[SciPy-User] random points within an ellipse

Erin Sheldon erin.sheldon@gmail....
Tue Aug 10 11:11:07 CDT 2010


Excerpts from nicky van foreest's message of Sun Aug 08 17:03:07 -0400 2010:
> Hi,
> 
> Good idea to include the graphical result.
> 
> I must admit that the results of the latest mail by David do not
> satisfy my "random intuition". There is a thick cloud around the
> origin, which seems unnatural to me.
> 
> Just for fun I implemented a rejection method that seems to give good
> results. From an algorithmic perspective my implementation is far from
> optimal (BTW, I welcome feedback on how to speed up code like this.)
> However, the rejection method does not seem that bad, roughly 20% of
> the numbers is rejected, and there is no need to compute cosines and
> the like. The resulting graph seems ok to me.
> 
> I modified the example of DG somewhat for the example below.

Nicky -

I think this version (from my original post) is fairly optimal:

  def get_random_ellipse(n, x0, y0):

      xout = numpy.zeros(n)
      yout = numpy.zeros(n)

      nkeep=0

      while nkeep < n:
          x=2*x0*(random.random(n-nkeep) - 0.5)
          y=2*y0*(random.random(n-nkeep) - 0.5)

          w,=where( ( (x/x0)**2 + (y/y0)**2 ) < 1 )
          if w.size > 0:
              xout[nkeep:nkeep+w.size] = x[w]
              yout[nkeep:nkeep+w.size] = y[w]
              nkeep += w.size

      return xout,yout




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