.isEmpty,
let var3 = jsonDict["var3"] as? String where !var3.isEmpty,
let var4 = jsonDict["var4"] as? String where !var4.isEmpty,

let var5 = jsonDict["var5"] as? String where !var5.isEmpty else {
print("Missing required variables")
return
}
print("vars are OK”)
// work with vars here
}

[Bruno Berisso]

You can also do something like:

func feedJSONdata(jsonDict: NSDictionary) {

    //Get a dictionary with values of type String (filter non string and nil values)

    let dictWithCuratedValues = jsonDict.filter { ($1 is String) }

    

    //Check if the curated version has valid data (ex: dictWithCuratedValues.count > numberOfFileds, etc)

    ....

}

I'm not sure if 'filter' is available in Swift 2, but for prior version you will need to add some extra code to give Dictionary the ability to filter his elements:

extension Dictionary {
    init(_ pairs: [Element]) {
        self.init()
        for (k, v) in pairs {
            self[k] = v
        }
    }

    func filter(includeElement: Element -> Bool) -> [Key: Value] {
        return Dictionary(Swift.filter(self, includeElement))
    }
}

}

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