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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Thu, 20 Sep 2012 13:28:12 -0600
Subject: Re: looping in array vs looping in a dic
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On Thu, Sep 20, 2012 at 1:09 PM, MRAB <python@mrabarnett.plus.com> wrote:
> for col in range(cols):
>     for row in range(rows):
>         cat = valuesCategory[row, col]
>         ras = valuesRaster[row, col]
>         totals[cat] += ras

Expanding on what MRAB wrote, since you probably have far fewer
categories than pixels, you may be able to take better advantage of
numpy's vectorized operations (which are pretty much the whole point
of using numpy in the first place) by looping over the categories
instead:

for cat in categories:
    totals[cat] += np.sum(valuesCategory * (valuesRaster == cat))