满足特定条件时添加消息（Java）

Question

Total newbie here, please forgive the silly question. As an exercise I had to make a program (using do and while loops) that calculates the average of the numbers typed in and exits when the user types 0. I figured the first part out :) The second part of the exercise is to change the program to display an error message if users types 0 before typing any other number. Can you kindly explain to me what is the easiest way to accomplish this? If you provide the code is great but I'd also like an explanation so I am actually understanding what I need to do. Thank you! Here is the code:

import java.util.Scanner;

public class totalave1 {
    public static void main(String[] args) {
        int number, average, total = 0, counter = 0;
        Scanner fromKeyboard = new Scanner(System.in);
        do {
            System.out.println("Enter number to calculate the average, or 0 to exit");
            number = fromKeyboard.nextInt();
            total = total + number;
            counter = counter + 1;
            average = (total) / counter;
        } while (number != 0);
        System.out.println("The average of all numbers entered is: " + average);
    }
}

Answer 1

The good news is that you probably have done the hardest part. :) However, I don't want to give too much away, so...

Have you learned about control flow? I assume you might have a little bit, as you are using do and while . I would suggest taking a look at the following Java documentation first: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/if.html

Then, look at your current solution and try to think what conditions you have that would lead you to display the error message, using if statements. How do you know the user typed a 0? How do you know it's the first thing they entered? Are there any variables that you have now that can help you, or do you need to create a new one?

I know this is not a code answer, but you did well in this first part by yourself already. Let us know if you need further hand.

Answer 2

The second part of the exercise is to change the program to display an error message if users types 0 before typing any other number.

It is not very clear :

• Do you you need to display a error message and the program stops ?
• Do you you need to display a error message and to force the input to start again ?

In the first case, just add a condition after this instruction : number=fromKeyboard.nextInt(); :

     do{
        System.out.println("Enter number to calculate the average, or 0 to exit");
        number=fromKeyboard.nextInt();
        if (number == 0 && counter == 0){
            System.out.println("Must not start by zero");
            return;
        }
       ...
      }  while (number!=0);

In the second case you could pass to the next iteration to take a new input. To allow to go to next iteration, just change the number from zero to any value different from zero in order that the while condition is true.

     do{
        System.out.println("Enter number to calculate the average, or 0 to exit");
        number=fromKeyboard.nextInt();
        if (number == 0 && counter == 0){
            System.out.println("Must not start by zero");
            number = 1;
            continue;
        }
       ...
      }  while (number!=0);

Answer 3

Here is a simple program that extends on yours but uses nextDouble() instead of nextInt() so that you can enter numbers with decimal points as well. It also prompts the user if they have entered invalid input (something other than a number):

import java.util.Scanner;

class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    System.out.println("Java_Paws's Average of Numbers Program");
    System.out.println("======================================");
    System.out.println("Usage: Please enter numbers one per line and enter a 0 to output the average of the numbers:");
    double total = 0.0;
    int count = 0;

    while(scanner.hasNext()) {
      if(scanner.hasNextDouble()) {
        double inputNum = scanner.nextDouble();
        if(inputNum == 0) {
          if(count == 0) {
            System.out.println("Error: Please enter some numbers first!");
          } else {
            System.out.println("\nThe average of the entered numbers is: " + (total / count));
            break;
          }
        } else {
          total += inputNum;
          count++;
        }
      } else {
        System.out.println("ERROR: Invalid Input");
        System.out.print("Please enter a number: ");
        scanner.next();
      }
    }
  }
}

Try it here!

Answer 4

Don't go down code after reading and if you cant then see the code.

First you have to learn about the flow control . Second you have to check whether user entered 0 after few numbers get entered or not, for that you have to some if condition . If current number if 0 and it is entered before anyother number then you have to leave rest of the code inside loop and continue to next iteration.

import java.util.Scanner;
public class totalave1
{
    public static void main (String[]args)
     {
          int number, average, total=0, counter=0;
          boolean firstTime = true;
          Scanner fromKeyboard=new Scanner (System.in);
          do{
               System.out.println("Enter number to calculate the average, or 0 to exit");
               number=fromKeyboard.nextInt();
               if(firstTime && number==0){
                     System.out.println("error enter number first");
                     number = -1;
                     continue;
               }
               firstTime = false;
               total=total+number;
               counter=counter+1;
               average=(total)/counter;
            } while (number!=0);
    System.out.println("The average of all numbers entered is: "+average);  
    }

}