[Rd] Dealing with R list objects in C/C++

[Wayne.Zhang at barclayscapital.com]

Many thanks for the quick reply Martin, your code works as expected.  Next I'd like to retrieve heterogeneous data from an SEXP object (let's just pretend it's the same type as the one what I'm constructing).  I'm sure the relevant APIs are defined in Rinternals.h, do we have API documentations for this header file somewhere?  

@Dirk: thanks for your help too.  I'm doing something very simple at the moment, so I prefer not to bring in Rinside/Rcpp if possible.

Thanks again,
Wayne 


-----Original Message-----
From: Martin Morgan [mailto:mtmorgan at fhcrc.org] 
Sent: Wednesday, January 26, 2011 10:04 PM
To: Zhang, Wayne: IT (NYK)
Cc: r-devel at r-project.org
Subject: Re: [Rd] Dealing with R list objects in C/C++

On 01/26/2011 02:56 PM, Wayne.Zhang at barclayscapital.com wrote:
> Hi,
> 
> I'd like to construct an R list object in C++, fill it with relevant data, and pass it to an R function which will return a different list object back.  I have browsed through all the R manuals, and examples under tests/Embedding, but can't figure out the correct way.  Below is my code snippet:
> 
>     #include <Rinternals.h>
> // Rf_initEmbeddedR and other setups already performed
> 
>     SEXP arg, ret;
> 
>     // this actually creates a pairlist.  I can't find any API that creates a list
> PROTECT(arg = allocList(3));

Allocate a list of length 3 via SEXPTYPE VECSXP

      PROTECT(arg = allocVector(VECSXP, 3));

> 
> // I want the first element to be type integer, second double, and third a vector.
>     INTEGER(arg)[0]  = 1;            // <- runtime exception: "INTEGER() can only be applied to a 'integer', not a 'pairlist'

set the first element of the list to an integer vector of length 1, and
assign a value

      SET_VECTOR_ELT(arg, 0, allocVector(INTSXP, 1));
      INTEGER(VECTOR_ELT(arg, 0))[0] = 1

or more succinctly

      SET_VECTOR_ELT(arg, 0, ScalarInteger(1));

>     REAL(arg)[1] = 2.5;               // control never reached here

and the second element

      SET_VECTOR_ELT(arg, 1, ScalarReal(2.5));

>     VECTOR_PTR(arg)[2] = allocVector(REALSXP, 4);

and for the third allocate a REALSXP and then fill

      SET_VECTOR_ELT(arg, 2, allocVector(REALSXP, 4));

next lines should be ok as REAL(VECTOR_ELT(arg, 2))[0] = 10.0; or with
less typing as

      double *x = REAL(VECTOR_ETL(arg, 2));
      x[0] = 10.0; x[1] = 11.0; x[2] = 12.0; x[3] = 13.0;

>     REAL(VECTOR_PTR(arg)[2])[0] = 10.0;
>     REAL(VECTOR_PTR(arg)[2])[1] = 11.0;
>     REAL(VECTOR_PTR(arg)[2])[2] = 12.0;
>     REAL(VECTOR_PTR(arg)[2])[3] = 13.0;
> 
>     PROTECT(call = lang2(install(entryPoint.c_str()), arg));

not sure where entryPoint.c_str() is coming from, but

     PROTECT(call = lang2(install("fun"), arg));

with some debate about whether install("fun") should be PROTECT'ed.

> 
> ret = R_tryEval(call, R_GlobalEnv, &errorOccurred);

likely PROTECT(ret = ...) while checking errorOccurred, etc.

Hope that helps,

Martin

> 
> 
> I'll be grateful if you can point me to any online docs/samples.
> 
> Thanks in advance,
> Wayne
> 
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