Thank you for your reply Jiefei. I think in theory your solution should work. I'll have to give them a try.
On Mon, 4 Nov 2019 23:41 Wang Jiefei, <szwj...@gmail.com> wrote: > Hi Morgan, > > My solutions might not be the best one(I believe it's not), but it should > work for your question. > > 1. Have you considered Rf_duplicate function? If you want to change the > value of `a` and reset it later, you have to have a duplication somewhere > for resetting it. Instead of changing the value of `a` directly, why not > changing the value of a duplicated `a`? So you do not have to reset it. > > 2. I think a pairlist behaves like a linked list(I might be wrong here and > please correct me if so). Therefore, there is no simple way to locate an > element in a pairlist. As for as I know, R defines a set of > convenient functions for you to access a limited number of elements. See > below > > ``` > #define CAR(e) ((e)->u.listsxp.carval) > #define CDR(e) ((e)->u.listsxp.cdrval) > #define CAAR(e) CAR(CAR(e)) > #define CDAR(e) CDR(CAR(e)) > #define CADR(e) CAR(CDR(e)) > #define CDDR(e) CDR(CDR(e)) > #define CDDDR(e) CDR(CDR(CDR(e))) > #define CADDR(e) CAR(CDR(CDR(e))) > #define CADDDR(e) CAR(CDR(CDR(CDR(e)))) > #define CAD4R(e) CAR(CDR(CDR(CDR(CDR(e))))) > ``` > > You can use them to get first a few arguments from a pairlist. Another > solution would be converting the pairlist into a list so that you can use > the methods defined for a list to access any element. I do not know which C > function can achieve that but `as.list` at R level should be able to do > this job, you can evaluate an R function at C level and get the list > result( By calling `Rf_eval`). I think this operation is relatively low > cost because the list should only contain a set of pointers pointing to > each element. There is no object duplication(Again I might be wrong here). > So there is no way to reset a pairlist to its first element? > 3. You can get unevaluated expression at the R level before you call the C > function and pass it to your C function( by calling `substitute` function). > However, from my vague memory, the expression would be eventually evaluated > at the C level even you pass the expression to it. Therefore, I think you > can create a list of unevaluated arguments before you enter the C function, > so your C function can expect a list rather than a pairlist as its > argument. This can solve both your second and third questions. > Correct me if I am wrong but does it mean that I will have to change "..." to "list(...)" and use .Call instead of .External? Also does it mean that to avoid expression to be evaluated at the R level, I have to use "list" or "substitute"? The function "switch" in R does not use them but manage to achieve that. switch(1, "a", stop("a")) #[1] "a" It is a primitive but I don't understand how it manage to do that. Best, Morgan > Best, > Jiefei > > > On Mon, Nov 4, 2019 at 2:41 PM Morgan Morgan <morgan.email...@gmail.com> > wrote: > >> Hi All, >> >> I have some questions regarding the R C API. >> >> Let's assume I have a function which is defined as follows: >> >> R file: >> >> myfunc <- function(a, b, ...) .External(Cfun, a, b, ...) >> >> C file: >> >> SEXP Cfun(SEXP args) { >> args = CDR(args); >> SEXP a = CAR(args); args = CDR(args); >> SEXP b = CAR(args); args = CDR(args); >> /* continue to do something with remaining arguments in "..." using the >> same logic as above*/ >> >> return R_NilValue; >> } >> >> 1/ Let's suppose that in my c function I change the value of a inside the >> function but I want to reset it to what it was when I did SEXP a = >> CAR(args); . How can I do that? >> >> 2/Is there a method to set "args" at a specific position so I can access a >> specific value of my choice? If yes, do you have an simple example? >> >> 3/ Let's suppose now, I call the function in R. Is there a way to avoid >> the >> function to evaluate its arguments before going to the C call? Do I have >> to >> do it at the R level or can it be done at the C level? >> >> Thank you very much in advance. >> Best regards >> Morgan >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-devel@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-devel >> > [[alternative HTML version deleted]] ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel