Stanford calculator app crashes with error "unexpectedly found nil" - ios

Question

I'm new to programming and I've started taking the stanford course on iTunes U for making an iPhone 8 app. They're using Xcode 6 and Swift 1 while I'm in El Capitan using Xcode 7 and Swift 2. I've found a few differences in code that Xcode has been able to pick up on and help me correct ("println" is now "print" for example), but I'm getting tripped up on one particular part of the code:
var displayValue: Double {
get {
return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
}
set {
display.text = "\(newValue)"
userIsInTheMiddleOfTypingANumber = false
}
}
I've double checked several times to make sure this is exactly how the teacher wrote it. His built correctly and functioned correctly, while mine builds correctly, but shows this fatal error when I try to operate it, "unexpectedly found nil while unwrapping an Optional value" (see screenshot for all the details).
Screenshot of the error
I've been looking around the internet and found a few similar examples, including one on this site (Stanford Calculator app keeps crashing), but after trying everything that was suggested I concluded that something must be unique in my system or I'm operating in a new Xcode/Swift environment than the others that had this problem. None of the solutions have resolved the problem, and all of them added errors of their own.
In responding to someone else's question someone suggested that we use this code to ensure that if "nil" is returned by "display" that it will provide "0" instead:
return (NSNumberFormatter().numberFromString(display.text) as? Double) ?? 0
I tried this, but it only gave me more errors, some seem to be related to Swift 2 (it required a comma after double, wanted me to unwrap display.text, and complained that there was an expected expression missing—maybe the suggested code was good in Swift 1??).

Answer 1

I've double checked several times to make sure this is exactly how the teacher wrote it. His built correctly and functioned correctly, while mine builds correctly, but shows this fatal error when I try to operate it, "unexpectedly found nil while unwrapping an Optional value"
I suspect that display is an IBOutlet property that needs to be connected to something in the user interface, probably a text field. If it's not connected, then even though your code is exactly the same, you'll get nil when you try to use its text property, and unwrapping that will cause the error you're seeing.
Whether or not the advice above actually solves your problem, what you really need to do is to set a breakpoint a line or two before the spot where the crash occurs and step through the code. Look at the variables involved and figure out where that nil value is coming from. You can work backward from there and figure out why the thing that you expect not to be nil is, in fact, nil. Learning to work that way will help you work out these kinds of problems when they occur (and they will occur again).

Answer 2

Not sure if you've found the answer or not by now, but I ran into a similar problem this morning and thought I'd share what I found.
While debugging, I entered two console logs to a simplified version of my operate like so:
case "x": if operandStack.count >= 2 {
print(" display before popping is: \(display.text!) ")
displayValue = operandStack.removeLast() * operandStack.removeLast()
print(" display after popping is: \(display.text!) ")
enter()
}
Display after popping came up as "newValue". I couldn't figure out what that meant at first, but realized that my issue is the setter. newValue is an optional that should be unwrapped i.e. "(newValue)!"
P.S. I opted to return:
return (display.text! as NSString).doubleValue
in my get.
Also, since newValue is unwrapped, keep in mind it will crash if display is set to nil.