Attempting to access an R list from within C code

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Hello R-help,

I am wondering if anyone can help me with this:

I want to access data in a list which has been passed
into a C function, but I cannot work out how to access
the values. How do I move from the given SEXP pointer
to the next object in the list? I have tried to use CDR,
but to no avail. The following code gives an
     'address (nil), cause unknown'
error:
Generally questions like this are better in the R-devel list, but I'll 
answer this one:

What R calls a list() isn't really a list in the Lisp sense, it's a 
vector.  You access elements in C code by index, e.g.

val = VECTOR_ELT(obj, i);

You set values using code like

SET_VECTOR_ELT(obj, i, newvalue);

Duncan Murdoch
R Code:
dyn.load("thelib.so")
list1<-list(c(1:3),c(11:13),c(21:23))
listLen<-length(list1)
.C("myfunc",list1,listLen)

C Code (kept in file thelib.c, compiled on Ubuntu x64
machine using R2.13.0):

void showList(SEXP *obj, int *size)
{
   SEXP *locObj=obj;
   int i;
   for(i=0;i<*size;i++){
     Rprintf("Entry is %d: value is %f",i,REAL(*locObj)[1]);
     *locObj=CDR(*locObj);
   }
   return;
}

Any help is greatfully appreciated!
Best wishes,
Dr. Cormac Long.

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