Thread: listing twin prime numbers help!

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{ int firstnum,secondnum;

     for(firstnum=1; firstnum<=499; firstnum++)
     {
       if(secondnum%firstnum)
           {
            secondnum=(firstnum*2)+1;
            printf("%d ",secondnum);
           }
       else
           {
                printf("done");
           }
      }
      system("PAUSE");
      return 0;
}

need help, i cant seem to find out how to list all the prime numbers from 1-1000...im not sure how to test it to check if its a prime number this code prints 9 for example but 9 is not prime, can some1 help me fix it?? i also edited this to look like this, which is better:

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{ int firstnum=1,secondnum=3;


      if(!(secondnum%firstnum))
      {
          for(firstnum=1; firstnum<=499; firstnum++)
          {
            secondnum=(firstnum*2)+1;
            printf("%d ",secondnum);
          }
      }
      else
          {
            printf("Doesn't work MANG!!!");
          }
      system("PAUSE");
      return 0;
}