algorithm for finding Pi in C

[aarklon]

Hi all,

see:- http://mathforum.org/library/drmath/view/54456.html

[pete]

aa*****@gmail.c om wrote:

Hi all,

see:- http://mathforum.org/library/drmath/view/54456.html

This works for me:

#include <float.h>
/*
** pi == (atan(1.0 / 3) + atan(1.0 / 2)) * 4
*/
static double fs_pi(void);

static double fs_pi(void)
{
unsigned n;
double a, b;
static double p;
static int initialized;

if (!initialized) {
initialized = 1;
n = 1;
a = 3;
do {
a /= 9;
b = a / n;
n += 2;
a /= 9;
b -= a / n;
n += 2;
p += b;
} while (b DBL_EPSILON / 4);
n = 1;
a = 2;
do {
a /= 4;
b = a / n;
n += 2;
a /= 4;
b -= a / n;
n += 2;
p += b;
} while (b DBL_EPSILON / 2);
p *= 4;
}
return p;
}

--
pete

[Richard Heathfield]

aa*****@gmail.c om said:

Hi all,

see:- http://mathforum.org/library/drmath/view/54456.html

Presumably you want Pi? Easy. It's about 3.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[spinoza1111]

On May 24, 2:44*pm, Richard Heathfield <[e-mail].i nvalidwrote:

aark...@gmail.c om said:

Hi all,

see:-http://mathforum.org/library/drmath/view/54456.html

Presumably you want Pi? Easy. It's about 3.

So we doan' need no arbitrary "unlimited" "limited only by time and
memory space" precision?

Wouldn't 3.14 be a better answer in all cases? That's what most
"normal geeks" remember from math class.

And isn't usually a #define const?

>
--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Richard Heathfield]

spinoza1111 said:

On May 24, 2:44 pm, Richard Heathfield <[e-mail].i nvalidwrote:

>aark...@gmail. com said:

Hi all,

see:-http://mathforum.org/library/drmath/view/54456.html

Presumably you want Pi? Easy. It's about 3.

So we doan' need no arbitrary "unlimited" "limited only by time and
memory space" precision?

It depends. Sometimes we do, and sometimes we don't.

Wouldn't 3.14 be a better answer in all cases?

No, not in all cases. In some cases, "about 3" is far superior, although
admittedly "just over 3" is even better. Case in point: you're parked by
the lake, looking at a map thereof. Using your thumb against the map scale
indicator and then against the lake, you can see that it's about a mile
across, and roughly circularish. There's a path all the way round. In this
kind of terrain (reasonably flat, for obvious reasons) you can manage,
say, 4mph. Your time, however, is not unlimited. Have you got time to walk
around the lake? In such a situation, taking pi as "three-and-a-bit" is
far more appropriate than the more pernickety 3.14.

Note that, as an estimate of pi, 3 is only about 4.5+% short. That's not
bad for a single digit.

And isn't usually a #define const?

No (but somehow I get the feeling that either I'm misinterpreting your
question, or you're going to misinterpret my answer, or perhaps both).

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[MisterE]

see:- http://mathforum.org/library/drmath/view/54456.html

Wow it gives all the worst ways to calculate pi, and it has nothing specific
to C. Its win-win!

[nembo kid]

pete ha scritto:

static d

Why all these 'static'?

[santosh]

nembo kid wrote:

pete ha scritto:

>static d

Why all these 'static'?

That's to prevent the objects from having external linkage, which is
mainly to control namespace pollution. IOW these identifiers are
visible only in this translation unit, from where they are defined
until the end of the unit, so you can reuse these identifiers for other
purposes elsewhere. The default linkage for functions and file scope
objects is external. The static qualifier in these cases suppress this.

[pete]

nembo kid wrote:

pete ha scritto:

>static d

Why all these 'static'?

There is no reason to calculate that one,
more than once.

--
pete

[Keith Thompson]

Richard Heathfield <rj*@see.sig.in validwrites:

Keith Thompson said:

>"Andy G." <an******@spamc op.netwrites:

<snip>

>> printf("#define PI %.32f\n", acos(-1.0));
return 0;
}

Output:

#define PI 3.1415926535897 931159979634685 4419

His program's first error is in the 17th digit (the 16th decimal place).

>#include <float.h>
#define PI
#3.14159265358 979232846264338 327950288419716 939937510582097 4944592

Your first error, however, is in the 16th digit (the 15th decimal place).
:-p

Whoops. s/232/323/

(His error was due to rounding; mine was a typo.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"