在java中的while循环之前添加程序退出

Question

I can't get the program to stop and display the message below if a -1 is entered to kill the loop before it starts. The program will also, not flow in to the while loop if a different integer is entered. I seem to have misplaced my curly brackets somewhere because the last 2 get errors.

public static void main(String[] args)
{
    double score;
    double total = 0.0;
    double average;
    int scoreCount = 0;

    // create the Scanner object. Name it stdin
    Scanner stdin = new Scanner(System.in);

    // title at the top of the output
    System.out.println (" score report");;

    do{
    //   read the first score
        System.out.printf ("Enter a score  (1-100, -1 to quit)"
            + ": ", scoreCount);
        score = stdin.nextDouble();

        if (score == -1)
        {
            System.out.println ("Nothing entered.");
        }           
    while((score = stdin.nextDouble()) != -1.0)
    {              
        if (score<-1 || score>100)
        {
        System.out.println ("Illegal score.  Try again");
        System.out.printf ("Enter a score  (1-100, -1 to quit)"
              + ": ", scoreCount);
        }
        else
        {
            System.out.printf ("Enter a score  (1-100, -1 to quit)"
              + ": ", scoreCount); 
            scoreCount++;
            total += score;
        }        
           // end of for loop
    average = total / scoreCount;       //equation
    System.out.printf ("\nThe average score for %d students is %8.2f\n",
                          scoreCount, average); 
    }
} // end of main    

} // end of class definition

Answer 1

Based on what I understand, the problem has a simple solution. To exit the loop, simply use break :

if (score == -1)
{
    System.out.println ("Nothing entered.");
    break;
}

Also, based on the code, the "do" statement isn't really required. Simply use a regular while loop.

Answer 2

You need a "while" condition at the end of your do loop. This is why it is called a do-while loop. Without the while condition, you should get a compile-time error. Here is an example:

double score;
double total = 0.0;
double average;
int scoreCount = 0;

// create the Scanner object. Name it stdin
Scanner stdin = new Scanner(System.in);

// title at the top of the output
System.out.println ("Score Report");

 do{
  //   read the first score
      System.out.println("Enter a score (0-100 or -1 to quit)" 
            + ": " + scoreCount);
      score = stdin.nextDouble();//Retrieve the score.

      if(score == -1) {
        System.out.println("Bye!");
       }

      if (score<-1 || score>100)//Here is the if statement that makes the user enter another score if it is illegal
      {
        System.out.println("Illegal score.  Try again");
        continue; //A continue statement will start the loop over again from the top!
      }
      else if(score >= 0 && score <= 100 && score != -1)
      {

            scoreCount++;
            total += score;
       }        
      // end of for loop
      average = total / scoreCount;       //equation
      System.out.println();
      System.out.printf ("\nThe average score for %d students is %8.2f\n",
                          scoreCount, average); 
      System.out.println();
}while(score != -1); //Runs "while" the score != -1

Here is a sample of the possible output of this program:

Score Report
Enter a score (0-100 or -1 to quit): 0
50.0


The average score for 1 students is    50.00

Enter a score (0-100 or -1 to quit): 1
50.0


The average score for 2 students is    50.00

Enter a score (0-100 or -1 to quit): 2
-90
Illegal score.  Try again
Enter a score (0-100 or -1 to quit): 2
50.0


The average score for 3 students is    50.00

Enter a score (0-100 or -1 to quit): 3
-1
Bye!


The average score for 3 students is    50.00

As you can see here, you if statement changes to:

if (score<-1 || score>100)//Here is the if statement that makes the user enter another score if it is illegal
        {
        System.out.println("Illegal score.  Try again");
        continue;
        }

The continue statement will force the loop to start at the beginning without executing the rest of the code in the loop. This will help you avoid invalid input. At the end of the do loop you also need this while condition:

 }while(score != -1);

Now the loop will only continue so long as the score is not equal to negative 1. If the score is equal to -1, you can also inform the user that they are exiting the program in your code:

if(score == -1) {
            System.out.println("Bye!");
        }

You can change your else statement to an else if statement to execute if the numbers entered are in the range of 0 to 100, or else the -1 entry would be counted as a score:

else if(score >= 0 && score <= 100 && score != -1)
        {

            scoreCount++;
            total += score;
        }        
           // end of for loop

You're also not exiting your loop like you say you are above. The average will be counted every time you iterate through your loop. Put the code that displays the average outside of your loop:

}while(score != -1);
    average = total / scoreCount;       //equation
    System.out.println();
    System.out.printf ("\nThe average score for %d students is %8.2f\n",
                      scoreCount, average);

Now the output will look something like this:

Score Report
Enter a score (0-100 or -1 to quit): 0
50.0
Enter a score (0-100 or -1 to quit): 1
70.0
Enter a score (0-100 or -1 to quit): 2
90.0
Enter a score (0-100 or -1 to quit): 3
-1
Bye!


The average score for 3 students is    70.00

If you want a message to be displayed ONLY when -1 is entered as the first option do:

if(score == -1 && scoreCount == 0) {
            System.out.println("Bye!");
        }

Answer 3

The code as posted will not compile. Here is a correct function to enter a bunch of numbers from 0 to 100 and calculate the average. The program will quit when -1 is entered. I have added comments to the code.

public static void main(String[] args)
{
    double score;
    double total = 0.0;
    double average;
    int scoreCount = 0;

    // create the Scanner object. Name it stdin
    Scanner stdin = new Scanner(System.in);

    // title at the top of the output
    System.out.println (" score report");

    //Request the first score
    System.out.printf ("Enter a score %d  (1-100, -1 to quit)"
            + ": ", scoreCount+1);
    //read the first score
    score = stdin.nextDouble();
    do{
        //check the entered score and decide what to do. 
        if (score == -1)
        {
            System.out.println ("Nothing entered.");
            break;
        } else if (score<-1 || score>100)
        {
            System.out.println ("Illegal score.  Try again");
            System.out.printf ("Enter a score %d  (1-100, -1 to quit)"
                    + ": ", scoreCount+1);
        }
        else
        {
            scoreCount++;
            total += score;
            // The entered score is good ask for the next one
            System.out.printf ("Enter a score %d  (1-100, -1 to quit)"
                    + ": ", scoreCount+1);
        }        
    }while((score = stdin.nextDouble()) != -1.0); // Here we read the input. 
    // end of the program.
    average = total / scoreCount;       //equation
    System.out.printf ("\nThe average score for %d students is %8.2f\n",
            scoreCount, average); 
} // end of main