返回到程序顶部重试

我正在自学 C#，当前章节的挑战要求我：

以您的上一个项目为例，创建其他方法来对传入其中的两个数字进行减法、乘法或除法。在除法中，检查第二个数字是否不为0，因为除以0是一个非法的数学概念。如果第二个数字是 0，则返回 0。

现在我写了下面的内容，我相信它满足所有标准。我不确定 IF 语句是否是最佳选择，但它确实有效。我还认为 SWITCH 也能达到这个目的。

那么第一个问题，IF 或 SWITCH 会更好吗？

第二个问题。在 ELSE 中，如果用户没有选择可用选项之一，我会给出通用失败消息，但我想做的是，如果调用 ELSE（不确定正确的术语是什么），我希望程序返回到开头并要求用户重试并显示第一个 Console.Writeline() 要求选择操作员。我知道这不是挑战的一部分，但它似乎是对程序的逻辑补充，并且想知道这是否可能而无需诉诸任何过于复杂的东西。

提前致谢！

        string whichOp;
        int firstNum, secondNum, result;

        Console.WriteLine("What Operator do you wish to use? [A]dd, [S]ubtract, [M]ultiply or [D]ivide?");

        whichOp = Console.ReadLine();

        whichOp = whichOp.ToLower();

        if (whichOp == "a")
        {
            Console.Write("You chose Addition.  Please choose your first number: ");
            firstNum = int.Parse(Console.ReadLine());
            Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
            secondNum = int.Parse(Console.ReadLine());
            result = Add(firstNum, secondNum);
            Console.WriteLine("You chose the number {0}.  {1} plus {2} equals {3}.", secondNum, firstNum, secondNum, result);
        }
        else if (whichOp == "s")
        {
            Console.Write("You chose Subtraction.  Please choose your first number: ");
            firstNum = int.Parse(Console.ReadLine());
            Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
            secondNum = int.Parse(Console.ReadLine());
            result = Sub(firstNum, secondNum);
            Console.WriteLine("You chose the number {0}.  {1} minus {2} equals {3}.", secondNum, firstNum, secondNum, result);
        }
        else if (whichOp == "m")
        {
            Console.Write("You chose Multiplication.  Please choose your first number: ");
            firstNum = int.Parse(Console.ReadLine());
            Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
            secondNum = int.Parse(Console.ReadLine());
            result = Mult(firstNum, secondNum);
            Console.WriteLine("You chose the number {0}.  {1} times {2} equals {3}.", secondNum, firstNum, secondNum, result);
        }
        else if (whichOp == "d")
        {
            Console.Write("You chose Division.  Please choose your first number: ");
            firstNum = int.Parse(Console.ReadLine());
            Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
            secondNum = int.Parse(Console.ReadLine());
            result = Div(firstNum, secondNum);
            Console.WriteLine("You chose the number {0}.  {1} divided by {2} equals {3}.", secondNum, firstNum, secondNum, result);
        }
        else
            Console.WriteLine("FAIL!  You did not choose an available option.");

        Console.ReadLine();
    }

    static int Add(int num1, int num2)
    {
        int theAnswer;

        theAnswer = num1 + num2;

        return theAnswer;
    }

    static int Mult(int num1, int num2)
    {
        int theAnswer;

        theAnswer = num1 * num2;

        return theAnswer;
    }

    static int Sub(int num1, int num2)
    {
        int theAnswer;

        theAnswer = num1 - num2;

        return theAnswer;
    }

    static int Div(int num1, int num2)
    {
        int theAnswer;

        if (num2 == 0)
            return 0;

        theAnswer = num1 / num2;

        return theAnswer;
    }

编辑：我采纳了这里的建议，并用 SWITCH 和 WHILE 重建了程序。有人还说，因为很多代码是相同的，所以我应该能够重用它。我喜欢这个想法，并将研究如何做到这一点。

        var retry = true;
        while (retry)
        {
            retry = false;

            string whichOp;
            int firstNum, secondNum, result;

            Console.WriteLine("What Operator do you wish to use? [A]dd, [S]ubtract, [M]ultiply or [D]ivide?");

            whichOp = Console.ReadLine();

            whichOp = whichOp.ToLower();

            switch (whichOp)
            {
                case "a":
                    Console.Write("You chose Addition.  Please choose your first number: ");
                    firstNum = int.Parse(Console.ReadLine());
                    Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
                    secondNum = int.Parse(Console.ReadLine());
                    result = Add(firstNum, secondNum);
                    Console.WriteLine("You chose the number {0}.  {1} plus {2} equals {3}.", secondNum, firstNum, secondNum, result);
                    Console.ReadLine();
                    break;
                case "s":
                    Console.Write("You chose Subtraction.  Please choose your first number: ");
                    firstNum = int.Parse(Console.ReadLine());
                    Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
                    secondNum = int.Parse(Console.ReadLine());
                    result = Sub(firstNum, secondNum);
                    Console.WriteLine("You chose the number {0}.  {1} minus {2} equals {3}.", secondNum, firstNum, secondNum, result);
                    Console.ReadLine();
                    break;
                case "m":
                    Console.Write("You chose Multiplication.  Please choose your first number: ");
                    firstNum = int.Parse(Console.ReadLine());
                    Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
                    secondNum = int.Parse(Console.ReadLine());
                    result = Mult(firstNum, secondNum);
                    Console.WriteLine("You chose the number {0}.  {1} times {2} equals {3}.", secondNum, firstNum, secondNum, result);
                    Console.ReadLine();
                    break;
                case "d":
                    Console.Write("You chose Division.  Please choose your first number: ");
                    firstNum = int.Parse(Console.ReadLine());
                    Console.Write("You chose the number {0}.  Please choose a second number: ", firstNum);
                    secondNum = int.Parse(Console.ReadLine());
                    result = Div(firstNum, secondNum);
                    Console.WriteLine("You chose the number {0}.  {1} divided by {2} equals {3}.", secondNum, firstNum, secondNum, result);
                    Console.ReadLine();
                    break;
                default:
                    Console.WriteLine("I'm sorry.  {0} is not an available option.  Please try again.", whichOp.ToUpper());
                    retry = true;
                    break;
            }
        }
    }

    static int Add(int num1, int num2)
    {
        int theAnswer;

        theAnswer = num1 + num2;

        return theAnswer;
    }

    static int Mult(int num1, int num2)
    {
        int theAnswer;

        theAnswer = num1 * num2;

        return theAnswer;
    }

    static int Sub(int num1, int num2)
    {
        int theAnswer;

        theAnswer = num1 - num2;

        return theAnswer;
    }

    static int Div(int num1, int num2)
    {
        int theAnswer;

        if (num2 == 0)
            return 0;

        theAnswer = num1 / num2;

        return theAnswer;
    }

(1) switch 通常是表达这种分支的更简洁的方式。 switch 的主要限制是它仅适用于编译时常量值（const 变量、文字字符串、整数、枚举等）。在这种情况下，这似乎不是问题，因此 switch 可能是更清晰、稍短的代码的不错选择。在性能敏感的代码中（这当然不是），单个 switch 可能比一堆 if 更快，因为程序将测试值并直接跳转到正确的情况，而不是针对每个 if 条件进行测试，直到达到匹配。

(2) 一个简单的方法是将整个程序包装在一个循环中：

var retry = true;
while (retry)
    retry = false;
    // your program
    else { // or default: if you're going with switch
        ...
        retry = true;
    }
}