[GRASS-user] vector attribute calculation in grass python

[Moritz Lennert]

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