[R] Sending lists from R to C using .C

Mercier Eloi emercier at chibi.ubc.ca
Mon Apr 16 22:21:00 CEST 2012

Hi Chris,

I personally found the documentation rather...indigestible. At least 3 
years ago when I developed my R package. It seems they have nicely 
improved this part of the documentation though.

I'm not an expert at all, but at least you will have the point of view 
of a beginner.

First, in your C or C++ file, redefine the type of you main function to 
SEXP so it returns an R-type object. Then change your object(s) to SEXP 
to be compatible with R. Don't forget to protect your variable(s) (and 
unprotect it at the end). (For C++ only, encapsulate your function with 
'extern "C"'). Also add this three headers in your file :
#include <R.h>
#include <Rinternals.h>
#include <Rdefines.h>

Run R CMD SHLIB on your C/C++ file. You should obtain a .so file.
In R, use dyn.load to load the .so file. Then you can use .Call.


Here is an example :

testC.c :
#include <R.h>
#include <Rinternals.h>
#include <Rdefines.h>

//extern "C" { // for C++ only
     SEXP myCfunction (SEXP myList) //this function return a SEXP object
         SEXP myResults; //definition of a new SEXP object, usable by R
         double tmp = 0;
         PROTECT(myResults = allocVector(REALSXP, 1)); //don't forget to 
protect your variable !

         for (int i=0; i<GET_LENGTH(myList); i++)
              tmp = tmp +  NUMERIC_VALUE(VECTOR_ELT(myList,i)); // refer 
to Rinternals.h for a complet list of types

         REAL(myResults)[0] = tmp;
         UNPROTECT(1); //free the variable
//} // end of extern "C"

Then run R CMD SHLIB testC.c

In R :
myResultsFromC <- .Call("myCfunction", myList=myList)
[1] 21

I hope this help.


On 12-04-15 12:11 AM, Prof Brian Ripley wrote:
> On 15/04/2012 05:22, chris90nz wrote:
>> Hi,
>> I am currently stuck on the following problem: I have a matrix where 
>> each
>> element is a list of numbers of differing lengths. I am wondering 
>> what would
>> be the best way to send this to my C function using .C?
> Follow the documentation ....
> This is not the list for discussing compiled code (see the posting 
> guide), nor have you told us what you tried.  But in fact there is 
> only one way to do this, and ?.C and 'Writing R Extensions' explain it 
> to you.
> I have no idea why you would have 'a list of numbers of differing 
> lengths'.  First, what is the length of a number?  Second, why not 
> store numbers in numeric vectors?  An example (as required by the 
> posting guide) would have helped compensate for an imprecise description.
>> Cheers,
>> Chris
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide 
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.

Eloi Mercier
Bioinformatics PhD Student, UBC
Paul Pavlidis Lab
2185 East Mall
University of British Columbia
Vancouver BC V6T1Z4

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