On 30-09-15 16:18, Anna Petrášová wrote:
On Wed, Sep 30, 2015 at 9:27 AM, Paulo van Breugel
<[email protected] <mailto:[email protected]>> 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?
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.
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.
Pietro
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