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

[Moritz Lennert]

On 01/10/15 00:05, Paulo van Breugel wrote:


On 30-09-15 17:52, Paulo van Breugel wrote:


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> 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> wrote:

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

Tried out again, but after running the code below, I end up with an
empty vector layer (no points and no attribute table). Same if I put the
vectormap.table.conn.commit() in the for cycle.

import grass.script as grass

from grass.pygrass.vector import VectorTopo

grass.run_command("v.random", output="testB", npoints=10, overwrite=True)

grass.run_command("v.db.addtable", map="testB", columns="GR INTEGER")

with VectorTopo('testB', mode='w') as vectormap:

     for feature in vectormap:

         feature.attrs['GR'] = 9

         vectormap.table.conn.commit()


Just a wild guess, here without looking at the library code, but could 
it be that it should be

VectorTopo('testB', mode='a')

instead of

VectorTopo('testB', mode='w')

Moritz
_______________________________________________
grass-dev mailing list
grass-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-dev