Re: [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.vanbreu...@gmail.com <mailto:p.vanbreu...@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.vanbreu...@gmail.com <mailto:p.vanbreu...@gmail.com>> wrote:



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

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