使NSNumberformatter().numberFromString返回可选 [英] Make NSNumberformatter().numberFromString return an optional

问题描述

我是新手.只是想知道如何将下面的代码转换为可选代码.

I'm pretty new to swift. Just wanted to know how to convert the code below to be an optional.

var displayValue:  Double {
    get {
        return NSNumberFormatter().numberFromString(display.text! as NSString)!.doubleValue
         }
    set {
        display.text = "\(newValue)"
        userIsInTheMiddleOfTypeingANumber = false
    }
}

这是来自斯坦福计算器讲座系列.讲师没有显示如何使其可选.我的理解是，当displayValue中有一个不能转换为Double的字符串(例如"Error")时，应用程序崩溃.问题在于displayValue需要显示可以在不同时间转换为Double的字符串.

this is from the stanford calculator lecture series. The lecturer didn't show how to make it optional. My understanding is that when there is a string in displayValue that can't be converted to a Double (like "Error") the app crashes. The problem is displayValue needs to show strings that can and can't be converted to Double at different times.

我知道之前也曾问过类似的问题，但找不到明确的答案.

I know similar questions have been asked before but I can't find a clear answer.

谢谢

推荐答案

无需使其返回可选值.您可以使用nil合并运算符在nil情况下返回0，如下所示:

No need to make it return an optional. You can use the nil coalescing operator to return 0 in case of nil as follow:

return (NSNumberFormatter().numberFromString(display.text) as? Double) ?? 0

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