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 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
