[Numpy-discussion] summing over more than one axis

[Joe Harrington]

On Thu, 19 Aug 2010 09:06:32 -0500, G?khan Sever <gokhansever at gmail.com> wrote:

>On Thu, Aug 19, 2010 at 9:01 AM, greg whittier <gregwh at gmail.com> wrote:
>
>> I frequently deal with 3D data and would like to sum (or find the
>> mean, etc.) over the last two axes.  I.e. sum a[i,j,k] over j and k.
>> I find using .sum() really convenient for 2d arrays but end up
>> reshaping 2d arrays to do this.  I know there has to be a more
>> convenient way.  Here's what I'm doing
>>
>> a = np.arange(27).reshape(3,3,3)
>>
>> # sum over axis 1 and 2
>> result = a.reshape((a.shape[0], a.shape[1]*a.shape[2])).sum(axis=1)
>>
>> Is there a cleaner way to do this?  I'm sure I'm missing something obvious.
>>
>> Thanks,
>> Greg
>>
>
>Using two sums
>
>np.sum(np.sum(a, axis=-2), axis=1)

Be careful.  This works for sums, but not for operations like median;
the median of the row medians may not be the global median.  So, you
need to do the medians in one step.  I'm not aware of a method cleaner
than manually reshaping first.  There may also be speed reasons to do
things in one step.  But, two steps may look cleaner in code.

--jh--