## See Also edit

## Discussion edit

Abstractly,*pi*is the ratio between the circumference and the diameter of a circle. It can be proven that the number is irrational, so you'll never get an exact representation of its value in Tcl.The best way to use it in Tcl is to create a global variable (called

*pi*of course) which contains a reasonably accurate approximation and then use that variable throughout your program.DKF: If you don't want to use a global var, a proc can also be used:

proc Pi {} {return 3.1415926535897931} ;# RS

atan provides a handy way to ask Tcl for the value of pi:

% expr {atan(1) * 4} 3.1415926535897931MGS: Actually, using acos() is (slightly) more efficient:

% set tcl_precision 17 17 % expr {acos(-1)} 3.1415926535897931

*Does anyone have any data on which method is preferable from a numerical point of view?*IDG: Both contain the assumption that the transcendental functions are accurate to the last ulp. In many math libraries this is not so. I think you are safer with a string representation:

set pi 3.1415926535897931AM: This is certainly recommendable: that way you can be sure to get a value which is platform-independent or at least highly predictable.GWM: Evaluating the algorithm below or looking up on the web (direct link: http://www.joyofpi.com/pi.html) shows the correct value of pi (to the precision given above) ends in the digit 2 (not 1). Pi = 3.141592653589793238462643383. The value given is however good enough to calculate the circumference of a sphere the mean radius of the earth to the size of an atom.

GS: (030927) Here is a small program ables to compute 2400 digits of pi:

# pi-2400.tcl # 2400 digits of pi with a spigot algorithm set e 0 for {set b 0} {$b <= 8400} {incr b} {set f($b) 2000} for {set c 8400} {$c > 0} {incr c -14} { set d 0 for {set b $c} {$b > 0} {incr b -1} { set g [expr 2*$b -1] set d [expr ($d*$b) + ($f($b)*10000)] set f($b) [expr round([expr fmod($d,$g)])] set d [expr $d/$g] } puts -nonewline [format "%.4i" [expr $e+($d/10000)]] flush stdout set e [expr round([expr fmod($d,10000)])] }It uses a spigot algorithm. More details in

*A spigot algorithm for the digits of pi*, Stanley Rabinowitz and Stan Wagon, American Mathematical Monthly, March 1995, pp195-203.MGS [2003/09/27]: Here's a more efficient version:

set e 0 for {set b 0} {$b <= 8400} {incr b} {set f($b) 2000} for {set c 8400} {$c > 0} {incr c -14} { set d 0 for {set b $c} {$b > 0} {incr b -1} { set g [expr {2 * $b - 1}] set d [expr {($d*$b) + ($f($b)*10000)}] set f($b) [expr {round(fmod($d,$g))}] set d [expr {$d / $g}] } puts -nonewline [format "%.4i" [expr {$e+($d/10000)}]] flush stdout set e [expr {round(fmod($d,10000))}] }AM: The efficiency is gained by putting the expressions in braces - then [expr] can parse it once and store the result.KBK: The following code is several times faster still. The improvements that gain further performance are:

- encapsulating code in a proc, to take advantage of the local variable table.
- using integer arithmetic rather than floating point throughout.
- using a list rather than a hash table to store the
*f*array.

proc pi3 {} { set n 2400 set e 0 set f {} for { set b 0 } { $b <= 8400 } { incr b } { lappend f 2000 } for { set c 8400 } { $c > 0 } { incr c -14 } { set d 0 set g [expr { $c * 2 }] set b $c while 1 { incr d [expr { [lindex $f $b] * 10000 }] lset f $b [expr {$d % [incr g -1]}] set d [expr { $d / $g }] incr g -1 if { [incr b -1] == 0 } break set d [expr { $d * $b }] } puts -nonewline [format %04d [expr { $e + $d / 10000 }]] flush stdout set e [expr { $d % 10000 }] } puts {} } puts [time pi3]

It might be amusing to implement the Bailey-Borwein-Plouffe algorithm, a digit-extraction algorithm for exactly calculating the digits of pi (http://mathworld.wolfram.com/BBP-TypeFormula.html , http://www.cecm.sfu.ca/~pborwein/). Fortran source code is available at the latter site under "My Papers on Pi" (direct link: http://www.cecm.sfu.ca/~pborwein/PISTUFF/Apistuff.html). Note that the algorithm provides hexadecimal digits, so conversion to decimal would have to be done explicitly if desired.Albert Davidson Chou HotFusionMan at Yahoo.comLars H: Note however that conversion hexadecimal to decimal requires knowing

*all*digits more significant than the one you want to convert, so knowing some hexadecimal digits around e.g magnitude 10^-10000 (i.e., around the 8305th hexadecimal) is not much help for computing the 10000th decimal of pi. Though the algorithm is still a very nice piece of mathematics, this renders it not very useful as a party trick.

KPV: see also Buffon's Needle for one of the earliest Monte Carlo simulation to compute the value of pi.

TV: For the handy, dandy, quick-result requiring, scientifically oriented etc, the tcl/tk front-ended mathematics packageMaxima has a arbitrary precision possibility built in, which quickly gave me this result, which I didn't verify, but it seems handy (and quick):

(C2) block([FPPREC:30],bfloat(%pi)); (D2) 3.14159265358979323846264338328B0comments on the accuracy ?

Sarnold: With math::bigfloat, you can ask (it is in tcllib):

package require math::bigfloat math::bigfloat::tostr [math::bigfloat::pi 100] => 3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117068The formuma used is :

Pi/4 = 44.atan(1/57) + 7.atan(1/239) - 12.atan(1/682) + 24.atan(1/12943)with the development of :

atan(1/N) = 1/N - 1/N^3 + 1/N^5 - 1/N^7 ...This is a formula of Strömer (http://fr.wikipedia.org/wiki/Pi). Of the few formulaes I investigated, it was the fastest asymptotically. Any how, in tcllib there is a math::constants package which you might want to use and to improve.

Jaf: I kind of like this one (we owe it to Ramanujan I think):

- Take the three first odd numbers 1,3,5
- Write each of them twice 113355
- Put a division sign in the middle: 113/355
- the inverse 355/113 is pi - 3e-7

expr acos(-1) => 3.141592653589793 expr 355/113.0 => 3.1415929203539825 expr 355/113.0-acos(-1) => 2.667641894049666e-7DKF: Well, not exactly. π is formally irrational, and so cannot be expressed as the sum of any finite number of fractions. (You can get very close though.)

"pi" is also the short name of a user on this wiki, pi31415.