Getting the error 'Unexpectedly found nil while unwrapping an Optional value' when using index to access the array elements

Question

I am calling the API service from my viewController using a private function.

private func updateCells() -> Void {
    let cities: [String] = ["New York", "London", "Tokyo", "Toronto", "Sydney", "Paris"]
    
    for cityName in cities {
        print(cityName)
        queryService.getSearchResults(cityName: cityName){results, errorMessage in
            
            if let results = results{
                self.myCity = results
                self.CityGrid.reloadData()
                self.list.append(results)
                print("Test -> name: \(results.name)")
                print("Test -> description: \(results.description)")
                print("Test -> currentTemp: \(Int(results.currentTemperature))")
                print(self.list[0])
                
            }
            
            if !errorMessage.isEmpty {
              print("Search error: " + errorMessage)
                let alert = UIAlertController(title: "Error", message: errorMessage, preferredStyle: UIAlertController.Style.alert)
                alert.addAction(UIAlertAction(title: "OK", style: UIAlertAction.Style.cancel, handler: nil))
                self.present(alert, animated: true, completion: nil)
            }
        }
    }
}

Here the QueryService class function to handle the API calls.

func getSearchResults(cityName: String, completion: @escaping QueryResult) {
    
    dataTask?.cancel()
    
    let url = URL(string: "http://api.openweathermap.org/data/2.5/weather?q=\(cityName)&units=metric&appid=zezfac1ecbe0511f1ac192add4ff112e")!
  
    
    dataTask = defaultSession.dataTask(with: url) { [weak self] data, response, error in
        
        
        defer {
            self?.dataTask = nil
        }
        
        if let error = error {
            self?.errorMessage += "Default Task Error: " + error.localizedDescription + "\n"
        } else if
            let data = data,
            let response = response as? HTTPURLResponse,
            response.statusCode == 200 {
            
            self?.updateRestults(data)
            
            DispatchQueue.main.async {
                completion(self?.location, self?.errorMessage ?? "")
            }
        }
        else if let res = response as? HTTPURLResponse,
                res.statusCode == 404{
            self?.errorMessage = "City Not Found"
            DispatchQueue.main.async {
                
                completion(self?.location, self?.errorMessage ?? "Not Found")
            }
            
            //print("City Not found")
        }
    }
    dataTask?.resume()
}

But I get the error Unexpectedly found nil while unwrapping an Optional value. But if I use a hard-coded string it works fine. What am I doing wrong here? Since the cities array is of type String,

Answer 1

The issue here is that the string "New York" which has spaces in it, and those spaces are put directly into the URL, which causes the initialization of the URL to fail. Spaces are not valid in URLs without first encoding them to %20.

You should change the line let cities: [String] = ... to var cities: [String] = ... and add this right line underneath it:

cities = cities.map { $0.replacingOccurrences(of: " ", with: "%20") }

This will replace every space in the city names with a %20 and store it back into the variable cities.