[Rd] Evaluating R expressions from C

[Robert Gentleman]

Hi Terry,

Terry Therneau wrote:
> I am currently puzzled by a passage in the R Extensions manual, section 5.10:
> 
> SEXP lapply(SEXP list, SEXP expr, SEXP rho)
>      {
>        R_len_t i, n = length(list);
>        SEXP ans;
>      
>        if(!isNewList(list)) error("`list' must be a list");
>        if(!isEnvironment(rho)) error("`rho' should be an environment");
>        PROTECT(ans = allocVector(VECSXP, n));
>        for(i = 0; i < n; i++) {
>          defineVar(install("x"), VECTOR_ELT(list, i), rho);
>          SET_VECTOR_ELT(ans, i, eval(expr, rho));
>        }
>     
> I'm trying to understand this code beyond just copying it, and don't find 
> definitions for many of the calls.  PROTECT and SEXP have been well discussed 
> previously in the document, but what exactly are
> 	R_len_t
> 	defineVar

        this function defines the variable (SYMSXP; one type of SEXP) of 
its first argument, to have the value given by its second argument, in 
the environment defined by its third argument. There are lots of 
variants, these are largely in envir.c


> 	install

   all symbols in R are unique (there is only one symbol named x, even 
though it might have bindings in many different environments). So to get 
the unique "thing" (a SYMSXP) you call install (line 1067 in names.c has 
a pretty brief comment to this effect). This makes it efficient to do 
variable look up, as we only need to compare pointers (within an 
environment), not compare names.

> 	VECTOR_ELT

     access the indicated element (2nd arg) of the vector (first arg)

> 	SET_VECTOR_ELT

      set the indicated element (2nd arg), of the vector (1st arg) to 
the value (3rd arg)

> 	
> The last I also found in 5.7.4, but it's not defined there either.  
> 
> So:
>    What do these macros do?  Some I could guess, like is.Environment; and I'm 
> fairly confident of R_len_t.  Others I need some help.
>    Perhaps they are elswhere in the document?  (My version of acrobat can't do 
> searches.)  Is there another document that I should look at first?
>    Why "isNewList"?  I would have guessed "isList".  What's the difference?

    "old lists" are of the CAR-CDR variant, and largely only used 
internally these days.  "new lists", are generic vectors, and are what 
users will almost always encounter (even users that program internals, 
you pretty much need to be messing with the language itself to run into 
the CAR-CDR variety).

    best wishes
     Robert

>    
>    	Thanks for any help,
>    		Terry Therneau
> 
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> 

-- 
Robert Gentleman, PhD
Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M2-B876
PO Box 19024
Seattle, Washington 98109-1024
206-667-7700
rgentlem at fhcrc.org