Stanford Calculator app keeps crashing

Question

I\'m taking the Stanford iPad and iPhone developer course online at Stanford using Swift and working on creating a Calculator application. (Still a bit new to programming.)

Answer 1

"found nil while unwrapping an Optional" means that you have a variable that may or maynot have a value, when you use the operator ! you are telling swift "Trust me there is a value in there" however if there is not swift will throw a exception as the one you just saw.

The best way to avoid this is checking before use:

if let value = display.text{
    //if your code get here value is safe to use
    NSNumberFormatter().numberFromString(value).doubleValue
}