[Numpy-discussion] subsampling array without loops

[Zachary Pincus]

> 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.

Perhaps you could take the relevant slices of the arrays with  
appropriate striding:

a = numpy.arange(128*512).reshape((128, 512))
top_left = a[::2, ::2]
top_right = a[::2, 1::2]
bottom_left = a[1::2, ::2]
bottom_right = a[1::2, 1::2]

tiles = numpy.array([top_left, top_right, bottom_left, bottom_right])
maxes = tiles.max(axis=0)

Similarly if you want overlapping tiles, you could leave out the  
final :2 in the slice specifications above.

Zach


> 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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