prime number problem

[Robert Fuhrman]

Hi, hope you can help me out here. I need to generate all the prime number factors for the number 999999 (3, 7, 11, 13, 37). Here is my algorithm:

step n k comment
------------------------------------------------
0 999999 2 not divisible by 2, k = k + 1
1 999999 3 n is divisible by 3, n = n / 3, print 3
2 333333 3 n is divisible by 3, n = n / 3, print 3
3 111111 3 n is divisible by 3, n = n / 3, print 3
4 37037 3 not divisible by 3, k = k + 1
5 37037 4 not divisible by 4, k = k + 1
6 37037 5 not divisible by 5, k = k + 1
7 37037 6 not divisible by 6, k = k + 1
8 37037 7 n is divisible by 7, n = n / 7, print 7
9 5291 7 not divisible by 7, k = k + 1
10 5291 8 not divisible by 8, k = k + 1
11 5291 9 not divisible by 9, k = k + 1
12 5291 10 not divisible by 10, k = k + 1
13 5291 11 n is divisible by 11, n = n / 11, print 11
14 481 11 not divisible by 11, k = k + 1
15 481 12 not divisible by 12, k = k + 1
16 481 13 n is divisible by 13, n = n / 13, print 13
17 37 13 STOP since 13^2 > 37 print 37


I had no problem generating all the factors for 999999 - but I can't seem to get my isPrime method to work, included below is a copy of my code. It won't compile because apparently I have erros on lines 18 and 33, 18 is the beginning of the isPrime method, and 33 apparently I need a ; but I can't figure out why I could possibly need one there. Hope you all can help me out. Thanks in advance

public class Primenum
{
public static void main (String[] args)
{

int n=999999;
System.out.println(n+ "'s factors are ");
for (int i = 1; i <= n / 2; i++)
{
if (n % i == 0 && isPrime(i))
{
System.out.print(i+ " ");
}
}



public isPrime()
{

int n=number;
int j;

for (j=2; j<i; j++)
if (j%i != 0)
System.out.print(" ");
j++;

else
j++;

}
}

}

[Garrett Rowe]

Hey Robert, there are a few things going on with this code. I've tried to highlight the errors I see, but there may be more.

   public static void main (String[] args)
   {
      int n=999999;
      System.out.println(n+ "'s factors are ");
      for (int i = 1; i <= n / 2; i++)
      {
         if (n % i == 0 && isPrime(i))
         {
            System.out.print(i+ " ");
         }
      }
//<--missing closing brace to close method    

   public isPrime()//<-- no return type, no parmeter
   {
      int n=number;//<--- this method has no variable called number.
      int j;
 
      for (j=2; j<i; j++)//<--this method has no variable i in scope
         if (j%i != 0)
            System.out.print(" ");//<--ends if statement and for loop
                                  //I'm not sure why you print an empty space here
 
      j++;//<--just increments j
 
      else//<--- else with no if
         j++;
 
   }
}

Garrett
[ March 06, 2006: Message edited by: Garrett Rowe ]

[Robert Fuhrman]

Hi Garrett, thanks for your input. Am I correct in what I am attempting to do though? In the main method I am trying to get all the factors of 999999 which I think is correct...my isPrime method is probably totally screwed up lol - what I am trying to do is take the factor of 999999 that I found in the main method and test it to see if its prime - I thought I could accomplish this by that for statement i have in there. I used i in the statement because i is the factor of n , then using that loop to test for its factors. Once I do test for it's factors and find them, do I need to make another method for output?Is this the right way to approach this problem or am I way off base?

thanks for your help

[Garrett Rowe]

The way your code is set up will work. For reusability though, I would consider making three seperate methods.

/**
  Returns all the factors of a given integer as an int array.
  @param number the number whos factors are to be determined
  @return all the factors of a given integer as an int array.
*/
public int[] getFactors(int number)
{
  //...
}
 
/**
  Returns true if the argument integer is a prime number.
  @param number the number that the method tests for "primeness".
  @return true if the argument integer is a prime number.
*/
public boolean isPrime(int number)
{
  //...
}
 
/**
  Returns all prime factors of the given integer as an int[].
  @param number the number whos prime factors are to be determined.
  @return all prime factors of the given integer as an int[].
*/
public int[] getPrimeFactors(int number)
{
  //would ideally call both previously defined methods
}

[Robert Fuhrman]

Garrett, I can't seem to get the factor from my main method to test in the isPrime method...any ideas?

[Garrett Rowe]

What does your main() method and isPrime() method look like. What problems are you having?