[SciPy-User] random points within an ellipse

josef.pktd@gmai... josef.pktd@gmai...
Tue Aug 10 11:26:10 CDT 2010


On Tue, Aug 10, 2010 at 12:11 PM, Erin Sheldon <erin.sheldon@gmail.com> wrote:
> 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

I would oversample if the rejection rate is around 20%

Josef


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