On 30/09/15 15:27, Paulo van Breugel wrote:
On Wed, Sep 30, 2015 at 11:53 AM, Pietro <[email protected]
<mailto:[email protected]>> wrote:
On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel
<[email protected] <mailto:[email protected]>> wrote:
>
>
> On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <[email protected]
<mailto:[email protected]>>
> wrote:
>>
>>
>>
>> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel
>> <[email protected] <mailto:[email protected]>> 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?
Pygrass has other options, but you can always call v.to.db with the -p
flag and read the output directly. See one example at [1].
Moritz
[1]
https://trac.osgeo.org/grass/browser/grass-addons/grass7/vector/v.centerline/v.centerline.py#L198
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