[GRASS-dev] Write array to (point) attribute table
Moritz Lennert
mlennert at club.worldonline.be
Thu Oct 1 00:15:44 PDT 2015
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.vanbreugel at 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.vanbreugel at gmail.com <mailto: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,
>>>
>>> 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
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