[SciPy-user] lognormal distribution

Stephen Walton stephen.walton at csun.edu
Mon Mar 7 14:04:58 CST 2005

Robert Kern wrote:

> If it's univariate, and you can write out the pdf or cdf as a 
> function, then I believe you can subclass scipy.stats.rv_continuous, 
> and it's rvs() method will numerically invert the cdf to generate it's 
> random numbers.

This is so cool!  I had a desire to generate values on (0,1) where 
values near 0.5 were less probable than values at the endpoints.  Here's 
the implementation:

import Numeric as Num
from scipy.stats.distributions import rv_continuous

# CDF for the sunspot generation function.  If we make it a subclass
# of rv_continuous we get sunspot.rvs for free :-)

class sunspot_gen(rv_continuous):
   def _pdf(self,x):
   def _cdf(self,x):
      return -(cos(pi*x)*pmin-cos(pi*x)*pmax-pmax*x*pi-pmin+pmax)/pi
sunspot = sunspot_gen(a=0.,b=1.,name='sunspot', longname='A sunspot 

Sunspot distribution

This distribution represents the probability of a subdivision of a sunspot
into two spots of size x and (1-x), where x is a value from sunspot.rvs(0).
The PDF has a high probability of x near 0 or 1 and a low probability of x
near 0.5.


My only question:  what should I replace the 'import Numeric as Num' 
with if I want to be able to work within the framework of using either 
Numeric or numarray?  'import numerix as Num' doesn't seem to work.

More information about the SciPy-user mailing list