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Thread: Need Help - Factoring & Prime Finding Code

  1. #1
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    Thumbs down Need Help - Factoring & Prime Finding Code

    Hello, I'm working on a project that outputs the numbers 2 100.
    Next to each number, it should list all the factors for that number.
    If the number has no factors, it should print prime.
    Ex.
    2 prime
    3 prime
    4 2
    5 prime

    Having some difficulty so far. I'm aware I need to use loops, but I'm not sure what to %, and by what to determine if it's a factor.

    This is my fail code so far: (I realize its horrible and is incomplete)
    public class Prime
    {
    public static void main(String[] args)
     {
     	findPrime(factors());
     	factors();
     }
     public static void findPrime(int k)
      {
      	int i = 2;
      	int ans = i%k;	
     	while(i<=100)
     	{
      		System.out.println(ans);
     		i++;
      	}
     
      }
    public static int factors()
      {
      	int k = 1;
      	for(k = 1; k <=100; k++)
      	{   		 	
      	}
      	return k;
      }
    }

    Any help whatsoever is greatly appreciated. Thanks!


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Need Help - Factoring & Prime Finding Code

    mmm... prime numbers

    The easiest way to do this is probably seeing if for some number 1<n<101, printing out any numbers that n%i == 0 (where i is a number from 2 to n-1). If nothing was printed out, then print out prime. You can keep track of nothing being printed out by having a boolean flag to denote the primality of the number.

    for (int n = 2; n <= 100; n++)
    {
        boolean isPrime = true; // assume n is prime
        System.out.print(n + " : ");
        for(int i = 2; i < n; i++)
         {
               if (n%i == 0)
               {
                          System.out.print(i);
                          isPrime = false;
               }
         }
         if (isPrime)
         {
              System.out.println("Prime");
         }
         else
         {
              System.out.println(""); // go to next line
    }

  3. The Following User Says Thank You to helloworld922 For This Useful Post:

    prodigytoast (November 4th, 2009)

  4. #3
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    Default Re: Need Help - Factoring & Prime Finding Code

    Thanks alot! That helped me so much, I appreciate it, however, I need to work on the factoring part now, with that current code, it is printing some pretty strange numbers next to the non-prime numbers.
    Ex.
    27 : 39
    28 : 24714
    29 : Prime
    30 : 23561015
    31 : Prime
    32 : 24816
    33 : 311
    34 : 217
    35 : 57
    36 : 234691218
    37 : Prime

  5. #4
    Super Moderator helloworld922's Avatar
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    Default Re: Need Help - Factoring & Prime Finding Code

    ahh, forgot... it's not printing out any spaces between the numbers

  6. #5
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    Default Re: Need Help - Factoring & Prime Finding Code

    Oh! haha I didn't realize those were factors! Thanks again!

  7. #6
    Senile Half-Wit Freaky Chris's Avatar
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    Default Re: Need Help - Factoring & Prime Finding Code

    lol nice one helloworld, always leave something out to confuse them. If it's solved coud you please mark it as so.

    Thanks,
    Chris
    chris[at]javaprogrammingforums[dot]com

    Prifysgol Bangor University, North Wales

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