Question

我正在进行各种数学训练计划。我试图实现一个for循环，以便用户选择的每个问题类型都会产生三个问题。如果用户输入正确的变量类型，这将完美地工作。但是，如果用户输入错误的变量/变量类型，那么它会以奇怪的方式循环。这很难解释，但是如果你接受了代码并尝试给出错误的输入，你很快就会看到。我不确定这是怎么造成的，所以我想我会在这里发帖。我已经注释掉了，只有一种操作类型（添加）可用，仅用于调试目的。如果您键入的内容不是程序所期望的内容。我对循环很陌生，并且在多种想法中尽我所能，但我很短暂。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

如果我输入＆＃39; z＆＃39;例如，程序打印else语句3次，而不是a，s，m，d或r。

假设3次是因为Config.NUMBER_OF_QUESTIONS是3，那是因为你在循环外执行String userLetter = stdin.nextLine(); ，所以userLetter的值永远不会改变。

如果您修复了代码的缩进，那么for循环的范围就会变得清晰，您将看到需要在循环中移动以下行：

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

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复I don't know而不是输入数字时，文字会保留。

当你问下一个问题时，上一个问题的（不好）答案仍然在缓冲区中，系统也会在下一个hasNextDouble()调用失败，依此类推，等等。

这是人们如何使用Scanner的主要缺陷之一。他们忘记了错误处理。

在这种情况下，答案是每行一个，所以每次你阅读答案时，你应该总是在事后调用nextLine()，在答案后丢弃任何额外的文字，或者如果不好则丢弃整行输入已经给出。

Loop仅打印Else语句

1 个答案: