How can I get my loop to not reprint its old value?

Question

Right now, when I get "Would you like to continue", if I press y (or Y), the program will just reprint the previous typed in value with the same question afterwards. How can I solve this? Thank you.

class PrimeNumber {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        int value = 0;
        char choice = 0;

        do {
            System.out.println("Enter integer ");
            value = console.nextInt();

            for (int i = 2; i < value; i++) {
                if (value % i == 0) { // checks if value is evenly divisible by any number
                    System.out.println(value + " is not a prime number ");
                } else {
                    System.out.println(value + " is a prime number ");
                }
                System.out.print("Would you like to continue y/n ");
                choice = console.next().charAt(0);
            }
        } while (choice == 'y' || choice == 'Y');
    }
}

Answer 1

You need to address the following things in your code:

1. You need to check only up to the square root of the number. Check Primality test.
2. You should break the loop as soon as it is found that the number is not prime.
3. You need to use console.nextLine() instead of console.nextInt() or console.next(). Check Scanner is skipping nextLine() after using next() or nextFoo()? for more details.

Code incorporating the comments mentioned above:

import java.util.Scanner;

public class PrimeNumber {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        int value = 0, i;
        char choice = 0;
        boolean valid;
        do {
            valid = true;
            System.out.print("Enter integer: ");
            try {
                value = Integer.parseInt(console.nextLine());
                for (i = 2; i <= Math.sqrt(value); i++) {
                    if (value % i == 0) { // checks if value is evenly divisible by any number
                        System.out.println(value + " is not a prime number ");
                        break;
                    }
                }
                if (i > Math.sqrt(value)) {
                    System.out.println(value + " is a prime number ");
                }
                System.out.print("Would you like to continue y/n: ");
                choice = console.nextLine().charAt(0);
            } catch (NumberFormatException e) {
                System.out.println("This is an invalid input. Try again.");
                valid = false;
            }
        } while (!valid || choice == 'y' || choice == 'Y');
    }
}

A sample run:

Enter integer: a
This is an invalid input. Try again.
Enter integer: 2
2 is a prime number 
Would you like to continue y/n: y
Enter integer: 91
91 is not a prime number 
Would you like to continue y/n: n

Answer 2

class PrimeNumber {
public static void main(String[] args) {
    Scanner console = new Scanner(System.in);
    int value = 0;
    char choice = 0;

    do {
        System.out.println("Enter integer ");
        value = console.nextInt();

        for (int i = 2; i < value; i++) {
            if (value % i == 0) { // checks if value is evenly divisible by any number
                System.out.println(value + " is not a prime number ");
                break;
            } if(i==value-1){
                System.out.println(value + " is a prime number ");
            }
             }
            System.out.print("Would you like to continue y/n ");
            choice = console.next().charAt(0);

    } while (choice == 'y' || choice == 'Y');
}
}

Answer 3

You are asking the user to answer y or n while still inside the for loop. Try debugging your app to see how it behaves while you're executing it.

Answer 4

I went through your program and fixed it, including the prime number loop. I made three changes:

1. I moved the end of the do-while loop so that it doesn't repeat things.
2. I fixed the prime number loop. To do this I added a boolean "isPrime" which is set to false whenever a number is found to be non-prime.

3. I changed the ending condition to entering a value of 0 or less. I did this because when I tested it, I kept accidentally entering my next number where it asked me if I wanted to continue.

   import java.util.Scanner;

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

       int value = 0;
       boolean isPrime = true;     // This needs to be set to true before the for loop
   
       do{
           System.out.println("Enter integer ");            
           value = console.nextInt();
   
           // End the program if a value is less than or equal to zero
           if (value <= 0) {
               System.out.println("bye bye");
               break;
           }
   
           isPrime = true;
           for(int i = 2; i < value; i++){
   
               if(value % i == 0){ // checks if value is evenly divisible by any number
                   System.out.println(value + " is not a prime number ");
   
                   // No sense in continuing this if it's not prime
                   isPrime = false;
                   break;
               }
           }
   
           // If the loop goes all the way through, then value is prime
           if (isPrime) {
               System.out.println(value + " is a prime number ");
           }
   
           System.out.println("value = " + value);
       } while (value > 0);
   
       console.close();
   }
   

   }

One thing that may help you is to get an IDE such as BlueJ. One thing that it does well is to show you the blocks of code.