[GRASS-dev] v.univar vs v.db.univar

[Moritz Lennert]

On 24/06/17 09:49, Moritz Lennert wrote:
>
>
> Le 23 juin 2017 08:38:43 GMT+02:00, Markus Metz <markus.metz.giswork at gmail.com> a écrit :
>> On Thu, Jun 22, 2017 at 4:01 PM, Moritz Lennert <
>> mlennert at club.worldonline.be> wrote:
>>>
>>> On 22/06/17 15:25, Moritz Lennert wrote:
>>>>
>>>> On 22/06/17 14:21, Markus Metz wrote:
>>>>>
>>>>>
>>>>>
>>>>> On Thu, Jun 22, 2017 at 10:53 AM, Moritz Lennert
>>>>> <mlennert at club.worldonline.be
>> <mailto:mlennert at club.worldonline.be>>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> On 21/06/17 22:19, Markus Metz wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Wed, Jun 21, 2017 at 4:30 PM, Moritz Lennert
>>>>>>> <mlennert at club.worldonline.be
>> <mailto:mlennert at club.worldonline.be>
>>>>>
>>>>> <mailto:mlennert at club.worldonline.be
>>>>> <mailto:mlennert at club.worldonline.be>>> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On 21/06/17 14:57, Markus Neteler wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Wed, Jun 21, 2017 at 11:54 AM, Moritz Lennert
>>>>>>>>> <mlennert at club.worldonline.be
>> <mailto:mlennert at club.worldonline.be>
>>>>>
>>>>> <mailto:mlennert at club.worldonline.be
>>>>> <mailto:mlennert at club.worldonline.be>>>
>>>>>>>
>>>>>>>
>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Do we really need v.db.univar ? Does it provide anything
>> else/better
>>>>>>>
>>>>>>>
>>>>>>> than
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> v.univar ?
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> The main difference between v.univar and v.db.univar is that
>> v.univar
>>>>>>> works with geometries while v.db.univar works with the attribute
>> table.
>>>>>>> If vector geometries share the same category value, or if some
>> vector
>>>>>>> geometries have more than one category value, the results of
>> v.univar
>>>>>>> and v.db.univar for the same column will be different. If you
>> want
>>>>>>> results with category (or class) as unit, use v.db.univar, if you
>> want
>>>>>>> results with vector geometry as unit, use v.univar.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Right. Didn't think about this. So they are actually
>> complementary.
>>>>>>
>>>>> [...]
>>>>>>
>>>>>>
>>>>>> However, in the case of multiple cat values per feature, it seems
>> that
>>>>>
>>>>> v.univar actually reads the attribute once for every
>> geometry/category
>>>>> combination, not only for each feature:
>>>>>
>>>>> According to the source code, yes.
>>>>>
>>>>> [...]
>>>>>>
>>>>>>
>>>>>> I don't know if this is the desired feature or if each geometry
>> should
>>>>>
>>>>> only be taken into account once, whatever its number of categories.
>> I
>>>>> personally would have expected the latter, if we consider v.univar
>> to be
>>>>> geometry-based.
>>>>>
>>>>> If each geometry should be taken into account only once, a
>> corresponding
>>>>> category / attribute value would need to be randomly picked.
>> Therefore I
>>>>> would prefer to consider all categories for each geometry.
>>>>
>>>>
>>>> Right.
>>>>
>>>>>>
>>>>>> All this definitely needs clearer documentation in the man page.
>>>>>
>>>>> Attached an attempt to amend the two respective man pages. MarkusM,
>> is
>>>>> this correct ? However, the above question concerning v.univar
>> probably
>>>>> should be checked before I commit this.
>>>>>
>>>>> According to the source code, v.univar reads attributes for all
>>>>> categories of a geometry.
>>>>
>>>>
>>>> Ok.
>>>>
>>>>>
>>>>> v.db.univar uses db.univar, not db.select, that's an error in the
>>>>> existing manual.
>>>>
>>>>
>>>> But db.univar uses db.select to access the data, so I don't consider
>>>> this an error in the manual...
>>>>
>>>>>
>>>>> I suggest to update the manuals first, then decide if the modules
>> should
>>>>> be modified.
>>>>
>>>>
>>>> Ok. See r71207 and r71208. Please check and tell me if I can
>> backport.
>>>
>>>
>>> Plus a further attempt at a better formulation for v.db.univar in
>> r71209.
>>
>> I have modified the description of v.univar regarding distance
>> calculations
>> in r71210,1. Backport now?
>
> +1
>
> But traveling so can't do it before next week.

Done in r71224.

Moritz