[GRASS-stats] Why does rgrass7::writeVECT() silently add a cat column if it doesn't exist?

[Tobias Pilz]

Hi Vero,

as I understand it, vector files in GRASS need a key column if they have 
an attribute table. But I don't see that this has to be a column named 
'cat'. However, GRASS is quite inconsistent here. Some functions ensure 
the existence of a 'cat' column, such as v.db.dropcolumn (just gives an 
error if you try to remove the 'cat' column), whereas other functions 
let one define other columns as key column (e.g. manually in the GUI or 
v.db.addtable).

The latter happened with my vector map, i.e. a different column was 
defined as key column. Therefore it is confusing why 
rgrass7::writeVECT() just adds the cat column and without any 
notification. This makes no sense to me and is annoying as my subsequent 
workflow produced unexpected results and I had to trace back the error 
which took a while.

 From my perspective as user it would be great if a more consistent 
solution regarding the treatment of key columns could be found, i.e. 
either always ensure the existence of a column named 'cat' or just 
accept if the user specifies a different key column.

Regards,

Tobias

Am 20.10.21 um 20:41 schrieb Veronica Andreo:
> Hello Tobias,
>
> The cat colum is the ID that GRASS uses for vectors, hence this is expected
> and right. The same happens if you import a gpgk or shape or any other
> vector file into GRASS. See
> https://grass.osgeo.org/grass78/manuals/vectorintro.html for further info
>
> HTH,
> Vero
>
>
>
> El mié, 20 oct 2021 a las 9:34, Tobias Pilz (<topilz at pik-potsdam.de>)
> escribió:
>
>> Dear list,
>>
>> when I use in R the writeVECT() command the resulting map in GRASS got
>> the 'cat' column silently added to its attribute table if it didn't exist.
>>
>> The vector object in question was previously imported to R via
>> readVECT() and had a different column defined as key column.
>>
>> Is this behaviour of writeVECT() intentional? If yes, why so and
>> wouldn't it make sense to give a warning?
>>
>> Note: I used rgrass7 version 0.2-6 together with GRASS 7.8.5 under Linux
>> (opensuse).
>>
>> Regards,
>>
>> Tobias
>>
>>
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-- 
Dr. Tobias Pilz

Climate Resilience - Research Department 2
Tel: +49 331 288 2539
E-mail: tobias.pilz at pik-potsdam.de

Potsdam Institute for Climate Impact Research
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www.pik-potsdam.de/members/topilz

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