Re: GMP precision problem - only getting 15 places of PI

[David Kastrup]

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