[R] allocating memory in C, not in R

[Sundar Dorai-Raj]

Sigal Blay wrote:
> Thank you for the fast reply.
> 
> Below is a simplified version of my c function that I am 
> currenctly using with a .C() call.
> The values that has to be returned to R are the three outVectors.
> If I need to convert .C to .Call,
> How do I do it based on myFunction below?
> 
> Thank you for your help.
>  
> 
> void myFunction(char **argv, int *inputVector, 
>   int *RoutVector1, double *RoutVector2, char **RoutVector3) {
>     int    *outVector1;
>     double *outVector2;
>     char   **outVector3;
>     int roof = 0;
>     roof = calculate_values ( argv[0], 
>                               inputVector, 
>                               &outVector1,
>                               &outVector2, 
>                               &outVector3   )
> 
>     for(i=0;i<roof;i++) {
>         RoutVector1[i] = outVector1[i];    
>         RoutVector2[i] = outVector2[i]; 
>     }             
>     return;
> }
> 
> 

Well, you could read "Writing R Extensions" (Section 4.8). Your function 
will have to change significantly, though. I would create a list within 
the C-function (called "val" below) and populate the RoutVectors with it.

(I can't remember whether you'll need more than #include <R.h>. Again, I 
direct you to the documentation.)

SEXP myFunction(SEXP argv, SEXP inputVector) {
   SEXP val;
   PROTECT(val = allocVector(VECSXP, 3));
   /* some code you'll have to write */
   UNPROTECT(1);
   return val;
}

# in R
RoutVectors <- .Call("myFunction", argv, inputVector)

--sundar

> On Wed, Sep 01, 2004 at 10:16:59PM +0100, Prof Brian Ripley wrote:
> 
>>On Wed, 1 Sep 2004, S. Blay wrote:
>>
>>
>>>I need to retrieve several vectors of various types from a call 
>>>to .C(), but I don't know their length in advance. 
>>>Is there a way to do this without allocating an excessive amount 
>>>of memory? 
>>>If so, an example would be very helpful.
>>
>>It would be very much easier to use .Call rather than .C.
>>
>>Alternatively, generate and allocate in C on one C call and retrieve on a
>>second, as rpart does.
>>
>>-- 
>>Brian D. Ripley,                  ripley at stats.ox.ac.uk
>>Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
>>University of Oxford,             Tel:  +44 1865 272861 (self)
>>1 South Parks Road,                     +44 1865 272866 (PA)
>>Oxford OX1 3TG, UK                Fax:  +44 1865 272595
> 
> 
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