java - 循环仅打印 else 语句

Question

我正在开展某种数学培训计划。我正在尝试实现一个 for 循环，以便用户选择的每个问题类型都会生成三个问题。如果用户输入正确的变量类型，这将非常有效。但是，如果用户输入错误的变量/变量类型，则会发生奇怪的循环。这很难解释，但如果你获取代码并尝试给它提供错误的输入，你很快就会发现。我不确定这是如何引起的，所以我想我应该在这里发帖。我已注释掉，以便只有一种操作类型(添加)可用，仅用于调试目的。如果您输入的内容与程序期望的内容不同，您将看到。我对循环还很陌生，并且已经尽力尝试了多种想法，但还是不够。谢谢!

    // Welcome
    System.out.println("Hello and welcome to the Math Trainer!\n======================================");
    System.out.println("Which math operation would you like to practice?");

    // Print options to select from
    System.out.println("    " + "[A]ddition"); 
    System.out.println("    " +"[S]ubtraction"); 
    System.out.println("    " + "[M]ultiplication");
    System.out.println("    " + "[D]ivision");
    System.out.println("    " + "[R]emainder");

    // Ask for user input on which to choose
    System.out.print("Enter your choice:" + " ");
    String userLetter = stdin.nextLine();

    // Calculate random values from seed and shift them within range
        for(int count = 0; count < Config.NUMBER_OF_QUESTIONS; count++){
        int ran1 = randGen.nextInt(Config.MAX_VALUE - Config.MIN_VALUE + 1);
        int ran2 = randGen.nextInt(Config.MAX_VALUE - Config.MIN_VALUE + 1);
        int ran1Shift = ran1 + Config.MIN_VALUE;
        int ran2Shift = ran2 + Config.MIN_VALUE;

        // Initialize different answers per operation
        double additionAnswer = (double)ran1Shift + ran2Shift;
        double subtractionAnswer = (double)ran1Shift - ran2Shift;
        double multiplicationAnswer = (double)ran1Shift * ran2Shift;
        double divisionAnswer = (double)ran1Shift / ran2Shift;
        double remainderAnswer = (double)ran1Shift % ran2Shift;



    // Prompt user with a question based upon random numbers and operation selection
    // Presentation of addition problems
    if(userLetter.equalsIgnoreCase("a")) {
        System.out.print("What is the solution to the problem:" + " " + ran1Shift + " " + "+" + " " + ran2Shift + " = ");
        if (stdin.hasNextDouble()) {
            double userNum = stdin.nextDouble();
            if (userNum == additionAnswer) {
                System.out.println("That is correct!");
            } else {
                System.out.println("The correct solution is: " + additionAnswer + ".");
            }
        } else {
            System.out.println("All solutions must be entered as decimal numbers.");
            System.out.println("The correct solution is " + additionAnswer + ".");
            }
        }

    else{
    System.out.println("I'm sorry, I only understand choices of: A, S, M, D, or R!");
    }
}

    // Program exit
    System.out.println("======================================");
    System.out.println("Thank you for using the Math Trainer!");
}

}

Answer 1

If I enter 'z' instead of a, s , m, d, or r, for example, the program prints the else statement 3 times.

假设3次是因为Config.NUMBER_OF_QUESTIONS是3，那是因为你执行String userLetter = stdin.nextLine(); outside 循环，因此 userLetter 的值永远不会改变。

如果修复代码的缩进，使 for 循环的范围变得清晰，您会发现需要在循环内移动以下行:

// Ask for user input on which to choose
System.out.print("Enter your choice:" + " ");
String userLetter = stdin.nextLine();

<小时/>

原始答案

hasNextDouble() 不会消耗任何内容，因此当您提出问题并且用户回答我不知道而不是输入数字时，文本会保留.

当您提出下一个问题时，上一个问题的(错误)答案仍在缓冲区中，并且系统下一个 hasNextDouble() 调用也会失败，依此类推，并且等等，...

这是人们使用扫描仪的主要缺陷之一。他们忘记了错误处理。

在这种情况下，每行给出一个答案，因此每次阅读答案时，都应该在之后调用 nextLine()，以丢弃答案后的任何额外文本，或者丢弃如果给出错误的输入，则整行。