[Numpy-discussion] convolve

Robert Kern rkern at ucsd.edu
Wed Jul 27 13:44:20 CDT 2005


Alan G Isaac wrote:
> On Wed, 27 Jul 2005, René Bastian apparently wrote: 
> 
>>Outcommenting
>>if len(data) < len(kernel):
>>   data, kernel = kernel, data
>>in convolve.py gives the correct result. 
> 
> The author is not listed for this numarray module,
> so we cannot directly ask why this behavior was chosen.

This behavior was chosen for performance reasons. Since convolution is 
commutative, it doesn't matter what order the arguments come in. I do 
not see the behavior René is claiming. Perhaps René can tell us what 
version of numarray he is using? or provide an example? I am using CVS 
as of a week ago, I think.

In [10]: import numarray

In [11]: import numarray.convolve

In [12]: a = numarray.arange(10) + 1.

In [13]: b = numarray.arange(5) + 2.

In [14]: c = a - 1.

In [15]: numarray.convolve.convolve(a,b)
Out[15]:
array([   2.,    7.,   16.,   30.,   50.,   70.,   90.,  110.,  130.,
         150.,  148.,  133.,  104.,   60.])

In [16]: numarray.convolve.convolve(b,a)
Out[16]:
array([   2.,    7.,   16.,   30.,   50.,   70.,   90.,  110.,  130.,
         150.,  148.,  133.,  104.,   60.])

In [17]: numarray.convolve.convolve(a,c)
Out[17]:
array([   0.,    1.,    4.,   10.,   20.,   35.,   56.,   84.,  120.,
         165.,  210.,  244.,  266.,  275.,  270.,  250.,  214.,  161.,
          90.])

In [18]: numarray.convolve.convolve(c,a)
Out[18]:
array([   0.,    1.,    4.,   10.,   20.,   35.,   56.,   84.,  120.,
         165.,  210.,  244.,  266.,  275.,  270.,  250.,  214.,  161.,
          90.])

-- 
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
  Are the graves of dreams allowed to die."
   -- Richard Harter




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