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Re: GMP precision problem - only getting 15 places of PI
From: |
David Kastrup |
Subject: |
Re: GMP precision problem - only getting 15 places of PI |
Date: |
Sun, 18 Sep 2005 19:02:17 +0200 |
User-agent: |
Gnus/5.11 (Gnus v5.11) Emacs/22.0.50 (gnu/linux) |
nmenuj@gmail.com writes:
> I installed the GNU MP package only today, and wanted to experiment
> with the C++ interface. Can someone tell me why I am unable to get more
> than 15 correct digits of PI with the following simple program? I
> experimented with the PREC variable and the loop upper bounds, but no
> luck - the bad digits start at the exact same spot.
>
> If someone has another multi-precision library installed, I request
> that you check the code against your library, to rule out a bug in the
> program. I suspect the problem is with a temporary assuming a smaller
> precision, but the mpf_set_default_prec should have addressed that...
>
> #include <iostream>
> #include <gmpxx.h>
>
> using namespace std;
> const int PREC=20000;
>
> mpf_class pival(0.0, PREC);
> mpf_class d2(1.0, PREC), d3(1.0,PREC);
> int main()
> {
> int i = 0;
> mpf_set_default_prec(PREC);
> d2 = mpf_class(1.0/2.0,PREC);
> pival = d2 - (d2/4.0)/3.0;
> for (i=5;i<10000;i+=4)
> {
> d2 /= 16.0;
> pival = pival + d2/i;
> pival = pival - (d2/4.0)/(i + 2);
> }
> cout << "def prec " << mpf_get_default_prec() << '\n';
> d3 = mpf_class(1.0/3.0, PREC);
^^^^^^^ Uh, oh!
> pival = pival + d3 - (d3/9.0)/3.0;
> for (i=5;i<10000;i+=4)
> {
> d3 /= 81.0;
> pival = pival + d3/i;
> pival = pival - (d3/9.0)/(i + 2);
> }
> cout.precision(100);
> cout << pival * 4.0 << '\n';
> return 0;
> }
--
David Kastrup, Kriemhildstr. 15, 44793 Bochum
- GMP precision problem - only getting 15 places of PI, nmenuj, 2005/09/18
- Re: GMP precision problem - only getting 15 places of PI,
David Kastrup <=
- Re: GMP precision problem - only getting 15 places of PI, nmenuj, 2005/09/19
- Re: GMP precision problem - only getting 15 places of PI, mensanator, 2005/09/19
- Re: GMP precision problem - only getting 15 places of PI, nmenuj, 2005/09/19
- Re: GMP precision problem - only getting 15 places of PI, mensanator, 2005/09/19
- Re: GMP precision problem - only getting 15 places of PI, nmenuj, 2005/09/21
- Re: GMP precision problem - only getting 15 places of PI, mensanator, 2005/09/21