Hi,

I have written a program that accepts a positive integer as input and determines whether or not the number is prime, it all works accept when a number is prime it should display " x is a prime number " and continue to prompt for user input ( loop ) but it will only display " please enter a number"

Could you have a look at my code and see what I am missing or what I have done wrong please?

Thank you!

Here is the code:

import java.io.*;

public class PrimalityTest
{

    public static void main(String[] args)throws IOException{


        // create a keyboard input stream
        BufferedReader stdin = new BufferedReader (new
                             InputStreamReader(System.in));

        // variables to store input
        int Input;



        do
        {

        // prompt for user input
        System.out.print("Please enter a number: ");
        Input = Integer.parseInt(stdin.readLine());

        // test for prime number

        for (int i = 2; i < Input; i++)
            if ((Input % i) == 0)
            {
                System.out.println(Input + " is not a prime number");
               return;
            }
        else   
        System.out.println(Input + " is a prime number");

        // if number is 0 then exit
        if (Input == 0)
        {
        System.out.println("Exiting program...");
        }
        else
        // if number is negative    
        if (Input <= -1)
        {
        System.out.println("Please enter a positive integer.");
        }

               }

        while (Input !=0);

        }
    }

Edited 3 Years Ago by Dani: Formatting fixed

Here is the completed code I worked out. Note - I expanded to a long to enable you to enter large numbers.

import java.io.*;

public class PrimalityTest {

    public static void main(String[] args) throws IOException {

        // create a keyboard input stream
        BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));

        // variables to store input
//     int Input;
        long Input;
        // Added boolean flag
        boolean isprime;

        do {
            // prompt for user input
            System.out.print("Please enter a number: ");
//            Input = Integer.parseInt(stdin.readLine());
            Input = Long.parseLong(stdin.readLine());
            
            isprime = true;    // Set/reset flag

            // test for prime number
            for (long i = 2; i < Input; i++) {
                if ((Input % i) == 0) {
                    System.out.println(Input + " is not a prime number");
                    System.out.println("It is divisable by: " + i);
                    isprime = false;
                    break;  // Break out of loop
                }
            }

            // See if we found a divisor
            if(isprime){
                System.out.println(Input + " is a prime number");
            }
            // if number is 0 then exit
            if (Input == 0) {
                System.out.println("Exiting program...");
            } else
            // if number is negative
            if (Input <= -1) {
                System.out.println("Please enter a positive integer.");
            }

        } while (Input != 0);

    }
}
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