On 09/02/2022 15:56, Bernardo Santos via grass-user wrote:
Dear Markus
Thanks for your answer. I thought about v.db.dropcolumn earlier.
However, I am trying to include this within a function, following a
workflow in which the input vector might have columns with different
names, in which case it would be good to be able to select the ones to
keep instead of the ones to remove.
For instance, let's say I want to be able to provide either of the
following input vectors:
- "vector1", with columns "a, b, c, d, e"
- "vector2", with columns "j, a, k, f, e"
Let's say I want to keep only "a" and "e".
If I use v.db.dropcolumn, I'll have to first list the names of all
columns (maybe with v.db.select), then match all the column names that
I do not want to keep ("b, c, d" in the first case, "j, k, f" in the
second), then drop them with v.db.dropcolumn. I thought just selecting
which ones I want to keep would be simpler and more straightforward,
if there is a tool for that.
So far I am doing a long step (I am creating a temporary vector to
avoid changing the input):
1) v.extract -t input=vector1 output=temp_vector (here I select only
the features, not attribute table)
2) v.db.addtable map=temp_vector columns=a,e (here I do not bother
with the names of the undesirable columns)
3) v.db.update map=temp_vector column=e value=1 (here I set a value
but I could take it from the original vector1)
Following what you suggest, I could do:
1) g.copy vector=vector1,temp_vector
2) (find out a way to list the undesirable column names)
Here's a possible solution. (Somewhat clunky, but workable)
You can get the column names using v.info -c, then grep out the column
names you want to keep. Then loop thru the column names that are left
and do v.db.dropcolumn for each.
i.e.:
# I have a vector "terraces" with 4 columns.
v.info -c terraces
Displaying column types/names for database connection of layer <1>:
INTEGER|cat
INTEGER|OBJECTID
DOUBLE PRECISION|Shape_Leng
INTEGER|Validated
#Keep only 2: "cat" and "Validated"
g.copy vect=terraces,terraces_tmp
Copying vector <terraces@Avdat> to current mapset as <terraces_tmp>
# Get a list of columns to drop
to_drop=`v.info -c terraces_tmp | cut -d'|' -f2 | grep -E -v 'Validate|cat'`
Displaying column types/names for database connection of layer <1>:
echo $to_drop
OBJECTID Shape_Leng
# Loop thru columns to drop and run v.db.dropcolumn on each
for col in $to_drop; do v.db.dropcolumn terraces_tmp col=${col}; done
# Result
v.info -c terraces_tmp
Displaying column types/names for database connection of layer <1>:
INTEGER|cat
INTEGER|Validated
HTH,
Micha
3) v.db.dropcolumn columns=list_undesidable_columns
Neither of the solutions is so straightforward, that's why I thought
there should be (or could be) another way.
In R, for instance, I can easily subset columns of an sf object using
dplyr::select(vector1, a, e). I guess in PostGIS it is also possible
to do that with "SELECT statements", even though I am less skilled
there. It would be great to have a module in GRASS to do it as well...
Best
Bernardo
Em quarta-feira, 9 de fevereiro de 2022 09:28:06 GMT+1, Markus Neteler
<nete...@osgeo.org> escreveu:
Hi Bernardo,
On Wed, Feb 9, 2022 at 1:39 AM Bernardo Santos via grass-user
<grass-user@lists.osgeo.org> wrote:
>
> Dear list,
>
> Is there a GRASS GIS command (maybe a v.db.* one) to subset, in a
single command, the columns of a vector?
>
> What I have: vector "vect" with 5 columns "a, b, c, d, e" in the
attribute table
> What I want: vector "vect_sub" with only, for instance, "a, c, e"
>
> I can use v.extract to subsample rows, but not columns. What is the
easiest way of doing that, what needing many commands (like creating
or copying the vector to a new one, creating a new attribute table,
then copying only the columns I want).
Isn't this simply
https://grass.osgeo.org/grass80/manuals/vector.html
--> v.db.dropcolumn - Drops a column from the attribute table
connected to a given vector map.
--> It offers single and multiple column dropping:
columns=name[,name,...]
> I looked for that in the documentation but did not found it so easily.
Please suggest where to improve the documentation.
Markus
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Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
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