algorithm for finding Pi in C

[aarklon]

Hi all,

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

[Jack Klein]

On Fri, 23 May 2008 20:53:16 -0700 (PDT), aa*****@gmail.c om wrote in
comp.lang.c:

Hi all,

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

Even better:

double pi = acos(-1);

Of course, if you want it outside of a function, you can't initialize
it with a function, so instead:

double pi;

int main(void)
{
pi = acos(-1);
/* stuff */
return 0;
}

Don't forget to include <math.h>, and you might have to do some extra
work in linking if you work on an antique system.

--
Jack Klein
Home: http://JK-Technology.Com
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[Andy G.]

On Sat, 24 May 2008 22:31:55 -0500, Jack Klein wrote:

On Fri, 23 May 2008 20:53:16 -0700 (PDT), aa*****@gmail.c om wrote in
comp.lang.c:

>Hi all,

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

Even better:

double pi = acos(-1);

Of course, if you want it outside of a function, you can't initialize it
with a function, so instead:

double pi;

int main(void)
{
pi = acos(-1);
/* stuff */
return 0;
}

Or take it a step further and copy and paste the output of the following
program into a suitable header file.

/* pi.c */
#include <math.h>
#include <stdio.h>
int main(void)
{
printf("#define PI %.32f\n", acos(-1.0));
return 0;
}

Output:

#define PI 3.1415926535897 931159979634685 4419

Don't forget to include <math.h>, and you might have to do some extra
work in linking if you work on an antique system.

s/antique/linux

jaysome@ubuntu:/tmp$ gcc -o pi pi.c
/tmp/ccIx5hjS.o: In function `main':
pi.c:(.text+0x1 c): undefined reference to `acos'
collect2: ld returned 1 exit status

Of course the way to fix this is explained in the C FAQ:

jaysome@ubuntu:/tmp$ gcc -o pi pi.c -lm

Regards
--
jay

[pete]

Andy G. wrote:

On Sat, 24 May 2008 22:31:55 -0500, Jack Klein wrote:

>On Fri, 23 May 2008 20:53:16 -0700 (PDT), aa*****@gmail.c om wrote in
comp.lang.c:

>>Hi all,

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

Even better:

double pi = acos(-1);

Of course, if you want it outside of a function, you can't initialize it
with a function, so instead:

double pi;

int main(void)
{
pi = acos(-1);
/* stuff */
return 0;
}

Or take it a step further and copy and paste the output of the following
program into a suitable header file.

/* pi.c */
#include <math.h>
#include <stdio.h>
int main(void)
{
printf("#define PI %.32f\n", acos(-1.0));
return 0;
}

Output:

#define PI 3.1415926535897 931159979634685 4419

That's wrong.
Pi is 3.1415926535897 932384626433832 7950

s/antique/linux

jaysome@ubuntu:/tmp$ gcc -o pi pi.c
/tmp/ccIx5hjS.o: In function `main':
pi.c:(.text+0x1 c): undefined reference to `acos'
collect2: ld returned 1 exit status

Of course the way to fix this is explained in the C FAQ:

jaysome@ubuntu:/tmp$ gcc -o pi pi.c -lm

Regards

--
pete

[Keith Thompson]

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

Or take it a step further and copy and paste the output of the following
program into a suitable header file.

/* pi.c */
#include <math.h>
#include <stdio.h>
int main(void)
{
printf("#define PI %.32f\n", acos(-1.0));
return 0;
}

Output:

#define PI 3.1415926535897 931159979634685 4419

#include <float.h>
#define PI 3.1415926535897 923284626433832 795028841971693 993751058209749 44592
#if LDBL_DIG 64
#error "Need more digits for PI"
#endif

Or you can write code to compute it for you if you're concerned that
the value might change.

--
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"

[Richard Heathfield]

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
#3.141592653589 792328462643383 279502884197169 399375105820974 944592

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

--
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

[vippstar]

On May 25, 11:15 am, Keith Thompson <[e-mail] rote:

Richard Heathfield <[e-mail].i nvalidwrites:

Keith Thompson said:

"Andy G." <andyg...@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
#3.141592653589 792328462643383 279502884197169 399375105820974 944592

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.)

.... typo? You don't remember PI up to all these decimal places, do you?

[CBFalconer]

vi******@gmail. com wrote:

Keith Thompson <[e-mail] rote:

>Richard Heathfield <[e-mail].i nvalidwrites:

>>Keith Thompson said:
"Andy G." <andyg...@spamc op.netwrites:

.... snip ...

>>

>>>>#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
#3.141592653 589792328462643 383279502884197 169399375105820 974944592

>>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.)

... typo? You don't remember PI up to all these decimal places,
do you?

Of course he does. These are c.l.c regulars you are speaking to.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
** Posted from http://www.teranews.com **

[Keith Thompson]

vi******@gmail. com writes:

On May 25, 11:15 am, Keith Thompson <[e-mail] rote:

>Richard Heathfield <[e-mail].i nvalidwrites:

Keith Thompson said:

>"Andy G." <andyg...@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.)

... typo? You don't remember PI up to all these decimal places, do you?

Yes. From memory:

3.1415926535897 932384626433832 795028841971693 993751058209749 445923078164

I used fewer digits above to avoid making the line too long.

--
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"

[James Dow Allen]

On May 24, 5:41*pm, Richard Heathfield <[e-mail].i nvalidwrote:

spinoza1111 said:

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

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

I'm disappointed no one in the thread mentioned the
almost unbelievable expression due to Ramanujan:
\frac{1}{\pi} = \frac{2\sqrt{2} }{9801}
\sum^\infty_{k= 0} \frac{(4k)!(110 3+26390k)}{(k!) ^4 396^{4k}}

Admittedly, this formula may have little interest for those
for whom "about 3" is an adequate approximation.

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).

Or perhaps even all three.

James Dow Allen

[Richard Heathfield]

James Dow Allen said:

On May 24, 5:41 pm, Richard Heathfield <[e-mail].i nvalidwrote:

>spinoza1111 said:

On May 24, 2:44 pm, Richard Heathfield <[e-mail].i nvalidwrote:
Presumably you want Pi? Easy. It's about 3.

I'm disappointed no one in the thread mentioned the
almost unbelievable expression due to Ramanujan:
\frac{1}{\pi} = \frac{2\sqrt{2} }{9801}
\sum^\infty_{k= 0} \frac{(4k)!(110 3+26390k)}{(k!) ^4 396^{4k}}

But someone did mention it. In case you missed the article, the message ID
is <4a************ *************** *******@f24g200 0prh.googlegrou ps.com-
check it out!

Admittedly, this formula may have little interest for those
for whom "about 3" is an adequate approximation.

<grinWell, "about 3" *is* an adequate approximation - on occasion. That
doesn't mean it's adequate for *all* uses.

--
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