在do-while循环Java中将变量重置为零

Question

I am working on an example using a do-while loop and switch statement. What I basically need is to accumulate numbers and depending on user input either add, substract, multiply or divide (mini calculator type).

The problem is when I ask the user to go back to the main menu the program does not reset the value as it is before the loop. The result is always the previous result.

Here is the code, it will explain it better.

import java.util.Scanner;
public class SwitchLoopNumbers{
public static void main(String[] args){

  Scanner scan = new Scanner(System.in);
  int numbers=0;
  int result=0;
  int option;
  boolean quit = true;
  String done="";
  do{
     System.out.println("CALCULATOR MENU");
     System.out.println("********** ****");
     System.out.println("\n1. Add");
     System.out.println("2. Substract");
     System.out.println("3. Multiply");
     System.out.println("4. Divide");

     System.out.println();
     System.out.print("Enter your option >> ");
     option = scan.nextInt();

     while(quit){

        switch(option){

           case 1: 

              System.out.print("Enter numbers, type 0 when done >> ");
              numbers = scan.nextInt();
              if(numbers==0)
              quit=false;
              result +=numbers;
              break;




           case 2:
              System.out.print("Enter numbers, type 0 when done >> ");
              numbers = scan.nextInt();
              result -=numbers;
              if(numbers==0)
              quit=false;
              break;

        }

     }





     System.out.println("The total is: "+result);
     System.out.println("Back to main menu ? y/n ");
     scan.nextLine();
     done = scan.nextLine(); 
     //I did reset numbers and result here to zero but it did not work
  }



  while("y".equalsIgnoreCase(done)); 
  System.out.println("Thank you for using calculator");

}
}

Answer 1

A couple things are going on here. To answer your question concisely, it's because you didn't reassign your variables before re-looping. Since you don't reassign result and quit, quit is false so it closes the loop, and result is unchanged so it then prints the same result. Try this:

 System.out.println("The total is: "+result);
 System.out.println("Back to main menu ? y/n ");
 scan.nextLine();
 done = scan.nextLine(); 
 numbers = 0;
 result = 0;
 quit = true;

I think it's the most straight-forward solution to your problem.

EDIT: I also wanted to add that using quit as the while condition seems a little counter-intuitive. If I saw a condition quit that was true, my assumption would be that it would break the loop, not continue it. You might make your code a bit clearer by designating more meaningful variable names. So instead of saying something like:

 boolean quit = true;
 while(quit) {
     //do stuff
     if (some_condition) {
         quit = false;
         //close loop
     }
 }

This may be a little clearer:

boolean quit = false;
 while(!quit) {
     //do stuff
     if (some_condition) {
         quit = true;
         //close loop
     }
 }

Just a general suggestion.

Answer 2

You can try to call main() again, but I'm not sure if it will work, solution can be make your own method eg. init() - where you will set vars into init state, and eg. work() , what will be remaining code :D

EDIT: you can make it this way

      import java.util.Scanner;

        public class main {
    //if you want work with result after user will write "y" in the end
            static int result = 0;

            public static void main(String[] args) {
                Scanner scan = new Scanner(System.in);
                int numbers = 0;

                int option;
                boolean quit = false;
                String done = "";
    //int result = 0; // if you want also init result 



                    // menu
                    System.out.println("CALCULATOR MENU");
                    System.out.println("********** ****\n");
                    System.out.println("1. Add");
                    System.out.println("2. Substract");
                    System.out.println("3. Multiply");
                    System.out.println("4. Divide");

                    // user menu input read
                    System.out.println();
                    System.out.print("Enter your option >> ");
                    option = scan.nextInt();

                    switch (option) {
                    case 1:
                        while (!quit) {
                            System.out.print("Enter numbers, type 0 when done >> ");
                            numbers = scan.nextInt();
                            if (numbers == 0) {
                                quit = true;
                            }
                            result += numbers; // result = result + numbers
                        }

                        break;
                    case 2:
                        while (!quit) {
                            System.out.print("Enter numbers, type 0 when done >> ");
                            numbers = scan.nextInt();
                            result -= numbers; // result = result - numbers
                            if (numbers == 0) {
                                quit = true;
                            }
                        }
                        break;
                    default:
                            System.out.println("Bad inpout");
                        break;
                    }


                System.out.println("The total is: " + result);
                System.out.println("Back to main menu ? y/n ");
                scan.nextLine();
                done = scan.nextLine();

//recursive call - run main() again 
                if (done.equals("y")) {
                    main(args);
                } else {
                    System.out.println("Thank you for using calculator");
                }

            }

        }