"Initialization of immutable value never used" but is actually used - swift2

Question

I keep getting this error of value is never used. I understand this error pops up with Swift 2.2 often, and it is because the value that is initialized is not used. BUT, I do use this value, and this error pops up 3 other times on errors that I do use and I don't know why I still get it.
Below is the code. "Difficulty" is he variable that the compiler says is not used, but as you can see from my code, it is in fact used. Anyone know why this happens?
class SettingsController: UIViewController {
// MARK: Properties
// Preferences for difficulty level of questions
let preferences = NSUserDefaults.standardUserDefaults()
let difficultyKey = "Difficulty"
let questionnumKey = "QuestionNum"
var difficulty: String = "EASY"
#IBOutlet weak var Easy: DLRadioButton!
#IBOutlet weak var Medium: DLRadioButton!
#IBOutlet weak var Hard: DLRadioButton!
override func viewDidLoad() {
super.viewDidLoad()
readUserDefaults()
setDifficulty()
}
func readUserDefaults(){
let difficulty = preferences.stringForKey(difficultyKey) // <--Error
}
func setDifficulty(){
if difficulty == "HARD"{
Hard.selected = true
}
else if difficulty == "MEDIUM"{
Medium.selected = true
}
else{
Easy.selected = true
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}

Answer 1

In readUserDefaults() it should be
difficulty = preferences.stringForKey(difficultyKey)
You need to remove the let: you already have created the difficulty variable earlier.
You also need to use ??, the "nil coalescing operator": preferences.stringForKey(difficultyKey) ?? "EASY", for example, to give a value even if the method call returns nil.
Note: answer made from the comments by #eric-d and #leo-dabus.