/usr/games/primes

[Ed Falk]

In article <8584 at brazos.Rice.edu> kelso at gwusun.gwu.edu (John Kelso) writes:

|Does anyone out there know the algorithm that it uses, or better yet,
|where I can get the source for it?

It uses the "Sieve of Eratosthenes".  It's a very simple algorithm:

    1) make a list of N numbers to test:
	2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
    2) walk through the list, crossing out all multiples of two:
	2 3   5   7   9    11    13    15    17    19    21    23    25
    3) walk through the list, crossing out all multiples of three:
	2 3   5   7        11    13          17    19          23    25
    4) Do nothing for four, because it's already been crossed out.
    5) walk through the list, crossing out all multiples of five:
	2 3   5   7        11    13          17    19          23

    keep doing this for all n <= sqrt(N) that are still in the list when
    you get to them.

In going through my toolbox, I see that I have two different
implementations of this.  Apparently it was easier to just write it again
than remember what I called it the first time.

Note that you don't actually need to allocate enough room for a list of
numbers, just for a list of flags indicating that this number is crossed
out or not.

Another optimization is to simply ignore even numbers everywhere.

/* simple sieve of Eratosthenes for primes */

#include <stdio.h>
#include <math.h>

main(argc,argv)
	int	argc ;
	char	*argv[] ;
{
register unsigned char	*array ;
register int	i ;
	int	lim, n, j ;

	n = atoi(argv[1]) ;

	array = (unsigned char *) malloc(n+1) ;

	for(i=1;i<=n;++i)
	  array[i] = 1 ;

	lim = (int) sqrt((float) n) ;

	for(j=2; j<=lim; ++j)
	  if( array[j] != 0 )
	  {
	    for(i=2*j; i<=n; i += j)
	      array[i] = 0 ;
	  }

	for(i=2; i<=n; ++i)
	  if( array[i] != 0 )
	    printf("%d\n",i) ;
}
	-ed falk, sun microsystems -- sun!falk, falk at sun.com
	"What are politicians going to tell people when the
	Constitution is gone and we still have a drug problem?"
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