how to generate pi in c

[Ian Smith]

On Tue, 9 Nov 2010, perryh at pluto.rain.com wrote:
 > Ian Smith <smithi at nimnet.asn.au> wrote:
 > > In freebsd-questions Digest, Vol 335, Issue 11, Message: 4
 > > On Sat, 06 Nov 2010 01:00:34 -0700 perryh at pluto.rain.com wrote:
 > >  > Julian Fagir <gnrp at physik.tu-berlin.de> wrote:
 > >  > > > Does anyone has a "generate-pi.c" source code?
 > >  > ...
 > >  > >   1 #include <stdlib.h>
 > >  > >   2 #include <string.h>
 > >  > >   3 #include <stdio.h>
 > >  > >   4 
 > >  > >   5 // Change this for a more accurate result.
 > >  > >   6 long max = 100000000;
 > >  > >   7 double a, b;
 > >  > >   8 double pi;
 > >  > >   9 long counter;
 > >  > >  10 long i;
 > >  > >  11 
 > >  > >  12 int main() {
 > >  > >  13     for (i = 0; i< max; i++) {
 > >  > >  14         a = drand48();
 > >  > >  15         b = drand48();
 > >  > >  16         if (a*a + b*b <= 1)
 > >  > >  17             counter++;
 > >  > >  18     }       
 > >  > >  19     pi = 4*counter;
 > >
 > > Surely that should be 'pi = 4 * counter / max;' otherwise even if the 
 > > integer counter were only 1 (of 100000000), pi would already be 4 :)
 > 
 > The part I snipped out included a note that it was only generating
 > the digits, not trying to show the decimal point placed properly.

Ah.

 > With that understanding, and as long as max is a (large-ish) power
 > of 10, the division is not needed.  (If the division were to be
 > inserted, at least one of its operands would need to be cast to
 > double, or pi would likely be reported as 3.0000 due to truncation.)

Ok, you can probably tell that I'm not really proficient in C; as an old 
Pascal programmer I'd have assumed an implied cast to pi's type, though 
I'd likely have specified 4.0 to make it clear.

 > An approach more in keeping with the original would involve using
 > sprintf, and then inserting the decimal point into the resulting
 > string :)

It's a bit O(N) to use for many digits .. still I like the approach.

cheers, Ian