how to generate pi in c

[Les]

On Sat, 2010-11-06 at 12:18 +0000, Marko Vojinovic wrote:
> On Sat, Nov 6, 2010 at 8:43 AM, Mogens Kjaer <mk at lemo.dk> wrote:
> >> http://www.kurims.kyoto-u.ac.jp/~ooura/pi_fft.html
> >>
> >> You'll need RAM to get many digits.
> >
> > 1.6 G decimals in 20 hours on a machine with 16G RAM, running
> > x86_64 Fedora 12.
> 
> Really, I'm curious, is there any real-world problem where anyone
> would actually *need* pi to a G decimal places? I mean, are these kind
> of computations actually useful for someone, or is it just a matter of
> "we have the power to do it, so let's do it" thing? Other than entry
> into the Guinness book of records, that is?
> 
> Or maybe there are still people who believe pi is rational rather than
> transcendent, and look for a cyclic repeat pattern in the decimals?
> ;-)
> 
> :-)
> Marko

PI is used for some kinds of random number generators and in some kinds
of encryption applications.  It is psuedo random because it is derived
from a static calculation, but it is random enough over large numbers of
bits for some specialized uses.

Also haveing it out to large numbers of places is useful for some kinds
of repetitive calculations in computational analysis, since each
multiplication loses essentially 1/2 bit.  If you have enough bits to
start with, then the loss won't show up in the final calculation.

Some kinds of simulations require enormous calculation chains, which in
turn means loss of accuracy.  It is not that the accuracy of 1G bits (or
bytes) of PI can be directly applied in the real world (getting A/D's
that work well down to 32 bits is a challenge.) but the loss of
precision in some types of research, in some real world chained
calculations, and in some matrix math does become an issue.

Regards,
Les H