[Numpy-discussion] subsampling array without loops

Catherine Moroney Catherine.M.Moroney@jpl.nasa....
Fri Aug 22 12:38:48 CDT 2008


I'm looking for a way to acccomplish the following task without lots
of loops involved, which are really slowing down my code.

I have a 128x512 array which I want to break down into 2x2 squares.
Then, for each 2x2 square I want to do some simple calculations
such as finding the maximum value, which I then store in a 64x256
array.

Here is the actual code involved.  It's only slightly more complex
than what I described above, since it also involves doing a bit of
masking on the 2x2 sub-arrays.

Any hints are much appreciated!  An inordinate amount of time is
being spent in this function and another one like it.

Catherine

     def calc_sdcm_at_rlra(self,iblock):

         npixl = self.nlineht/self.nlinerl
         npixs = self.nsmpht/self.nsmprl

         sdcm_out = numpy.array([Constants.CLOUDMASK_NR] 
*self.nlinerl*self.nsmprl,'int8') \
                         .reshape(self.nlinerl,self.nsmprl)

         for ilinerl in range(0,self.nlinerl):
             for ismprl in range(0,self.nsmprl):

                 height = self.data[iblock].height[ilinerl*2:ilinerl*2 
+2, ismprl*2:ismprl*2+2]
                 sdcm   = self.data[iblock].sdcm[ilinerl*2:ilinerl*2 
+2, ismprl*2:ismprl*2+2]
                 source = self.data[iblock].heightsrc 
[ilinerl*2:ilinerl*2+2, ismprl*2:ismprl*2+2]

                 mask1 = (source == Constants.HEIGHT_STEREO)
                 mask2 = ( (source == Constants.HEIGHT_SURFACE) | \
                           (source == Constants.HEIGHT_DEFAULT) )

                 if (mask1.any()):
                     loc = height[mask1].argmax()
                     sdcm_out[ilinerl,ismprl] = sdcm[mask1].ravel()[loc]
                 elif (mask2.any()):
                     loc = height[mask2].argmax()
                     sdcm_out[ilinerl,ismprl] = sdcm[mask2].ravel()[loc]

         return sdcm_out



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