Hi Michel,Thanks! I think I got the suggestions by Anna, Pietro and Moritz working, but this certainly looks like an handy function to have at hand. One possible disadvantage if using this in a script to be shared with others is that it would add an dependency on pandas, wouldn't it?
Cheers, Paulo On 01-10-15 10:01, Michel Wortmann wrote:
Hi Paulo,I had the same problem some time ago. I am using pandas for a lot of things and discovered the quickest way to append a column to a vector table is using pandas' .to_sql dataframe method. Here is my full function for it:import pandas as pa dataframe = pa.DataFrame( ... ) def appendColumns(vecttbl,dataframe,join=None,layer=1): '''Upload a DataFrame (Series should be first converted into a sql compliant datafram) to a grass vector table. Join is a valid column in vecttbl or if None take categories. Always joins on df.index, which is not uploaded.''' # get sqlite.db info db = grass.vector_db(vecttbl)[layer] con = sqlite3.connect(db['database']) # decide join indeces itbl = {False:db['key'],True:join}[join!=None] # make series to dataframe dataframe.to_sql('pandas_temp',con,if_exists='replace',index_label='ix') # index label? # join columns to vecttbl grass.run_command('v.db.join',map=vecttbl,column=itbl, other_table='pandas_temp',other_column='ix', subset_columns=','.join(map(str,dataframe.columns))) # remove temp table cur = con.cursor() cur.execute('DROP TABLE IF EXISTS pandas_temp') con.commit(); con.close() returnThe dataframe.to_sql line is essentially all you need, the rest of the function is just prepping.Hope it helps, Michel On 10/01/2015 12:05 AM, 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.vanbreu...@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.vanbreu...@gmail.com <mailto:p.vanbreu...@gmail.com>> wrote: On Wed, Sep 30, 2015 at 11:53 AM, Pietro <peter.z...@gmail.com> wrote: On Wed, Sep 30, 2015 at 9:51 AM, Paulo van Breugel <p.vanbreu...@gmail.com> wrote: > > > On Wed, Sep 30, 2015 at 2:02 AM, Anna Petrášová <kratocha...@gmail.com> > wrote: >> >> >> >> On Tue, Sep 29, 2015 at 6:09 PM, Paulo van Breugel >> <p.vanbreu...@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.yGreat, 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-classesbut 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.ipynbThanks, I will have a look at it.Pietro_______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev_______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
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