Parsing JSON with a nested array in Swift 3

Question

I found this question on parsing JSON in Swift 3 to be very helpful, but I noticed my JSON structure had an array for the "weather" key (see red arrow). I was able to parse other parts of the JSON output, but this array caused problems.

Question: Why am I not able to use the [String:Any] pattern that worked on other parts of this JSON data?.

This is my error in the console: Could not cast value of type '__NSSingleObjectArrayI' (0x112e04be0) to 'NSDictionary' (0x112e05108).

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let url = URL(string: "http://api.openweathermap.org/data/2.5/weather?q=London,uk&appid=MYAPIKEY")!

        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
            if error != nil {
                print(error)
            }else {
                if let urlContent = data {
                    do {
                        let parsedData = try JSONSerialization.jsonObject(with: urlContent, options: .allowFragments) as! [String:Any]
                        print(parsedData)
                       let currentCondions = parsedData["main"] as! [String:Any]

                        for (key, value) in currentCondions {
                            print("\(key) - \(value)")
                        }

                        let locationInfo = parsedData["sys"] as! [String:Any]
                        for (key, value) in locationInfo {
                            print("\(key) - \(value)")
                        }

                        let weatherMain = parsedData["weather"] as! [String:Any]
                        print(weatherMain)

                    } catch {
                       print("JSON processessing failed")
                    }//catch closing bracket
                }// if let closing bracket
            }//else closing bracket
        }// task closing bracket
        task.resume()
    }
}

Your issue isn't clear. Where in your code specifically is your issue? What is the actual issue? Compile error? Runtime error? Unexpected results? Be specific. — rmaddy, Nov 28 '16 at 22:13
I updated the code and the question to show the issue in console. I get a "Thread 8: signal SIGABRT" with the following error in the console: Could not cast value of type '__NSSingleObjectArrayI' (0x112e04be0) to 'NSDictionary' (0x112e05108). — brickm, Nov 28 '16 at 22:17

score 3 · Accepted Answer · answered Nov 28 '16 at 22:19

The error makes the issue fairly clear (that, and looking at the output at the start of your question. The value for the "weather" key is not a dictionary. It's an array of dictionary.

So this:

let weatherMain = parsedData["weather"] as! [String:Any]

needs to be:

let weatherMain = parsedData["weather"] as! [[String:Any]]

As a side note, every use of ! in your app is a crash waiting to happen. You should be safely unwrapping and safely casting values that might not actually be what you think. I strongly suggest you spend some quality time reviewing the sections on optionals, type casting, and optional chaining in the The Swift Programming Language book.

thanks for the quick reply and explaining the dictionary vs. array of dictionary. This was my first attempt at JSON ever. Your answer was spot on. I am a learner and can get stumped easily. I'll take your advice on not force unwrapping. I should get in the habit of wrapping it in something with error handling. — brickm, Nov 28 '16 at 23:59

Parsing JSON with a nested array in Swift 3

1 Answers1