R devel - Jul 2006 - send a list from R to C

Hello at all !

What is the correct form to give a list form R (e.g. list(c(-1,1),
"TestFile") ) to C. I have no problems, if I don't use character
and I have
also no problems to give a character vector form R to C. But,  if I combine
real and strings in a list then I don't know the correct syntax.

I try it with following (an example):

SEXP writeFile(SEXP headerR){
Image *SaveImage;

PROTECT(headerR = AS_VECTOR(headerR));
SaveImage->Xmin = (float) REAL(VECTOR_ELT(headerR, 0))[0];
SaveImage->Ymin = (float) REAL(VECTOR_ELT(headerR, 0))[1];
strcpy(SaveImage->Description, CHAR(STRING_ELT(headerR, 1)));
}

where

typedef struct {
  float Xmin;
  float Ymin;
  char Description[80];
  } Image;

But with this form the characters are doesn't correct (in this case
Description). Perhaps a problem is that I protect the list with AS_VECTOR.
Or is it not possible to comitted a list from R to C that contain numeric
values and characters ?
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On Tue, 18 Jul 2006, J?rn Schulz wrote:
> 
> Hello at all !
> 
> What is the correct form to give a list form R (e.g. list(c(-1,1),
> "TestFile") ) to C. I have no problems, if I don't use
character and I have
> also no problems to give a character vector form R to C. But,  if I combine
> real and strings in a list then I don't know the correct syntax.
> 
> I try it with following (an example):
> 
> SEXP writeFile(SEXP headerR){
> Image *SaveImage;
> 
> PROTECT(headerR = AS_VECTOR(headerR));
> SaveImage->Xmin = (float) REAL(VECTOR_ELT(headerR, 0))[0];
> SaveImage->Ymin = (float) REAL(VECTOR_ELT(headerR, 0))[1];
> strcpy(SaveImage->Description, CHAR(STRING_ELT(headerR, 1)));
> }
> 
> where
> 
> typedef struct {
>   float Xmin;
>   float Ymin;
>   char Description[80];
>   } Image;
> 
> But with this form the characters are doesn't correct (in this case
> Description). Perhaps a problem is that I protect the list with AS_VECTOR.
> Or is it not possible to comitted a list from R to C that contain numeric
> values and characters ?
You intended the second element of the list, so you need

CHAR(STRING_ELT(VECTOR_ELT(headerR, 1), 0)))

that is X[[2]][1], not X[1] (assuming X to be a character vector).

[This is all in `Writing R Extensions', with examples.  The R-devel 
version of R would have given you an informative warning about the misuse 
of SET_VECTOR_ELT.]

-- 
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

O.k., thanks. I think, I tried your suggesion before I post here, but I made
a mistake in my first approach. Now, it opperate. 

Again, thank you for the reply.
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