JB>   to avoid the sqrt() you could if you're working
JB>   sequentially you can store square of the prime above
JB>   thats too big to be used yet and when you reach that
JB>   number go the the next prime in your list etc...
   I did one using an integer based square root function which
   only checks one less than or one greater than multiples of
   six, which all primes seem to be for some reason. :)
   This finds all primes < 32766 in 2.6 seconds on my 486sx25.
   On my Pentium, it takes less than 0.3 seconds. The resulting
   array of primes is sufficient for checking further primes up
   to squared(32749). This is using only integers, using only
   DOS and 640K, and using only standard malloc(). It should be
   rather simple to go much farther with a big number library
   and a huge allocation utility.
 /*_|_|   PRIMES.C
 _|_|_|   A program to find all the primes < 32,766.
 _|_|_|   No warrantee or guarantee is given or implied.
 _|_|_|   Released   PUBLIC DOMAIN   by Kurt Kuzba.  (10/2/96)*/
#include 
#include 
#include 
int rootsqr(int val)
{
   unsigned long ndx, cmp, dif;
   ndx = cmp = dif = (unsigned long)val;
   while((ndx * ndx) >  cmp)    ndx -= (dif >>= (dif > 1));
   while((ndx * ndx) <= cmp)    ndx++;
   return (int)--ndx;
}
int main(int c, char **v)
{
   int ndx = 2, check, prime, current, limit, *primes, show = 0;
   float runtime = (float)clock();
   primes = malloc(3600 * sizeof(int));
   if(primes == NULL)
     return puts("Malloc failure");
   primes[0] = 1, primes[1] = 2, primes[2] = 3;
   if(c > 1 && (v[1][0] == 's' || v[1][0] == 'S')) show++;
   for(current = 6; current < 32766; current += 6)
   {
      check = prime = 1;
      limit = rootsqr(current - 1);
      while(prime && primes[++check] <= limit)
         prime = (int)((current - 1) % primes[check]);
      if(prime)
         if(show)
            printf("%8d", primes[++ndx] = current - 1);
         else
            primes[++ndx] = current - 1;
      check = prime = 1;
      limit = rootsqr(current + 1);
      while(prime && primes[++check] <= limit)
         prime = (int)((current + 1) % primes[check]);
      if(prime)
         if(show)
            printf("%8d", primes[++ndx] = current + 1);
         else
            primes[++ndx] = current + 1;
   }
   runtime = ((float)clock() - runtime) / CLK_TCK;
   printf("\n%d Primes found in %.2f seconds", ndx + 1, runtime);
   return 0;
}
 /*_|_|   end   PRIMES.C   */

> ] I don't believe they knew... I was Long John Silver.........

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