[R] subsetting from C code, do_subset

[Gabor Csardi]

On Sat, Apr 22, 2006 at 07:33:56PM +0100, Prof Brian Ripley wrote:
> On Sat, 22 Apr 2006, Gabor Csardi wrote:
> 
> >Dear List,
> >
> >do you know a way for subsetting an R object from C code? I would need the
> >equivalent of x[s]. I've found that the do_subset internal function does
> >this, but don't really know how to call it properly. I've two SEXP's, one
> >for the object and one for the index.
> >
> >I'm thinking of creating an enviroment, place the two SEXP's in it and then
> >just parse and run the corresponding R code, but there might be simpler
> >way...
> 
> That's overkill.  You know where your objects are so you can set up a call 
> with e.g. lang2 and eval it in the right place.  The code for e.g. optim 
> might give you some ideas.

Thanks, i've checked the optim code. The only difference is that i don't
have a function or expression object for '[' itself, and i couldn't manage
to find a public entry point which can create one for me.

> do_subset is not a public entry point, and may well not be visible to 
> you in R >= 2.3.0.

That is true, it is better to extract the entry from R_FunTab then?
Of course i don't need this if i use eval, lang2, etc. But for that i need a
function which can create a function object for '[', something like do_call,
but do_call is not a public entry point either....

> [I'd say this was an R-devel question, BTW.]

Maybe. I've posted it here because i don't want to 'develop' R, only use it
as a user. But you're right that most people here might be not interested in
R internals.... I there was an R-help-advenced list i would have posted it
there. 

Gabor

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-- 
Csardi Gabor <csardi at rmki.kfki.hu>    MTA RMKI, ELTE TTK