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

[Paulo van Breugel]

On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.zamb at gmail.com> wrote:

> On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
> <p.vanbreugel at gmail.com> wrote:
> >
> >
> > On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratochanna at gmail.com>
> > wrote:
> >>
> >>
> >>
> >> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
> >> <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?




>
> Pietro
>
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