[Rd] Converting SEXP to primitive C types

[Prof Brian Ripley]

On Tue, 11 Jul 2006, Michael Petrovich wrote:

> 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.

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.

> 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 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)
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