[Rd] eval a SYMSXP from C

[Duncan Murdoch]

On 12-04-16 5:53 PM, Whit Armstrong wrote:
> Can someone offer some advice on how to properly evaluate a SYMSXP
> from a .Call ?
>
> I have the following in R:
>
> variable xn, with an attribute "mu" which references the variable mu
> in the global environment.
>
> I know "references" is a loose term; mu was defined in this fashion as
> a way to implement deferred binding:
>
> foo<- function(x,mu) {
>      attr(x,"mu")<- substitute(mu)
>      x
> }
>
> mu<- 2.0
> xn<- foo(rnorm(100),mu)
>
>> typeof(attr(xn,"mu"))
> [1] "symbol"
>> eval(attr(xn,"mu"))
> [1] 2
>>
>
> In a .Call, I am attempting to eval the SYMSXP as follows:
>
> SEXP mu_ = Rf_getAttrib(x_,Rf_install("mu"));
>
> if(TYPEOF(mu_)==SYMSXP) {
>    mu_ = Rf_eval(Rf_lang1(mu_),R_GlobalEnv);
> }
>
> However, when running this code, I get the following error:
> Error in logp(xn) : could not find function "mu"
>
> Do I need to create an expression of c("get", "mu") to force the name
> lookup to evaluate the SYMSXP?
>

Rf_lang1(mu_) will produce mu(), not mu.  I think you just want to 
evaluate mu_.

There are lots of possible types of object returned by substitute(mu), 
but you should be able to evaluate all of them.  You probably don't want 
to evaluate them in R_GlobalEnv, you want to evaluate them in the 
environment where the call was made to foo, so that you'll get local 
variables if it was called from a function.

Duncan Murdoch