Using a do while loop to determine a prime number?

Question

0.00/5 (No votes)

See more:

Hi, I have wrote a simple program to determine a prime number using a do while loop but am having trouble getting the correct output. Any help would be appreciated. I have a feeling it is something to do with the updates in my loops?

What I have tried:

C#

public class DoWhile{
    public static void main(String[]args)
    {
        int num=13;
        do
        {
            System.out.println(num + " is a prime number");
            num++;
        }
        while(num%2==1);
        
        if(num%2==1)
        {
        do
        { 
            System.out.println(num+ " is NOT a prime number");
            num++;
        }
        while (num%2==0);
    }
}
}

Posted 5-Nov-16 5:49am

Member 12833355

Updated 11-May-21 9:17am

Add a Solution

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2016-11-05T06:12:00

Solution 1

No, it has to do with that not being a prime number detector algorithm, or even close.
Have a look at this: Sieve of Eratosthenes - Wikipedia[^] and consider trying to implement that.

Posted 5-Nov-16 6:12am

OriginalGriff

Patrice T · Answer 2 · 2016-11-05T06:35:00

No, with some corrections this program will tell if a number is odd or even.
This how your program should look:

Java

public class DoWhile{
    public static void main(String[]args)
    {
        int num=13;
        int last=100;
        do
        {
            if(IsPrime(num))
            {
                System.out.println(num + " is a prime number");

            }
            else
            {
                System.out.println(num+ " is NOT a prime number");

            }
            num++;
        }
        while(num <==last);
    }
}

You need to define the function IsPrime that will check if a number is prime or not.

If you have difficulties to understand what you program do, use the debugger, it will show you.

You should learn to use the debugger as soon as possible. Rather than guessing what your code is doing, It is time to see your code executing and ensuring that it does what you expect.

The debugger allow you to follow the execution line by line, inspect variables and you will see that there is a point where it stop doing what you expect.
Debugger - Wikipedia, the free encyclopedia[^]
http://docs.oracle.com/javase/7/docs/technotes/tools/windows/jdb.html[^]
https://www.jetbrains.com/idea/help/debugging-your-first-java-application.html[^]

The debugger is here to show you what your code is doing and your task is to compare with what it should do.