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72 lines
2.0 KiB
Plaintext
72 lines
2.0 KiB
Plaintext
/*
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* pix - iterative method of finding the number of primes less than x
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*
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* Copyright (C) 1999 Landon Curt Noll
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*
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* Calc is open software; you can redistribute it and/or modify it under
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* the terms of the version 2.1 of the GNU Lesser General Public License
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* as published by the Free Software Foundation.
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*
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* Calc is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General
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* Public License for more details.
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*
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* A copy of version 2.1 of the GNU Lesser General Public License is
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* distributed with calc under the filename COPYING-LGPL. You should have
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* received a copy with calc; if not, write to Free Software Foundation, Inc.
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* 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
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*
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* @(#) $Revision: 29.1 $
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* @(#) $Id: pix.cal,v 29.1 1999/12/14 09:15:31 chongo Exp $
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* @(#) $Source: /usr/local/src/cmd/calc/cal/RCS/pix.cal,v $
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*
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* Under source code control: 1996/07/09 03:14:14
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* File existed as early as: 1996
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*
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* chongo <was here> /\oo/\ http://reality.sgi.com/chongo/
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* Share and enjoy! :-) http://reality.sgi.com/chongo/tech/comp/calc/
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*/
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/*
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* Here is an iterative method of finding the number of primes less than
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* or equal to a given number. This method is from "Computer Recreations"
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* June 1996 issue of Scientific American.
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*
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* NOTE: For reasonable values of x, the builtin function pix(x) is
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* much faster. This code is provided because the method
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* is interesting.
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*/
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define pi_of_x(x)
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{
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local An; /* A(n) */
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local An1; /* A(n-1) */
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local An2; /* A(n-2) */
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local An3; /* A(n-3) */
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local primes; /* number of primes found */
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local n; /* loop counter */
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/*
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* setup
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*/
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An1 = 2;
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An2 = 0;
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An3 = 3;
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primes = 1;
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/*
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* main A(n+1)=A(n-1)+A(n-2) sequence loop
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*/
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for (n = 3; n < x; ++n) {
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An = An2 + An3;
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An3 = An2;
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An2 = An1;
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An1 = An;
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if (An % n == 0)
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++primes;
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}
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return primes;
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}
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