[Numpy-discussion] bug in numpy.mean() ?

Kathleen M Tacina Kathleen.M.Tacina@nasa....
Tue Jan 24 12:53:27 CST 2012


I have confirmed this on a 64-bit linux machine running python 2.7.2
with the development version of numpy.  It seems to be related to using
float32 instead of float64.   If the array is first converted to a
64-bit float (via astype), mean gives an answer that agrees with your
looped-calculation value: 3045.7472500000002.  With the original 32-bit
array, averaging successively on one axis and then on the other gives
answers that agree with the 64-bit float answer to the second decimal
place.


In [125]: d = np.load('data.npy')

In [126]: d.mean()
Out[126]: 3067.0243839999998

In [127]: d64 = d.astype('float64')

In [128]: d64.mean()
Out[128]: 3045.747251076416

In [129]: d.mean(axis=0).mean()
Out[129]: 3045.7487500000002

In [130]: d.mean(axis=1).mean()
Out[130]: 3045.7444999999998

In [131]: np.version.full_version
Out[131]: '2.0.0.dev-55472ca'



--
On Tue, 2012-01-24 at 12:33 -0600, K.-MichaelA wrote:

> I know I know, that's pretty outrageous to even suggest, but please 
> bear with me, I am stumped as you may be:
> 
> 2-D data file here:
> http://dl.dropbox.com/u/139035/data.npy
> 
> Then:
> In [3]: data.mean()
> Out[3]: 3067.0243839999998
> 
> In [4]: data.max()
> Out[4]: 3052.4343
> 
> In [5]: data.shape
> Out[5]: (1000, 1000)
> 
> In [6]: data.min()
> Out[6]: 3040.498
> 
> In [7]: data.dtype
> Out[7]: dtype('float32')
> 
> 
> A mean value calculated per loop over the data gives me 3045.747251076416
> I first thought I still misunderstand how data.mean() works, per axis 
> and so on, but did the same with a flattenend version with the same 
> results.
> 
> Am I really soo tired that I can't see what I am doing wrong here?
> For completion, the data was read by a osgeo.gdal dataset method called 
> ReadAsArray()
> My numpy.__version__ gives me 1.6.1 and my whole setup is based on 
> Enthought's EPD.
> 
> Best regards,
> Michael
> 
> 
> 
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion

-- 
--------------------------------------------------
Kathleen M. Tacina
NASA Glenn Research Center
MS 5-10
21000 Brookpark Road
Cleveland, OH 44135
Telephone: (216) 433-6660
Fax: (216) 433-5802
--------------------------------------------------
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