Do While 循环在测试 while 循环之前无法识别扫描仪输入

Question

import java.util.Scanner;

public class Switch {


    private static void case1() {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter side A");
        String a = input.next();

        System.out.println("Enter side B");
        String b = input.next();

        double number1 = Double.parseDouble(a);
        double number2 = Double.parseDouble(b);


        double A2 = number1 * number1;
        double B2 = number2 * number2;

        System.out.println(A2);
        System.out.println(B2);

        double Csq = A2 + B2;

        double C = Math.sqrt(Csq);

        System.out.println("C equals " + C);
        input.close();
    }

    private static void case2() {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter side A");
        String a = input.next();

        System.out.println("Enter side C");
        String c = input.next();

        double number1 = Double.parseDouble(a);
        double number2 = Double.parseDouble(c);


        double A2 = number1 * number1;
        double C2 = number2 * number2;

        System.out.println(A2);
        System.out.println(C2);

        double b1 = C2 - A2;

        double B = Math.sqrt(b1);

        System.out.println("B equals " + B);
        input.close();
    }

    private static void case3() {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter side B");
        String b = input.next();

        System.out.println("Enter side C");
        String c = input.next();

        double number1 = Double.parseDouble(b);
        double number2 = Double.parseDouble(c);


        double B2 = number1 * number1;
        double C2 = number2 * number2;

        System.out.println(B2);
        System.out.println(C2);

        double a1 = C2 - B2;

        double A = Math.sqrt(a1);

        System.out.println("A equals " + A);
        input.close();
    }


    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        String cont = "y";

        do {

            try{
                System.out.println("Welcome to the Pythagorean Theorm Program!!");
                System.out.println("");
                System.out.println("Choose one of the following:");
                System.out.println("");
                System.out.println("1. A2 + B2 = X");
                System.out.println("2. A2 + X = C2");
                System.out.println("3. X + B2 = C2");

                int choice = input.nextInt();

                switch (choice) {

                    case 1: case1();
                            break;
                    case 2: case2();
                            break;
                    case 3: case3();
                            break;
                }
            }
            catch(Exception e) {
                System.out.println("error");
            }

            System.out.println("Do you want to continue?");
            String more = input.next();
        } while (more.equals(cont));



    }
}

我需要在 while 循环之前进行用户输入，这样它就会知道程序是否要继续。问题是while 条件里面的更里面看不到扫描仪之前输入的一行。有什么建议吗？

Answer 1

将 String more 声明放在 while 循环之外：

`public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        String cont = "y";
        String more = null;


        do {

            try{
                System.out.println("Welcome to the Pythagorean Theorm Program!!");
                System.out.println("");
                System.out.println("Choose one of the following:");
                System.out.println("");
                System.out.println("1. A2 + B2 = X");
                System.out.println("2. A2 + X = C2");
                System.out.println("3. X + B2 = C2");

                int choice = input.nextInt();

                switch (choice) {

                    case 1: case1();
                            break;
                    case 2: case2();
                            break;
                    case 3: case3();
                            break;
                }
            }
            catch(Exception e) {
                System.out.println("error");
            }

            System.out.println("Do you want to continue?");
            more = input.next();
        } while (more.equals(cont));



    }`

Answer 2

变量more 只在do while 循环的范围内。一种解决方案是在循环外声明更多内容，仅在内部更新其值。

像这样……

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    String cont = "y";
    String more = null;

    do {
        .
        .// code here
        .

        System.out.println("Do you want to continue?");
        more = input.next();
    } while (more.equals(cont));

发生这种情况是因为在 do-while 构造中声明的任何变量都只在同一个循环内的范围内，不能在循环外或在循环条件下访问。

在循环外和 main 内声明变量可以访问 main 方法内的任何位置。

Answer 3

您的问题是变量范围，您在 do 语句中定义了更多的字符串，然后在它之外添加了 while 条件，要修复它，您需要将范围更改为 main 函数的本地，以执行此操作只需在执行之前将其添加到您的代码中

 public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    String cont = "y";
    String more = null;
    do {
    // rest of code

你还需要去掉 "String" 关键字来定义更多的数据类型，但应该就是这样。

尽管如果您的目标是在他们需要再次使用该程序时重复该程序，为什么不将整个 main 函数封装在 while 循环中，而条件取决于末尾的用户输入例如循环

boolean more = True;
While(more = True){
    // your current code
    // then check to see if they want to go again, if not , change more to false
}

希望对你有帮助