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

[Moritz Lennert]

On 30/09/15 15:27, Paulo van Breugel 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?

Pygrass has other options, but you can always call v.to.db with the -p 
flag and read the output directly. See one example at [1].

Moritz

[1] 
https://trac.osgeo.org/grass/browser/grass-addons/grass7/vector/v.centerline/v.centerline.py#L198