[Numpy-discussion] random_array.poisson(0) gives -1 and -2

Sebastian Haase haase at msg.ucsf.edu
Wed May 19 12:18:01 CDT 2004


On Wednesday 19 May 2004 12:03 pm, you wrote:
> On Wed, 2004-05-19 at 14:25, Sebastian Haase wrote:
> > Hi,
> >
> > the random_array poisson functions returns negative values if mean=0:
> > >>>from numarray import random_array as ra
> > >>> ra.seed(x=1, y=1)
> > >>> ra.poisson(0)
> >
> > 5
> >
> > >>> ra.poisson(0)
> >
> > -2
> >
> > My "math book" tells me that it should be always zero.
> > This seems to be a constructed case, but I'm using this to put
> > "quantum statistic" into a simulated image:
> > obj = na.array( something )
> > imageFromDetector = ra.poisson( obj ) + gaussianNoiseArray
> > The object array might have lots of zeros surrounding the "actual
> > object". Thinking of a fluorescent object sending out photons it makes
> > sense to not get any photons at all from 'empty' regions.
> > I'm using numarray 0.8;
>
> I tried this on Fedora-1 i386 with Python-2.3.3 and it returned zero
> consistently.  What platform are you on?
>
> Regards,
> Todd

I running debian (Woody)
$ uname -a
Linux baboon 2.4.18 #1 Tue Dec 16 14:11:01 PST 2003 i686 unknown
$python -v
<snip>
Python 2.2.1 (#1, Feb 28 2004, 00:52:10)
[GCC 2.95.4 20011002 (Debian prerelease)] on linux2

and I get this:
>>> ra.poisson(0, 100000).min()
-4
>>> ra.poisson(0, 100000).min()
-4
>>> ra.poisson(0, 100000).mean()
1.9383
>>> ra.poisson(0, 100000).mean()
1.9607
>>> ra.poisson(0, 100000).max()
29
>>> ra.poisson(0, 100000).max()
28

Thanks for checking,
Sebastian




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