[GRASS-dev] Write array to (point) attribute table
Paulo van Breugel
p.vanbreugel at gmail.com
Wed Sep 30 15:05:28 PDT 2015
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()
>>
>>> 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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