[GRASS-user] vector attribute calculation in grass python

Alassane_Toure · August 29, 2016, 3:27pm

Group,

I am in the process of developing a python/grass code to calculate an attribute value based on other attributes. The following error comes up:

NameError: name ‘par_sqf’ is not defined

Here is the code…

some predefined variables

###########################
import os
import sys
import subprocess
import shutil
import binascii
import tempfile
import shutil

import gdal
from gdalconst import *
…
…
grass.run_command(‘v.rast.stats’,map=‘parcel’, raster=‘constr_clean1’,method=‘number’,column_prefix=‘c’)

grass.run_command(‘v.db.addcolumn’, map=‘parcel’, columns=‘par_sqf double precision’)
grass.run_command(‘v.to.db’, map=‘parcel’, option=‘area’, columns=‘par_sqf’, unit=‘feet’)
grass.run_command(‘v.db.addcolumn’,map=‘parcel’,columns=‘pchng int’)

grass.read_command(‘v.db.select’,map=‘parcel’,columns=‘c_number’)
grass.read_command(‘v.db.select’,map=‘parcel’,columns=‘par_sqf’)
pchng = range(len(par_sqf))
for i in range(len(par_sqf)):
pchng[i]=+int((0.25*c_number[i]/par_sqf[i])*100)

for i in range(len(par_sqf)):
query=“UPDATE parcel SET pchng=” + str(pchng[i]) + " WHERE cat = " + str(i+1)
grass.write_command(“db.execute”, stdin = query)

…
…

Your insight is appreciated.

Alassane

mlennert · August 29, 2016, 3:32pm

On 29/08/16 17:27, alassane toure wrote:

Group,
I am in the process of developing a python/grass code to calculate an
attribute value based on other attributes. The following error comes up:

NameError: name 'par_sqf' is not defined

Here is the code...

# some predefined variables
###########################
import os
import sys
import subprocess
import shutil
import binascii
import tempfile
import shutil

import gdal
from gdalconst import *
...
grass.run_command('v.rast.stats',map='parcel',
raster='constr_clean1',method='number',column_prefix='c')

grass.run_command('v.db.addcolumn', map='parcel', columns='par_sqf
double precision')
grass.run_command('v.to.db', map='parcel', option='area',
columns='par_sqf', unit='feet')
grass.run_command('v.db.addcolumn',map='parcel',columns='pchng int')

grass.read_command('v.db.select',map='parcel',columns='c_number')
grass.read_command('v.db.select',map='parcel',columns='par_sqf')
pchng = range(len(par_sqf))

It seems that you are trying to get the length of a variable par_sqf that is not defined in the code.

Moritz

Alassane_Toure · August 29, 2016, 6:25pm

Dear Moritz & group,

I am a newbi in python programming. Following Moritz’s instructions i added the followings…

par_sqf=
c_number=
pchng=

but pchng stayed NULL for all rows (see below)!

Inline image 2

Any suggestions how to resolve this?

Regards,

Alassane---------- Forwarded message ----------
From: alassane toure <atourej@gmail.com>
Date: Mon, Aug 29, 2016 at 9:27 AM
Subject: vector attribute calculation in grass python
To: GRASS user list <grass-user@lists.osgeo.org>

Group,

I am in the process of developing a python/grass code to calculate an attribute value based on other attributes. The following error comes up:

NameError: name ‘par_sqf’ is not defined

Here is the code…

some predefined variables

###########################
import os
import sys
import subprocess
import shutil
import binascii
import tempfile
import shutil

import gdal
from gdalconst import *
…
…
grass.run_command(‘v.rast.stats’,map=‘parcel’, raster=‘constr_clean1’,method=‘number’,column_prefix=‘c’)

grass.run_command(‘v.db.addcolumn’, map=‘parcel’, columns=‘par_sqf double precision’)
grass.run_command(‘v.to.db’, map=‘parcel’, option=‘area’, columns=‘par_sqf’, unit=‘feet’)
grass.run_command(‘v.db.addcolumn’,map=‘parcel’,columns=‘pchng int’)

grass.read_command(‘v.db.select’,map=‘parcel’,columns=‘c_number’)
grass.read_command(‘v.db.select’,map=‘parcel’,columns=‘par_sqf’)
pchng = range(len(par_sqf))
for i in range(len(par_sqf)):
pchng[i]=+int((0.25*c_number[i]/par_sqf[i])*100)

for i in range(len(par_sqf)):
query=“UPDATE parcel SET pchng=” + str(pchng[i]) + " WHERE cat = " + str(i+1)
grass.write_command(“db.execute”, stdin = query)

…
…

Your insight is appreciated.

Alassane

Blumentrath_Stefan1 · August 29, 2016, 7:23pm

Hei Alassane,
Can't you just do:

db.execute sql="UPDATE parcel SET pchng=int((0.25*c_number/par_sqf)*100)"

Cheers,
Stefan
________________________________________
Von: grass-user [grass-user-bounces@lists.osgeo.org] im Auftrag von alassane toure [atourej@gmail.com]
Gesendet: Montag, 29. August 2016 20:25
An: GRASS user list
Betreff: [GRASS-user] Fwd: vector attribute calculation in grass python

Dear Moritz & group,
I am a newbi in python programming. Following Moritz's instructions i added the followings..

par_sqf=
c_number=
pchng=

but pchng stayed NULL for all rows (see below)!
[Inline image 2]
Any suggestions how to resolve this?

Regards,
Alassane
---------- Forwarded message ----------
From: alassane toure <atourej@gmail.com<mailto:atourej@gmail.com>>
Date: Mon, Aug 29, 2016 at 9:27 AM
Subject: vector attribute calculation in grass python
To: GRASS user list <grass-user@lists.osgeo.org<mailto:grass-user@lists.osgeo.org>>

Group,
I am in the process of developing a python/grass code to calculate an attribute value based on other attributes. The following error comes up:

NameError: name 'par_sqf' is not defined

Here is the code...

# some predefined variables
###########################
import os
import sys
import subprocess
import shutil
import binascii
import tempfile
import shutil

import gdal
from gdalconst import *
...
...
grass.run_command('v.rast.stats',map='parcel', raster='constr_clean1',method='number',column_prefix='c')

grass.run_command('v.db.addcolumn', map='parcel', columns='par_sqf double precision')
grass.run_command('v.to.db', map='parcel', option='area', columns='par_sqf', unit='feet')
grass.run_command('v.db.addcolumn',map='parcel',columns='pchng int')

grass.read_command('v.db.select',map='parcel',columns='c_number')
grass.read_command('v.db.select',map='parcel',columns='par_sqf')
pchng = range(len(par_sqf))
for i in range(len(par_sqf)):
pchng[i]=+int((0.25*c_number[i]/par_sqf[i])*100)

for i in range(len(par_sqf)):
query="UPDATE parcel SET pchng=" + str(pchng[i]) + " WHERE cat = " + str(i+1)
grass.write_command("db.execute", stdin = query)

...
...

Your insight is appreciated.

Alassane

(attachments)

Alassane_Toure · August 29, 2016, 10:22pm

It works great…thanks Stefan!

···

On Mon, Aug 29, 2016 at 1:23 PM, Blumentrath, Stefan <Stefan.Blumentrath@nina.no> wrote:

Hei Alassane,
Can’t you just do:

db.execute sql=“UPDATE parcel SET pchng=int((0.25*c_number/par_sqf)*100)”

Cheers,
Stefan

Von: grass-user [grass-user-bounces@lists.osgeo.org] im Auftrag von alassane toure [atourej@gmail.com]
Gesendet: Montag, 29. August 2016 20:25
An: GRASS user list
Betreff: [GRASS-user] Fwd: vector attribute calculation in grass python

Dear Moritz & group,
I am a newbi in python programming. Following Moritz’s instructions i added the followings…

par_sqf=
c_number=
pchng=

but pchng stayed NULL for all rows (see below)!
[Inline image 2]
Any suggestions how to resolve this?

Regards,
Alassane
---------- Forwarded message ----------
From: alassane toure <atourej@gmail.com mailto:[atourej@gmail.com](mailto:atourej@gmail.com)>
Date: Mon, Aug 29, 2016 at 9:27 AM
Subject: vector attribute calculation in grass python

To: GRASS user list <grass-user@lists.osgeo.org mailto:[grass-user@lists.osgeo.org](mailto:grass-user@lists.osgeo.org)>

Group,
I am in the process of developing a python/grass code to calculate an attribute value based on other attributes. The following error comes up:

NameError: name ‘par_sqf’ is not defined

Here is the code…

some predefined variables

###########################
import os
import sys
import subprocess
import shutil
import binascii
import tempfile
import shutil

import gdal
from gdalconst import *
…
…
grass.run_command(‘v.rast.stats’,map=‘parcel’, raster=‘constr_clean1’,method=‘number’,column_prefix=‘c’)

grass.run_command(‘v.db.addcolumn’, map=‘parcel’, columns=‘par_sqf double precision’)
grass.run_command(‘v.to.db’, map=‘parcel’, option=‘area’, columns=‘par_sqf’, unit=‘feet’)
grass.run_command(‘v.db.addcolumn’,map=‘parcel’,columns=‘pchng int’)

grass.read_command(‘v.db.select’,map=‘parcel’,columns=‘c_number’)
grass.read_command(‘v.db.select’,map=‘parcel’,columns=‘par_sqf’)
pchng = range(len(par_sqf))
for i in range(len(par_sqf)):
pchng[i]=+int((0.25*c_number[i]/par_sqf[i])*100)

for i in range(len(par_sqf)):
query=“UPDATE parcel SET pchng=” + str(pchng[i]) + " WHERE cat = " + str(i+1)
grass.write_command(“db.execute”, stdin = query)

…
…

Your insight is appreciated.

Alassane

mlennert · August 30, 2016, 9:17am

Dear Alassane,

Please keep conversations on the list.

On 29/08/16 19:49, alassane toure wrote:

Dear Moritz,
I am a newbi in python programming. Following your instructions i added
the followings..

par_sqf=
c_number=
pchng=

but pchng stayed NULL for all rows (see below)!

You set pchng = range(len(par_sqf)),
but par_sqf is only defined as a Python variable by

par_sqf=

so it is normal that it is NULL.

Don't mix up Python variables and vector map column names.

If you want to set the attribute column pchng to contain a value that is equal to the length of the value of the attribute column par_sqf (i.e. count the number of digits (plus decimal point) in the area value), then you have to use v.db.update:

v.db.update map=parcel column=pchng qcol="length(par_sqf)"

Although I don't understand what the sens of such a length would be.

Maybe it would be easier if you explained us what you are trying to do, instead of only how you are doing it.

Moritz

Alassane_Toure · August 30, 2016, 3:15pm

Problem solved! Suggestion was made to use db.execute and it worked without variable declaration. Thanks for your insight!

On Aug 30, 2016 3:17 AM, “Moritz Lennert” <mlennert@club.worldonline.be> wrote:

Dear Alassane,

Please keep conversations on the list.

On 29/08/16 19:49, alassane toure wrote:

Dear Moritz,
I am a newbi in python programming. Following your instructions i added
the followings…

par_sqf=
c_number=
pchng=

but pchng stayed NULL for all rows (see below)!

You set pchng = range(len(par_sqf)),
but par_sqf is only defined as a Python variable by

par_sqf=

so it is normal that it is NULL.

Don’t mix up Python variables and vector map column names.

If you want to set the attribute column pchng to contain a value that is equal to the length of the value of the attribute column par_sqf (i.e. count the number of digits (plus decimal point) in the area value), then you have to use v.db.update:

v.db.update map=parcel column=pchng qcol=“length(par_sqf)”

Although I don’t understand what the sens of such a length would be.

Maybe it would be easier if you explained us what you are trying to do, instead of only how you are doing it.

Moritz