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swift - 一元运算符 '--' 不能应用于 '@lvalue Int?' 类型的操作数(又名 '@lvalue Optional' )

转载 作者：IT王子 更新时间：2023-10-29 05:12:59 26 4
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我刚刚将我的应用程序代码更新到最新版本的 Swift，并且我有这个功能:

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
                }
            }
        }
    }

但是我在以下行收到一条错误消息:

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

Xcode 给我以下错误:

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

我尝试在线搜索答案，但仍然找不到任何可以帮助我修复此错误的信息。

我也很好奇 @lvalue Int (aka '@lvalue Optional<Int>') 类型的错误消息到底意味着什么？我以前从未见过这种数据类型，也不知道如何相应地解决问题。

提前致谢。

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