[Numpy-discussion] Questionable reduceat behavior

[Wes McKinney]

On Sun, Aug 14, 2011 at 11:58 AM, Wes McKinney <wesmckinn at gmail.com> wrote:
> On Sat, Aug 13, 2011 at 8:06 PM, Mark Wiebe <mwwiebe at gmail.com> wrote:
>> Looks like this is the second-oldest open bug in the bug tracker.
>> http://projects.scipy.org/numpy/ticket/236
>> For what it's worth, I'm in favour of changing this behavior to be more
>> consistent as proposed in that ticket.
>> -Mark
>>
>> On Thu, Aug 11, 2011 at 11:25 AM, Wes McKinney <wesmckinn at gmail.com> wrote:
>>>
>>> I'm a little perplexed why reduceat was made to behave like this:
>>>
>>> In [26]: arr = np.ones((10, 4), dtype=bool)
>>>
>>> In [27]: arr
>>> Out[27]:
>>> array([[ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True],
>>>       [ True,  True,  True,  True]], dtype=bool)
>>>
>>>
>>> In [30]: np.add.reduceat(arr, [0, 3, 3, 7, 9], axis=0)
>>> Out[30]:
>>> array([[3, 3, 3, 3],
>>>       [1, 1, 1, 1],
>>>       [4, 4, 4, 4],
>>>       [2, 2, 2, 2],
>>>       [1, 1, 1, 1]])
>>>
>>> this does not seem intuitively correct. Since we have:
>>>
>>> In [33]: arr[3:3].sum(0)
>>> Out[33]: array([0, 0, 0, 0])
>>>
>>> I would expect
>>>
>>> array([[3, 3, 3, 3],
>>>       [0, 0, 0, 0],
>>>       [4, 4, 4, 4],
>>>       [2, 2, 2, 2],
>>>       [1, 1, 1, 1]])
>>>
>>> Obviously I can RTFM and see why it does this ("if ``indices[i] >=
>>> indices[i + 1]``, the i-th generalized "row" is simply
>>> ``a[indices[i]]``"), but it doesn't make much sense to me, and I need
>>> work around it. Suggestions?
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>>> NumPy-Discussion at scipy.org
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>>
>>
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>>
>
> Well, I certainly hope it doesn't get forgotten about for another 5
> years. I think having more consistent behavior would be better rather
> than conforming to a seemingly arbitrary decision made ages ago in
> Numeric.
>
> - Wes
>

just a manual hack for now where I needed it...

https://github.com/wesm/pandas/blob/master/pandas/core/frame.py#L2155