[GRASS-dev] Write array to (point) attribute table

[Paulo van Breugel]

On 30-09-15 17:47, Anna Petrášová wrote:
>
>
> On Wed, Sep 30, 2015 at 11:20 AM, Paulo van Breugel 
> <p.vanbreugel at gmail.com <mailto:p.vanbreugel at gmail.com>> wrote:
>
>
>
>     On 30-09-15 16:18, Anna Petrášová wrote:
>>
>>
>>     On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel
>>     <p.vanbreugel at gmail.com <mailto:p.vanbreugel at gmail.com>> wrote:
>>
>>
>>
>>         On Wed, Sep 30, 2015 at 11:53 AM, Pietro
>>         <peter.zamb at gmail.com <mailto:peter.zamb at gmail.com>> wrote:
>>
>>             On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
>>             <p.vanbreugel at gmail.com <mailto:p.vanbreugel at gmail.com>>
>>             wrote:
>>             >
>>             >
>>             > On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová
>>             <kratochanna at gmail.com <mailto:kratochanna at gmail.com>>
>>             > wrote:
>>             >>
>>             >>
>>             >>
>>             >> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
>>             >> <p.vanbreugel at gmail.com
>>             <mailto:p.vanbreugel at gmail.com>> wrote:
>>             >>>
>>             >>> This must be a very basic question, but I can't find
>>             an easy/direct way
>>             >>> to do this. In python, if I have an array with values
>>             with a length equal to
>>             >>> the number of rows in an attribute table of a (point)
>>             vector layer, how can
>>             >>> I write those values to a new column in that
>>             attribute table. I can of
>>             >>> course first create the column, but than how to
>>             update that column with the
>>             >>> values in the array?
>>             >>
>>             >>
>>             >> it should be pretty easy to do with pygrass,
>>             unfortunately there is no
>>             >> example on assigning attributes in the official
>>             documentation [1],  but it
>>             >> should be pretty easy, something like that (not tested):
>>             >>
>>             >> with VectorTopo('myvector', mode='w') as vectormap:
>>             >>     for feature in vectormap:
>>             >>  feature.attrs['mycolumn'] = value
>>             >>
>>             >>
>>             > Thanks, but that seems to write the vector back without
>>             attribute table
>>
>>             You have to save the changes in the database out from
>>             your cycle, with:
>>
>>             vectormap.table.conn.commit()
>>
>>
>>         Thanks Pietro. I am, however, not sure I understand (I tried
>>         to use it, but thanks to my limited experience in Python /
>>         pygrass not much luck). Just to be more specific, I am trying
>>         to create a script that divides points in training and test
>>         groups, similar to v.kcv, but with points clustered in space.
>>         E.g.,
>>
>>         # Create vector
>>         grass.run_command("v.random", output="testB", npoints=10,
>>         overwrite=True)
>>         grass.run_command("v.db.addtable", map="testB", columns="X
>>         DOUBLE PRECISION,Y DOUBLE PRECISION,GR INTEGER")
>>         grass.run_command("v.to.db", map="test", option="coor",
>>         columns="X,Y")
>>
>>         # Create groups
>>         vectmap = 'test'
>>         cvals = array(grass.vector_db_select(vectmap, layer = int(1),
>>         columns = 'X,Y')['values'].values()).astype(np.float)
>>         centroids,_ = kmeans(cvals,2)
>>         idx,_ = vq(cvals,centroids)
>>
>>         # write results to tabel
>>         Now I would like to write idx to the column 'GR' in the
>>         attribute table of 'test'.
>>
>>         p.s. I am first creating the XY columns now, but is there a
>>         function to get the coordinates (cvals) in pygrass directly?
>>
>>
>>     I don't fully understand the example,
>
>     Thanks for the quick response. I basically have a list with values
>     (idx in the example above) which I like to add as a column to the
>     attribute table of an existing vector (point layer). The length of
>     idx is equal to the number of rows in the attribute table. The
>     solution of Anna seems like an elegant solution (and easier and
>     more flexible than other solutions I tried using e.g., sqlite3).
>     However, as I wrote, I end up with a vector without attribute
>     table. You wrote that I "have to save the changes in the database
>     out from your cycle, with: vectormap.table.conn.commit()". I am,
>     however, not sure what you mean with 'out from your cycle' or how
>     to implement that.
>
>
> probably after the for cycle ends you would call this 
> 'vectormap.table.conn.commit()', if it doesn't works, try to put it in 
> the cycle (I am not sure what is supposed to work).

I did try both, with no luck so-far. I'll give it another try though, 
perhaps I did something else wrong.
>
>
>>     but yes, you can get coordinates:
>>
>>     with VectorTopo('myvector', mode='w') as vectormap:
>>         for feature in vectormap:
>>     print feature.x
>>           print feature.y
>
>     Great, thanks. I did not find this in the manual. If it is not
>     there, perhaps it would be something worth including? I would not
>     mind providing a text, but I am not sure what would be the best
>     way to do that.
>
>
> Technically, it's in the manual:
> https://grass.osgeo.org/grass70/manuals/libpython/pygrass_vector.html#geometry-classes
>
> but we are lacking more examples of often used constructions. Some 
> other examples are available in the recent workshop we did:
>
> https://github.com/wenzeslaus/python-grass-addon/blob/master/02_pygrass_library.ipynb

Thanks, I will have a look at it.
>
>
>>
>>
>>             Pietro
>>
>>
>>
>
>

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