[Rd] Timing of SET_VECTOR_ELT

[Simon Urbanek]

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.
>> 
>> Cheers,
>> Simon
> 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).

Cheers,
Simon