Unary operator '--' cannot be applied to an operand of type '@lvalue Int?' (aka '@lvalue Optional')

Question

I just updated my app's code to the latest version of Swift and I have this function:

func setupGraphDisplay() {

        //Use 7 days for graph - can use any number,
        //but labels and sample data are set up for 7 days
        //let noOfDays:Int = 7

        //1 - replace last day with today's actual data
        graphView.graphPoints[graphView.graphPoints.count-1] = counterView.counter

        //2 - indicate that the graph needs to be redrawn
        graphView.setNeedsDisplay()

        maxLabel.text = "\((graphView.graphPoints).max()!)"
        print((graphView.graphPoints).max()!)

        //3 - calculate average from graphPoints
        let average = graphView.graphPoints.reduce(0, +)
            / graphView.graphPoints.count
        averageWaterDrunk.text = "\(average)"

        //set up labels
        //day of week labels are set up in storyboard with tags
        //today is last day of the array need to go backwards

        //4 - get today's day number
        //let dateFormatter = NSDateFormatter()
        let calendar = Calendar.current
        let componentOptions:NSCalendar.Unit = .weekday
        let components = (calendar as NSCalendar).components(componentOptions,
            from: Date())
        var weekday = components.weekday

        let days = ["S", "S", "M", "T", "W", "T", "F"]

        //5 - set up the day name labels with correct day
        for i in (1...days.count).reversed() {
            if let labelView = graphView.viewWithTag(i) as? UILabel {
                if weekday == 7 {
                    weekday = 0
                }
                labelView.text = days[(weekday--)!]
                if weekday! < 0 {
                    weekday = days.count - 1
                }
            }
        }
    }

However I am getting an error message at the following line:

labelView.text = days[(weekday--)!]

Where Xcode is giving me the following error:

Unary operator '--' cannot be applied to an operand of type '@lvalue Int?' (aka '@lvalue Optional<Int>')

I tried searching online for answers but I still couldn't find anything that would help me fix this error.

I was also curious what the error message exactly means by type @lvalue Int (aka '@lvalue Optional<Int>')? I have never seen this data type before and don't know how to solve the problem accordingly.

Thanks in advance.

`--` and `++` have been removed from Swift. Use `weekday -= 1` on the line immediately above. — par, Feb 02 '17 at 21:59
You're going to have to learn to stop using the C-style increment/decrement, also C-style for loops — Pierce, Feb 02 '17 at 22:02

oroom · Accepted Answer · 2017-02-02T22:09:56.637

43

Answer is very simple. ++ and -- was removed from Swift 3. But += and -= remained

About Optional<Int> this is longer version for Int? definition. In Swift Optional defined as public enum Optional<Wrapped> : ExpressibleByNilLiteral

edited Feb 02 '17 at 22:09

answered Feb 02 '17 at 22:04

oroom

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This is yet another hideous error message. If the operators don't exist, why is it complaining about the type they're applied to (which implies that they do exist for some types) rather than just plain stating that the operator isn't valid? – Jules Jul 20 '18 at 15:09

Unary operator '--' cannot be applied to an operand of type '@lvalue Int?' (aka '@lvalue Optional')

1 Answers1