[Rd] Questions on the R C API

Morgan Morgan morg@n@em@||box @end|ng |rom gm@||@com
Tue Nov 5 08:35:57 CET 2019

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, <szwjf08 using 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,
> Jiefei
> On Mon, Nov 4, 2019 at 2:41 PM Morgan Morgan <morgan.emailbox using 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
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