Skip to content

SEXP size management.

4 messages · Charles Danko, Oleg Sklyar

Original
Charles Danko
# 
Hi,

Trying to decrease the size of a SEXP variable without reassigning
values individually in a loop.

So far, I've tried using Realloc, as the follow source demonstrates:
SEXP dothis() {
	SEXP Rblah;
	PROTECT(Rblah = NEW_INTEGER(6));

	int* blah = INTEGER(Rblah);
	blah[0] = 1;
	blah[1] = 2;
	blah[2] = 3;

	Realloc(Rblah, 3, INTEGER);

	UNPROTECT(1);
	return(Rblah);
}

According to the documentation, I think that this should work, however
it returns an error on compile: "test.c:17: error: expected expression
before ')' token" (line 17 refers to the Realloc line).

Another solution that will suit my needs is managing the variable in C
and assigning the pointer to an R type in the end.  The following code
gets at this, but int* and SEXP, INTEGER are incompatible:
SEXP dothat() {
	int* blah = malloc(3);// = INTEGER(Rblah);
	blah[0] = 1;
	blah[1] = 2;
	blah[2] = 3;

	SEXP Rblah;
	PROTECT(Rblah = blah);

	UNPROTECT(1);
	return(Rblah);
}

Any suggestions for someone still new to SEXP memory management?

Thanks,
Charles
Oleg Sklyar
# 
Rblah in your example is a SEXP structure. Realloc is an interface to C 
realloc and is not intended to resize SEXP structures -- it is used to 
resize user-controlled memory (which is generally not created by 
allocVector/INTEGER).

You would expect a call like:

int * val;
...
Realloc(val, 20, int);

Best,
Oleg
Charles Danko wrote:

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Charles Danko
    

    

      
      #
    

  


  

      
Hi, Oleg,

Thanks very much for your answer!  I was indeed confused; thinking
that Realloc was intended to be an analog to realloc (except for SEXP
variables).

Is there a way to do either of the following:
-- resize the SEXP directly (for my purpose, I ONLY need to decrease
the size), or
-- assign it to to a C pointer that was allocated and used previously
in the function (and can thus be resized with realloc).

Thanks again,
Charles

        
On Wed, Mar 5, 2008 at 11:14 AM, Oleg Sklyar <osklyar at ebi.ac.uk> wrote:

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Oleg Sklyar
    

    

      
      #
    

  


  

      
Charles Danko wrote:

            

      
      
      
    

        
Not that I know.

            

      
      
      
    

        
Well you can get a pointer to the underlying data, basically with 
INTEGER, REAL etc macros, but resizing that vector is not a good idea as 
you do not know about underlying processes in R connected with memory 
management etc.

I think that whether you want to increase or decrease the size, you will 
end up with recreating a SEXP var. Well, you can keep your data outside 
of SEXP's, like in STL vectors etc and then move it to SEXP at the end.