[Rd] Timing of SET_VECTOR_ELT
simon.urbanek at r-project.org
Mon Apr 1 21:11:49 CEST 2013
On Apr 1, 2013, at 2:54 PM, Terry Therneau wrote:
> On 04/01/2013 12:44 PM, Simon Urbanek wrote:
>> On Apr 1, 2013, at 1:10 PM, Terry Therneau wrote:
>>> Assume a C program invoked by .Call, that returns a list.
>>> Near the top of the program we allocate space for all the list elements. (It is my habit to use "xyz2" for the name of the R object and "xyz" for the pointer to its contents.)
>>> PROTECT(means2 = allocVector(REALSXP, nvar));
>>> means = REAL(means2);
>>> PROTECT(u2 = allocVector(REALSXP, nvar));
>>> u = REAL(u2);
>>> PROTECT(loglik2 = allocVector(REALSXP, 2));
>>> loglik = REAL(loglik2);
>>> PROTECT(rlist = mknamed(VECSXP, outnames));
>>> Can I assign the individual elements into rlist using SET_VECTOR_ELT at this point, or do I need to wait until the end of the program, after I've filled in means[i], u[i], etc.? I likely depends on whether I'm assigning a pointer or a copy.
>> You're assigning a pointer, so it doesn't matter.
>> FWIW, you can avoid all the PROTECTion mess if you alloc+assign, e.g.
>> SEXP rlist = PROTECT(mknamed(VECSXP, outnames));
>> SEXP means = SET_VECTOR_ELT(rlist, 0, allocVector(REALSXP, nvar));
>> since you only need to protect the parent object.
> Neat trick. I take it that SET_VECTOR_ELT returns a pointer to the object that was just created? I haven't found a description of the function in any of the documents, only examples of its use, so this is a surprise to me.
> Lacking documentation, can I count on it in the future?
Well, it's a pretty fundamental function, so if its behavior changed, the whole world would collapse ;) so if you can't rely on SET_VECTOR_ELT then I don't know what else you can rely on. Its return value is also used in R itself, so it's not an obscure use. It may look a bit scary as the upper-case may suggest it's a macro, but R-exts clarifies that it is a function so the above is ok (and it's frequently used with allocations in R itself).
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