[GRASS-dev] Direct link from GRASS to R via C

[Vaclav Petras]

On 16 July 2013 09:59, Rainer M Krug <Rainer at krugs.de> wrote:
>
> Vaclav Petras <wenzeslaus at gmail.com> writes:
>
>> Hi,
> Hi,
>
>>
>> you probably know, I've once started the discussion on this topic on
>> grass-stats ML (very simple test code included) [1]. And I've also
>> tried some basic example and experienced some problems [2].
>
> To my shame, I must say I overlooked your threads. I am aware of the
> problem which Roger discussed, therefore I think it would be the best to
> write a function in GRASS which is exported and is doing the access and
> simply returns the column / block / complete raster (i.e. a *very basic*
> exported raster access api for grass).
>
> This function could be called then from Rcpp and it would be much easier
> then writing the complete access function in Rcpp (as you mention below,
> I also think that the header files locations might be a problem).
>
>
>> To the
>> information you provided I would add that the new functionality have
>> to be written in C++ since Rcpp is only for C++.
>
> I am always confused by these two and how they are linked together - can
> I use C libraries from Rcpp, as described above?
>
Yes you can, see my example. Generally, you can call C functions from
C++ code and create C wrapper for your C++ library (you can find
documentation everywhere, note especially the extern "C" declaration).

> As you already have done some work (could you please mail it to me as I
> can't access the attachment via gmane?), I would very much appreciate
> your input - also my C / C++ experience is quite limited and input from
> that side would be brilliant.
>
I have only that small example (attached) but it contains all you
need, however there were also some problems [2].

> Thanks,
>
> Rainer
>
>>
>> Vaclav
>>
>> [1] http://lists.osgeo.org/pipermail/grass-stats/2012-July/001422.html
>> [2] http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2012-July/004055.html
>>
>>
>> On 15 July 2013 17:30, Rainer M Krug <Rainer at krugs.de> wrote:
>>> The following message is a courtesy copy of an article
>>> that has been posted to gmane.comp.gis.grass.devel as well.
>>>
>>> Hi
>>>
>>> I posted on my blog an outline of an idea of how spatial data could be
>>> easily loaded from GRASS into R and written back.
>>>
>>> (see http://rmkrug.wordpress.com/2013/07/15/grassrlink-1/)
>>>
>>> In a nutshell, the idea would be to use the C functions in GRASS to
>>> write a function which returns a single column, column range or whole
>>> raster. This function could then be called from R (via Rcpp) to get the
>>> data into R without having to worry about intermittent exports and
>>> imports via the hdd.
>>>
>>> I would ike to have some input from the GRASS developer community what
>>> they think about such a function. This function should be usable as
>>> "stand alone", and not requiring the opening and buffer allocations et
>>> al as in the r.example, but rather simply take the mapset, raster name,
>>> column(s) to read, if MASK should be respected, ... arguments.
>>>
>>> I was thinking that it would be useful to have such a function in GRASS,
>>> as the compilation together with GRASS, in the same line as a module,
>>> would be quite easy.
>>>
>>> Initially, I was thinking about read and write support for rasters,
>>> which then could be extended to vectors and 3d-rasters and possibly even
>>> the temporal data.
>>>
>>> Could you give feedback on what y9ou think about the idea, and how you
>>> think it could be realized (simplicity to install would be important).
>>>
>>> Cheers and thanks,
>>>
>>> Rainer
>>>
>>> --
>>> Rainer M. Krug
>>>
>>> email: RMKrug<at>gmail<dot>com
>>> _______________________________________________
>>> grass-dev mailing list
>>> grass-dev at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/grass-dev
>
>
> --
> Rainer M. Krug
>
> email: RMKrug<at>gmail<dot>com
>
> _______________________________________________
> grass-dev mailing list
> grass-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-dev
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