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

[Anna Petrášová]

On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel <p.vanbreugel at gmail.com>
wrote:

>
>
> 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?
>
>
> I don't fully understand the example, but yes, you can get coordinates:

with VectorTopo('myvector', mode='w') as vectormap:
    for feature in vectormap:
        print feature.x
        print feature.y


>
>
>>
>> Pietro
>>
>
>
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