Working on a calculator in swift and I'm stuck - ios

Question

I'm new to coding and just learning now. swift code and I'm running into a snag and can't seem to figure it out, so I've been flowing along with the Stanford YouTube channel on swift coding and they are doing a calculator. Currently we are trying to get double value.
#IBOutlet weak var display: UILabel!
var userIsInTheMiddleOfTypingANumber = false
#IBAction func appendDigit(sender: UIButton) {
let digit = sender.currentTitle!
if userIsInTheMiddleOfTypingANumber {
display.text = display.text! + digit
} else {
display.text = digit
userIsInTheMiddleOfTypingANumber = true
}
}
var operandStack: Array<Double> = Array<Double>()
#IBAction func enter() {
userIsInTheMiddleOfTypingANumber = false
operandStack.append(displayValue)
print("operandStack = \(operandStack)")
}
var displayValue: Double {
get {
return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
// This is my problem and comes back with exc_bad_instruction (code=exc_i387_invop any help would be greatly appreciated on this! thank you for your time!
}
set {
display.text = "\(newValue)"
userIsInTheMiddleOfTypingANumber = false
}
}

Answer 1

In addition to Martin R: Actually you have two big problems and both are related to your unwrapping of optionals.
First is this line:
#IBOutlet weak var display: UILabel!
The problem is the !. An #IBOutlet links to a view in a storyboard. But depending on the stage the GUI is at it may be that this displaywon`t exist yet. So this should be a optional marked with ? not !. Then your compiler would complain about this line:
return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
He would complain that display probably does not exist and that you can`t just do that. You would probably "solve" the problem like this:
return NSNumberFormatter().numberFromString(display!.text!)!.doubleValue
But this will either fail if displayist not ready or of the NSNumberFormatter can`t convert the string to a number. And you will never know why. So stop using forced unwrapping. You have different tools to solve your problem though. But at first you should think about your problem. Your problem is in the end that not every string can be converted to a double. So you have to deal with this at some point. There are countless possibilities to do so. But as a starter, if you have code that will crash if an optional (can be nil value) is actually nil then you can use this pattern:
if let thisValueIsNotNil = dontKnowIfItIsNil {
// Do something with thisValueIsNotNil - it is save
}
Edit: The core problem an mechanism are pretty well explained IMO in this blog post:
http://appventure.me/2014/06/13/swift-optionals-made-simple/