[GRASS-user] log function for SQLite

[Moritz Lennert]

Le 9 mars 2017 19:01:32 GMT+01:00, Nikos Alexandris <nik at nikosalexandris.net> a écrit :
>
>NikosAlexandris wrote:
>>>>>>>> Dear list,
>>>>>>>> where is the extension for SQLite that contains the
>(mathematical) log
>>>>>>>> function available to download?  As referenced in
>>>>>>>> https://grass.osgeo.org/grass73/manuals/v.db.update.html.
>
>Helmut Kudrnovsky:
>
>>>>>>>> I think the reference in the manual requires a source or a link
>to it.
>>>>>>> just found something in the wiki:
>>>>>>>
>https://grasswiki.osgeo.org/wiki/Build_SQLite_extension_on_windows
>
>Nikos:
>
>>>>> For Linux:
>>>>> gcc extension-functions.c -fPIC -shared -o
>libextensionfunctions.so
>>>>> Note,
>>>>> 1) there is still a "compile" warning for the code (no time to
>hunt)
>>>>> 2) The log() function does not like, naturally, 0 values. But this
>can
>>>>> be workedaround with a where SQL clause.
>>>>> Can we not integrate this into GRASS?  It feels awkward to have to
>spend
>>>>> so much time for a log or similar function to use in a v.db.update
>call.
>
>Moritz:
>
>>>> -1
>>>>
>>>> I don't think we should start interalizing solutions to issues
>related
>>>> to database backend choice.
>
>Nikos:
>
>>> We can of course integrate a log and an "sqrt" function for
>v.db.update,
>>> right?
>
>Moritz:
>
>>You mean within the v.db.update code ? But why log and sqrt and not
>others ?
>>As said, I don't think we should go down that road.
>
>You mean to say also no in intgrating, yes, in v.db.update some
>"common"
>functions?  Why not?  Log and SqRt are just common functions.  We have
>all of the basic descriptive statistics and we can do pretty much
>everything with mapcalc.  Why not so for vector attributes, independent
>from tha backend?
>
>If there is no complication in integrating in v.db.update some math,
>then why not use all of the math functions defined in the contributed
>library for sqlite?

The difficulty is not the functions as such, but the parser you need to interpret what the user writes.

But hey, if you want to go down that road, go ahead. I just don't think that compiling the functions once for SQLite is such a hassle as to make worthwhile... 

Moritz


>
>Note, d.vect has an "-r" flag. I want to get these values in a new
>column, so I can share the vector and this sqrt-ed attribute outside of
>GRASS.
>
>Cheers, Nikos