java - Java 新手，Do-While 循环问题

Question

我对 Java 比较陌生，在运行这个 do-while 循环时遇到一些麻烦。

问题1

Write a program that allows the user to convert a temperature given in degrees from either Celsius to Fahrenheit or Fahrenheit to Celsius. Use the following formulas: Degrees_C = 5 (Degrees_F − 32) / 9 Degrees_F = (9 (Degrees_C) / 5) + 32 Prompt the user to enter a temperature and either a C or c for Celsius or an F or f for Fahrenheit. Convert the temperature to Fahrenheit if Celsius is entered, or to Celsius if Fahrenheit is entered. Display the result in a readable format. If anything other than C, c, F, or f is entered, print an error message and stop.

问题2

Let’s continue Problem 1, but use a loop so the user can convert other temperatures. If the user enters a letter other than C or F—in either uppercase or lowercase—after a temperature, print an error message and ask the user to reenter a valid selection. After each conversion, ask the user to type Q or q to quit or to press any other key to repeat the loop and perform another conversion

public class Lab_4 {

    public static void main(String arg[]) {
        Scanner input = new Scanner(System.in);
        double F; //Fahrenheit
        double C; //Celsius
        String method; 
        boolean keepGoing = true; 
        do {            
            System.out.println("Choose a method:  ");
            System.out.println("(F)  Fahrenheit to Celsius.  ");
            System.out.println("(C)  Celsius to Fahrenheit.  ");
            System.out.println("(Q)  Exit the loop.  ");
            method = input.nextLine();
            if (method.charAt(0) == 'f' || method.charAt(0) == 'F') {
                System.out.println("Method F");
                System.out.println("Enter the temperature in Fahrenheit:  ");
                F = input.nextDouble();
                C = 5 * (F - 32) / 9;
                System.out.println("Temperature in Celsius:  " + C); }
            if (method.charAt(0) == 'c' || method.charAt(0) == 'C') {
                System.out.println("Method C");
                System.out.println("Enter the temperature in Celsius:  ");
                C = input.nextDouble();
                F = (9 * C / 5) + 32;
                System.out.println("Temperature in Fahrenheit:  " + F); }
            if (method.charAt(0)== 'q' || method.charAt(0)== 'Q') {
                keepGoing = false; 
                System.out.println("Exiting the loop!  "); }
            else {
                //if index 0 doesn't equal to C, F, Q, it's out of range. 
                System.out.println("Method is out of range!  ");
            }
        }while(keepGoing = true);
        input.close();
    }
}

循环将继续进行，直到我输入 Q 或 q 退出。所以我必须输入 Q 才能退出循环，但在获得转换值后我立即收到一条错误消息，它根本不运行循环。该程序只是不执行 while 循环。

enter image description here

尝试2，仍然错误

package Lab_4;

import java.util.Scanner;

    public class Lab_4 {

        public static void main(String arg[]) {
            Scanner input = new Scanner(System.in);
            double F; //Fahrenheit
            double C; //Celsius
            String method; 
            boolean keepGoing = true; 
            do {            
                System.out.println("Choose a method:  ");
                System.out.println("(F)  Fahrenheit to Celsius.  ");
                System.out.println("(C)  Celsius to Fahrenheit.  ");
                System.out.println("(Q)  Exit the loop.  ");
                method = input.nextLine();
                if (method.charAt(0) == 'f' || method.charAt(0) == 'F') {
                    System.out.println("Method F");
                    System.out.println("Enter the temperature in Fahrenheit:  ");
                    F = input.nextDouble();
                    C = 5 * (F - 32) / 9;
                    System.out.println("Temperature in Celsius:  " + C); }
                else if (method.charAt(0) == 'c' || method.charAt(0) == 'C') {
                    System.out.println("Method C");
                    System.out.println("Enter the temperature in Celsius:  ");
                    C = input.nextDouble();
                    F = (9 * C / 5) + 32;
                    System.out.println("Temperature in Fahrenheit:  " + F); }
                else if (method.charAt(0)== 'q' || method.charAt(0)== 'Q') {
                    keepGoing = false; 
                    System.out.println("Exiting the loop!  "); }
                else {
                    //if index 0 doesn't equal to C, F, Q, it's out of range. 
                    System.out.println("Method is out of range!  ");
                }
            }while(keepGoing);
            input.close();
        }
    }

Answer 1

您有一个拼写错误:

}while(keepGoing = true);

应该是:

}while(keepGoing == true);

或者只是:

}while(keepGoing);

说明

keepGoing = true ，即使在条件内，也会分配 keepGoing至true并返回其值(在本例中始终为 true)。

另一方面，keepGoing == true要求比较keepGoing和true值(value)相等。