Swift calculate value of UITextFields and return v

问题:

Using swift and Xcode 6, I'm trying to return a value calculated based on content of few UITextFields.

I have declared variables

var Number1 = Field1.text.toInt()
var Number2 = Field2.text.toInt() 

var Duration = Number1*Number2

Mylabel.text = String ("\(Duration)")

The idea is to capture duration from few UI Fields and based on calculation of those values assign that to a variable as well as display it on a label.

In line: var Duration = Number1*Number2

Challenge is that I have is that when performing multiplication Xcode highlight error: Value of optional type 'Int?' not unwrapped; did you mean to use '!' or '?'?

回答1:

The toInt() method returns an optional value because the string it is trying to convert may not contain a proper value. For instance, these strings will be converted to nil: "house", "3.7","" (empty string).

Because the values may be nil, toInt() returns an optional Int which is the type Int?. You can't use that value without unwrapping it first. That is why you are getting the error message. Here are two safe ways to handle this:

You need to decide what you want to do when a value can't be converted. If you just want to use 0 in that case, then use the nil coalescing operator (??) like so:

let number1 = field1.text.toInt() ?? 0
// number1 now has the unwrapped Int from field1 or 0 if it couldn't be converted

let number2 = field2.text.toInt() ?? 0
// number2 now has the unwrapped Int from field2 or 0 if it couldn't be converted

let duration = number1 * number2

mylabel.text = "\(duration)"

If you want your program to do nothing unless both fields have valid values:

if let number1 = field1.text.toInt() {
    // if we get here, number1 contains the valid unwrapped Int from field1

    if let number2 = field2.text.toInt() {
        // if we get here, number2 contains the valid unwrapped Int from field2

        let duration = number1 * number2
        mylabel.text = "\(duration)"
    }
}

So, what did the error message mean when it said did you mean to use a !. You can unwrap an optional value by adding a ! to the end, but you must be absolutely sure the value is not nil first or your app will crash. So you could also do it this way:

if number1 != nil && number2 != nil {
    let duration = number1! * number2!
    mylabel.text = "\(duration)"
}     

回答2:

Get the value of textFields in var, multiply them, and then set the value of label. Try this:

var Number1 = Field1.text.toInt()
var Number2 = Field2.text.toInt() 

var Duration = Number1 * Number2

Mylabel.text = NSString(format:"%d",Duration!)as String;

Hope this helps.. :)