[Rd] append/concatenate an element to a list in C-language

[Oleg Sklyar]

Hi.

I believe it is virtually impossible, maybe even not virtually: if you
manage to do so somehow, please report it as a bug because such things
must be impossible as they break the integrity of R and data.

Forget about changing size in place: it is C and 'realloc' would be
about the only way to do it, which would not be smart even if you can
find a pointer to what you want to change. However, consider you want to
copy (after all it is a list, which is an array of pointers and you
might think you only need to copy pointers, which is cheap even for a
huge list). Consider a simple example:
you allocVector(VECSXP,n+1); then either memcpy n elements (if you
manage to get a pointer say with DATAPTR, and R-API is smart not to
allow you to get it that simple) or you simply SET_VECTOR_ELT(new, i,
VECTOR_ELT(old, i)) and then add the last one. You know what would be
wrong about this idea? You cannot be sure that pointers to elements of
the old list are not modified/deleted elsewhere, thus affecting your new
list! Even if you overwrite the list in place, providing such software
to users is error prone as users do not know about your ideas of keeping
everything tidy.

The only right way to replicate (if you want to do it in C) is copy and
copy duplicating each element of the list, i.e.:

SEXP f(SEXP old, SEXP v) {
SEXP new; int i,nprotect=0;
PROTECT(new=allocVector(VECSXP,n+1)); nprotect++;
for(i=0;i<n;i++)
  SET_VECTOR_ELT(new, i, Rf_duplicate(VECTOR_ELT(old, i)));
SET_VECTOR_ELT(new, n, v);
/* set names here if you need */
UNPROTECT(1);
return new;

What you could do, if you really need such functionality. Provide R with
external pointer to a data structure that you maintain in your C (I
would suggest C++) code and use C++ STL for dynamic updates (either
lists or vectors, whatever suits better). But the overhead of writing
the interface will take more than copying a list of any size!

Finally: void f(SEXP list, SEXP vector); is a malformed definition which
will lead to Segmentation Fault. Please read "Writing R extensions"
first.

Best,
Oleg

-  
Dr Oleg Sklyar * EMBL-EBI, Cambridge CB10 1SD, UK * +441223494466


On Thu, 2007-10-18 at 19:41 +0200, Robert Castelo wrote:
> dear people,
> 
> i need to code a function in C working in R and receives two R SEXP
> objects as parameters, where one is a list and another is a vector of
> integers:
> 
> void f(SEXP list, SEXP vector) {
> 
>   ...
> 
>   return list;
> }
> 
> 
> and it should return the given list with the integer vector concatenated
> at the end (as the last element of the list). the list can be really big
> so i would not like to create a new list from scratch and copy all the
> elements, including the additional one. i'm also interested in knowing
> how should i properly handle protections of the SEXP objects when doing
> this.
> 
> i've been looking at the R source code for everything that has to do
> with CAR, CDR, CONS, and even found functions with promising names like
> listAppend or GrowList but i have not been able to figure this out nor i
> haven't been able to find any reference on how to do this by googling
> all around, so any help will be very much appreciated.
> 
> 
> thanks a lot!!!
> 
> robert.
> 
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