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URGENT - PRIME CHECKER ??

4 replies on 1 page. Most recent reply: Apr 12, 2004 12:26 AM by Martijn Veening

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nicola

Posts: 3
Nickname: nicola
Registered: Apr, 2004

URGENT - PRIME CHECKER ??

Posted: Apr 10, 2004 2:55 AM

Hello, I am new to all this Java and have been desperately trying to work out how to write a primechecker program.

I need to accept a positive integer as input and determine whether the number is prime by using the formula:

An integer number n is prime if it is not evenly divisible by any integer number k(starting at 2), such that k[/] squared <= n.

The program should accept only positive numbers, and an error message should display if it is negative that is entered. The prompts should continue util the number entered is 0.

I would appreciate anyones help. I really want to understand how this works. Thank You

twc

Posts: 129
Nickname: twc
Registered: Feb, 2004

Re: URGENT - PRIME CHECKER ??

Posted: Apr 10, 2004 12:36 PM

Have you written any code or are you expecting us to do your homework for you? I don't mind helping you find errors in your code or your logic, but I'm not doing anyones homework for them.

nicola

Posts: 3
Nickname: nicola
Registered: Apr, 2004

Re: URGENT - PRIME CHECKER ??

Posted: Apr 11, 2004 6:36 AM

I am sorry if it came across that way. I didn't mean to have it all done for me - how dreadful!! I would never learn like that.

I do have code I have done for it. I have been having difficulty trying to interpret where I use the do while code and wher I use the for loops. I keep trying different combinations and just can't seem to make it all happen. I have read all my notes and reread them a zillion times but none of the information provides on how to do it when you need to use a combination of them all.

Just some guidance on what I have already done would be appreciated. Here is the code:

 import java.io.*;
class PrimeChecker
{
   public static void main(String[] args) throws IOException
   {
   
   BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
   int num=0, i=2, sqrtnum;
   
   
   do
   {
      System.out.println("Enter a number:");
      num = Integer.parseInt(stdin.readLine());
      while (num!=0)                    
        {
          sqrtnum = (int)Math.sqrt(num);
      while (i <= sqrtnum);
      if (num%i == 0)
      i++;
      if (i == sqrtnum+1)
      
    System.out.println(num + " is a prime number");
   else
   System.out.println(num + "is not a prime number");
      }
      
      break;
      }
       while (num >= 0);
       System.out.println("You entered a negative number");
      
             
   }
   }

nicola

Posts: 3
Nickname: nicola
Registered: Apr, 2004

I HAVE DONE THE WORK - NEED GUIDANCE !!!

Posted: Apr 11, 2004 11:04 PM

I have gotten this far with my code. I have researched this all day and have spent over 10 hours on it. I have got the program as far as providing the right answer as to whether it is prime or not and if the number entered is negative a warning message appears and asks for the number to be re-entered.

2 things I have been unable to accomplish.
1: for the program to continually loop and ask for a number to be entered, providing an answer each time until a zero is entered.
and
2: for the program to exit when zero is typed in.

please help me, I obviously have some do, whiles, ifs, and fors wrong!! I have tried so many different combinations.

Here is the code I have so far:

import java.io.*;
class PrimeChecker
{
   public static void main(String[] args) throws IOException
   {
   
      BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
      int num = 0;
      int i=2;
      int sqrtnum; 
      
      while (num <=0)          
      {
      
      System.out.println("Enter a number:");
         num = Integer.parseInt(stdin.readLine());
         while (num!=0)
         {
            if (num < 0)
            System.out.println("You entered a negative number");
            break;
         }
      }      
      sqrtnum = (int)Math.sqrt(num);
      while (i <= sqrtnum)
         {
            if (num%i == 0)
            break;
            i++;
        }
      
      if (i == sqrtnum+1)
        System.out.println(num + " is a prime number");
      else
        System.out.println(num + " is not a prime number");
   }
}

Martijn Veening

Posts: 1
Nickname: martijn
Registered: Apr, 2004

Re: I HAVE DONE THE WORK - NEED GUIDANCE !!!

Posted: Apr 12, 2004 12:26 AM

You missed something trivial indeed.
Your brackets don't include the right stuff. The closing bracket right before sqrtnum=(int)Math.sqrt(num) line should not be there. You probably missed that because you have a tab on the line num=Integer.parseInt(stdin.readline()), which does not belong there.
So delete that ending-bracket, and add it at the end of the while-loop, just before the last two ending-brackets.
Furthermore, you can optimalize the while loop by rewriting it to
while (i<sqrtnum && num%i!=0) i++
The while-loop then ends as soon as it has found a divisor, your algorithm goes on after that until sqrtnum has been reached.
Also, first try checking divisibility by 2, and then increment i from 3 with steps of 2 (i+=2 instead of i++).
This excludes the unnescessary checks of 4,6,8 etc if 2 was not a divisor (if num%2!=0, num%4!=0).
Hope this helped you.

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