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Converting SEXP to primitive C types

3 messages · Michael Petrovich, Brian Ripley, Hin-Tak Leung

Original
Michael Petrovich
# 
I'm new to R development, so this may be a trivial question.

In C, How do you convert a SEXP value returned by an R function to a  
primitive C type (int, double, char, array, etc.)? I've tried using  
the INTEGER(x), VECTOR_ELT(x,i), and similar functions found in /src/ 
include/Rinternals.h, but doing so results in incorrect results or a  
seg-fault.

For example, I modified /tests/Embedding/Rerror.c so that it tries to  
print the value returned by the function defined in error.R:

val = Test_tryEval(e, &errorOccurred);
printf("Returned: {%d}\n", VECTOR_ELT(val, 0) );

Where error.R is contains:

foo <-
function(x=3, y=6)
{
   z <- x * y
   z
}

But the output is just "0", not the correct "18". Any ideas?

Thanks for your help!
Brian Ripley
#
On Tue, 11 Jul 2006, Michael Petrovich wrote:

            

      
      
      
    

        
See `Writing R Extensions', the documentation for that file.  You cannot 
just guess that an object has the appropriate SEXPTYPE: you have to check 
it (or coerce to it).  In your example it is likely to be a REALSXP (it 
could be an INTSXP), so you need to cover all possible cases.

            

      
      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Hin-Tak Leung
    

    

      
      #
    

  


  

      
Michael Petrovich wrote:

            

      
      
      
    

        
Prof Ripley had already pointed you to the "Writing R extensions" 
manual, which explains this stuff in more details, and also hinted
that most probably it is REALSXP instead of yours guesses. So I am just 
going to give a somewhat concrete example - you want to be
doing something like this instead:

val = Test_tryEval(e, &errorOccurred);

switch( TYPEOF(val) ) {
case REALSXP:
printf("Returned: {%f}\n", REAL(val)[0]);
break;
case INTSXP:
printf("Returned: {%d}\n", INTEGER(val)[0]);
break;
case STRSXP:
printf("Returned: {%s}\n", CHAR(STRING_ELT(val, 0)));
break;
default:
printf("Don't know what to do about type %d\n", TYPEOF(val));
}