Re: error: Value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?import Founda
NSNumberFormatter.stringFromNumber() is declared to return an optional string (written "String?" in the headers), so that the method can return nil if there's a problem formatting the number. (I believe the main error that can occur is that the value you're formatting might be lower than the minimum or higher than the maximum, but I could be wrong.)
Swift requires you to handle this error case in some way. Adding an "!" tells Swift that you don't think it should be possible for this call to fail, and that if it does, your app should crash. Other options include substituting a default value with the "??" operator or moving the number formatting call into an "if let" or (in Swift 2) "guard let" statement to handle the nil case.