Friday, July 1, 2011

Approximating pi

This script uses the following formula
to approximate the value of pi with a fixed number of correct digits.
import math
def pi(digits):
 k = 0
 pi = 0.0
 e = 1.0
 tol = pow(10,-digits) # is minimum error that we want to accept
 while e > tol:
  pi_old = pi
  pi += (2*pow(-1,k)*pow(3,.5-k))/(2*k+1)
  e = abs(pi-pi_old) # current error
  k += 1
  print '%0.16f %20.16f' % (pi,e)
 print '\nerror',e,'\niterations ',k
 print '%0.16f' % pi,'result'
 print '%0.16f' % math.pi,'real pi'

pi(8) # approximating pi with 8 correcet digits
During the execution you can see the current approximated value on the left column and the difference with the approximation at the previous step on the right column. At the end, the difference between the approximated value and the value provided by the math python module will be printed.
$ python pi.py 
3.4641016151377544   3.4641016151377544
3.0792014356780038   0.3849001794597506
3.1561814715699539   0.0769800358919501
3.1378528915956800   0.0183285799742738
3.1426047456630846   0.0047518540674045
3.1413087854628832   0.0012959602002014
3.1416743126988376   0.0003655272359544
3.1415687159417840   0.0001055967570536
3.1415997738115058   0.0000310578697218
3.1415905109380802   0.0000092628734256
3.1415933045030817   0.0000027935650015
3.1415924542876463   0.0000008502154354
3.1415927150203800   0.0000002607327336
3.1415926345473140   0.0000000804730660
3.1415926595217138   0.0000000249743999
3.1415926517339976   0.0000000077877162

error 7.78771624965e-09 
iterations  16
3.1415926517339976 result
3.1415926535897931 real pi

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