How to convert string to array in Swift 3

I don't know about the actual type of the "option" in your code, so I set up a fake Elem struct to represent it. The remaining logic is independent of the type as long as you provide a conversion logic to and from String.

struct Elem {
    // let's say this is your element type in your array
    let foo: Int;
}

extension Elem: CustomStringConvertible {
    var description: String {
        // provide a logic to convert your element to string
        return "\(foo)";
    }
}

let arrayToSave = [
    Elem(foo: 1),
    Elem(foo: 2),
    Elem(foo: 3)
]

extension Elem {
    init(string: String) {
        // provide a function to construct your element type from a string
        self.init(foo: Int(string)!)
    }
}

let stringToSave = arrayToSave.map { $0.description }.joined(separator: "|")

// save this string

// at some point retrieve it from database, which hopefully same as the saved one

let retrivedString = stringToSave;

let retrivedArray = retrivedString.split(separator: "|").map { Elem(string: String($0)) }

print(retrivedArray) // [1, 2, 3]

Here below is my array inserting into database (TestQuestion) as a String :

let testQuestionModel : TestQuestion = NSEntityDescription.insertNewObject(forEntityName: "TestQuestion", into: AppDelegate.getContext()) as! TestQuestion
testQuestionModel.optionsArray = "\(question["options"] as! NSArray)"

No, and No.
You are using -description method of an array to save it. Clearly no. What's wrong? Apple can't affirm that in next OS release, it won't add an extra character. In some more complex cases, it's added <NSArray <0x address> or stuff similar like that.

Suggestion 1:
Modify your entity to have an ARRAY (or usually a Set) of String. Learn about Core-Data relationship (but that's clearly a DataBase basic knownledge). A relationship one to many should be the thing to do.You could even keep in memory what were the choices, by adding for creating the entity Options, with a String property name (name of the option), another one boolean isChecked, etc.

Suggestion 2:
If you have a limited number of options (like says one to 5), add 5 options string to your entity, and iterate to set them

testQuestionModel.option1 = question["option"][0]
testQuestionModel.option2 = question["option"][1] (if it's of course not out of range for the array)
...

Suggestion 3: Not really recommended (in my opinion it's missing the whole advantage of the database, especially fetch and predicates, on previous sample you could fetched easily which options were checked), but if you still want to save them as a String, save them as JSON (ie. stringified). In pseudo code (I'm not sure about the exact syntax, there are no fail safe like try/catch, optional/wrapping):

let options = questions["options"] as [String]
let jsonData = JSONSerialization.data(withJSONObject: (question["options"], options:[])
let jsonString = String.init(data:jsonData encoding:.utf8)

To retrieve them:

let options = JSONSerialization.jsonObject(with data: myJSONString.data(encoding:.utf8), options:[]) as [String]

done using Library SwiftyJSON.

 if let dataFromString = yourString?.data(using: String.Encoding.utf8, allowLossyConversion: false) {
        do{
            let json = try JSON(data: dataFromString)
            print(json)
            let arrayValue = json.rawValue as! NSArray

            print(arrayValue)
        } catch{
            print(error.localizedDescription)

        }
    }

Source: https://github.com/SwiftyJSON/SwiftyJSON