Question

我已经制作了一个解决二次方程式的应用程序（只是为了看看能否），但是，如果用户意外地没有为其中一个插槽输入值，我不希望应用程序崩溃。请帮忙。

 @IBAction func solveButton(sender: AnyObject) {

        let a:Double! = Double(textFieldA.text!)      // textfieldA is UITextField
        let b:Double! = Double(textFieldB.text!)    // textfieldB is UITextField
        let c:Double! = Double(textFieldC.text!)   // textFieldC is UITextField

        // This is for if the a value is nil
        if a == nil && b == nil && c == nil{

            errorTextField.text = "Please do not leave an empty value, all quadratic equations must have 3 values!"
            answerLabel.text = " "

            answerLabelNegative.text = " "
            whatEquation.text = " "
            return

        } else {

            let a:Double! = Double(textFieldA.text!)      // textfieldA is UITextField
            let b:Double! = Double(textFieldB.text!)    // textfieldB is UITextField
            let c:Double! = Double(textFieldC.text!)   // textFieldC is UITextField
            let x:Double = (b * b)
            let y:Double = -4 * a * c //Here I get EXC_BAD_INSTRUCTION error
            let xy:Double = x + y
            let az:Double = sqrt(xy)
            let finalTopPlus:Double = -b + az
            let finalTopMinus:Double = -b - az
            let aTwo:Double = 2 * a
            let finalEquationPositive:Double = finalTopPlus / aTwo
            let finalEquationNegative:Double = finalTopMinus / aTwo

            answerLabel.text = "Positive x = \(finalEquationPositive)"
            answerLabelNegative.text = "Negative x = \(finalEquationNegative)"

            let aValue = a < 0 ? "-" : " "
            let bValue = b < 0 ? " " : "+"
            let cValue = c < 0 ? " " : "+"
            whatEquation.text = "Equation: \(aValue) \(a)x² \(bValue) \(b)x \(cValue) \(c)"
        }

Answer 1

您的if语句会检查所有值是否为nil。您想查看其中是否有nil。

将if更改为：

if a == nil || b == nil || c == nil {

我没有在Swift中编程，但我得到的印象是，您应该进行许多其他改进，以验证值不是nil。

Answer 2

在Swift中，您应该仔细考虑是否要强制解包（！）值或有条件地解包值（？） - 强制解包nil会给您一个运行时异常。

例如，选择您的第一行。这应该是：

let a = Double(textFieldA.text!)

这将导致a成为Double? - 即它是可选的，因此它可能是零。强制解包textFieldA.text值是可以的，因为这将由您的故事板设置（或者您创建文本字段），此处的零值表示您需要调查和解决的严重错误。

一旦你有选项，你可以使用Swift guard语句来检查nil。使用guard语句后，您可以放心使用！强制拆开价值，因为你知道它们不是。

@IBAction func solveButton(sender: AnyObject) {

    guard let a = Double(textFieldA.text!), 
          let b = Double(textFieldB.text!),
          let c = Double(textFieldC.text!) else {
        errorTextField.text = "Please do not leave an empty value, all quadratic equations must have 3 values!"
        answerLabel.text = " "
        answerLabelNegative.text = " "
        whatEquation.text = " "
        return
    }

    let x = (b * b)
    let y = -4 * a * c 
    let xy = x + y
    let az = sqrt(xy)
    let finalTopPlus= -b + az
    let finalTopMinus = -b - az
    let aTwo= 2 * a
    let finalEquationPositive = finalTopPlus / aTwo
    let finalEquationNegative = finalTopMinus / aTwo
    answerLabel.text = "Positive x = \(finalEquationPositive)"
    answerLabelNegative.text = "Negative x = \(finalEquationNegative)"

    let aValue = a < 0 ? "-" : " "
    let bValue = b < 0 ? " " : "+"
    let cValue = c < 0 ? " " : "+"
    whatEquation.text = "Equation: \(aValue) \(a)x² \(bValue) \(b)x \(cValue) \(c)"
}

请注意，在Swift中，您只需要定义变量或常量的类型（如果无法推断）。 Double()返回一个可选的Double，添加两个双打的结果是Double，因此您不需要声明该常量将是Double。

在我的方程求解程序中，输入nil值会崩溃应用程序

2 个答案: