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

[Paulo van Breugel]

On 01-10-15 09:15, Moritz Lennert wrote:
> 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')

That is not a valid option; "Mode not supported. Use one of: 'r', 'rw', 
'w' ".  This did make me try again though, but now with mode='rw', which 
worked:

idx= array([0, 1, 0, 0, 0, 0, 1, 1, 0, 1])

i = -1

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

     for feature in vectormap:

         i = i + 1

         feature.attrs['GR'] = idx[i]

     vectormap.table.conn.commit()


Thanks all for the help/advices

> Moritz