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From: jroth@allvax.enet.dec.com (Jim Roth)
Newsgroups: comp.compression,sci.math
Subject: Re: A program for computing Pi
Message-ID: <4277@ryn.mro4.dec.com>
Date: 3 Apr 91 16:08:31 GMT
Sender: guest@ryn.mro4.dec.com
Followup-To: comp.compression
Organization: Digital Equipment Corporation
Lines: 50


In article <1991Apr3.070614.11682@agate.berkeley.edu>, greg@garnet.berkeley.edu (Greg Kuperberg) writes...
>In article <1991Apr3.014832.15021@linus.mitre.org> bs@faron.mitre.org (Robert D. Silverman) writes:
>>In article <AVENGER.91Apr2185942@wpi.WPI.EDU> avenger@wpi.WPI.EDU (Samuel Joseph Pullara) writes:
>>>Could someone please send me a program that will calculate PI to any
>>>arbitrary number of digits?  Thanks in advance...
>>Let me add my opinion.
>>Please do NOT comply with the above request.
> 
>That's a great idea!  Here is a simple C program to compute n digits of
>Pi in time O(n^2).

Here's an even shorter program :-)

Stan and I once sat down and wrote a one line TECO macro that did this
calculation!

/*  1000 digits of PI					    */
/*  'Spigot' algorithm origionally due to Stanly Rabinowitz */

#include stdio
#include math

main()
{
  int d = 4, r = 10000, n = 251, m = 3.322*n*d;
  int i, j, k, q;
  static int a[3340];

  for (i = 0; i <= m; i++) a[i] = 2;
  a[m] = 4;

  for (i = 1; i <= n; i++) {
    q = 0;
    for (k = m; k > 0; k--) {
      a[k] = a[k]*r+q;
      q = a[k]/(2*k+1);
      a[k] -= (2*k+1)*q;
      q *= k;
    }
    a[0] = a[0]*r+q;
    q = a[0]/r;
    a[0] -= q*r;
    printf("%04d%s", q, i & 7 ? "  " : "\n");
  }
}

>Enjoy the program!

- Jim