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Path: utzoo!watmath!clyde!cbosgd!ukma!sambo
From: sambo@ukma.UUCP (Inventor of micro-S)
Newsgroups: net.micro
Subject: Re: Z-100 vs IBM-AT Challenge Update
Message-ID: <1805@ukma.UUCP>
Date: Thu, 23-May-85 19:51:45 EDT
Article-I.D.: ukma.1805
Posted: Thu May 23 19:51:45 1985
Date-Received: Sat, 25-May-85 00:25:20 EDT
Organization: U of Kentucky, Mathematical Sciences, Lexington KY
Lines: 41
Summary: A couple simple modifications to the prime number generator posted

earlier produces a 10% speed increase on the VAX 11/750
Expires: 
References: <10787@brl-tgr.ARPA>
Sender: 
Reply-To: sambo@ukma.UUCP (Inventor of micro-S)
Followup-To: 
Distribution: 
Organization: Univ. of KY Mathematical Sciences
Keywords: 

/*
 * Yet Another Prime number generator
 */

#define MAXPRIME 3300

#define MAXN 32767

int index,
    dividend,
    product,
    inc = 4, /* increment to add to n - oscillates between 2 and 4 */
    n = 5,
    nprime = 1,
    prime[MAXPRIME] = {2, 3};

main()
{
    /* note that n is never divisible by 3 because of how it is incremented */
    for ( ; (nprime < MAXPRIME-1) && (n <= MAXN); n += (inc = 4 - inc + 2)) {
	/* start with index = 2 instead of 1 to avoid dividing by three */
	for (index = 2, dividend = n ;
	     (index < nprime) && (dividend > prime[index]); index++)
	    if ((product = (dividend = n / prime[index]) * prime[index]) == n)
	        break;
	if (product != n)
	    prime[++nprime] = n;
    }
    printf("There are %d primes less than %d. The largest is %d.\n",
            nprime + 1, n, prime[nprime]);
}