Swift Convert Optional String to Int or Int32 (Unwrapping optionals question)

Question

I am trying to read a string and convert it to an int. I have a solution but it seems way too complicated. I guess I am still trying to wrap my head around unwrapping.

I have posted code below along with the compiler errors that I get with each solution.

In this example I try to read a string from UserDefaults and convert to an integer value.

static func GetSelectedSessionNum() -> Int32 {
    var sessionNum : Int32 = 0
    let defaults = UserDefaults.standard

    let optionalString: String? = defaults.string(forKey: "selectedSessionNum")
    // this works but it's too complicated
    if let string = optionalString, let myInt = Int32(string) {
        return myInt
    }
    return 0

    // Error : optional String? must be unwrapped to a value of type 'String'
    let t : String = defaults.string(forKey: "selectedSessionNum")

    if let s : String = defaults.string(forKey: "selectedSessionNum") {
        // error - Int32? must be unwrapped to a value of Int32
        return Int32(s)
    }
    return 0
}

Answer 1

You need to cast to non optional Int32 in order to match your return type.

You can use any optional binding approach, or change your return type to Int32?

Answer 2

If you want an uncomplicated solution save selectedSessionNum as Int

static func getSelectedSessionNum() -> Int32 {    
    return Int32(UserDefaults.standard.integer(forKey: "selectedSessionNum"))
}

otherwise double optional binding

if let string = UserDefaults.standard.string(forKey: "selectedSessionNum"), let myInt = Int32(string) {
    return myInt
}

or the nil coalescing operator

if let string = UserDefaults.standard.string(forKey: "selectedSessionNum") {
    return Int32(string) ?? 0
}

is the proper way

Answer 3

If you want to avoid optional bindings, you can use flatMap, when called on Optional's it allows you to convert one optional to another:

return UserDefaults.standard.string(forKey: "selectedSessionNum").flatMap(Int32.init) ?? 0

You'd also need the ?? (nil coalescing operator) to cover the scenarios where either the initializer fails, or the value is not present in user defaults.