Swift中Optional（“ nil”）的含义是什么？

Question

Here is my code. If defaults did not ever save the lat, lng, then give it a lat lng.

The problem is error:

fatal error: unexpectedly found nil while unwrapping an Optional value

when using self.lat and self.lng after the code below. Every time console print out is

the saved gps is Optional("nil")

What's the meaning for Optional("nil") and the difference from Optional(nil) ?

I am using Xcode 7.1 and Swift 2.

var lat : String = ""
var lng : String = ""

let defaults = NSUserDefaults.standardUserDefaults()
if  defaults.stringForKey("lat") != nil  {
    self.lat = defaults.stringForKey("lat")!
    self.lng = defaults.stringForKey("lng")!
    print ("the saved gps is \(defaults.stringForKey("lat"))")
} else {
    self.lat = "34.7009333"
    self.lng = "135.4942047"
    print ("the init gps is  \(self.lat) , \(self.lng)")
}

Answer 1

it show the actual value...

let digit = sender.currentTitle!

suppose your sender value is 10 then output is 10.

suppose your sender value is nil then app carsh.

it show the optional value

   let digit = sender.currentTitle

suppose your sender value is 10 then output is optional("10").

suppose your sender value is nil then app is not carsh.

& display value like this optional("nil")