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

[Martin Morgan]

On 1/27/2011 1:03 PM, Wayne.Zhang at barclayscapital.com wrote:
> 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?
Hi Wayne -- Your best bet might be sections 5 and 6 of

     RShowDoc("R-exts")

or the books Dirk mentioned; see also Rdefines.h. Martin
> @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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-- 
Dr. Martin Morgan, PhD
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N.
PO Box 19024 Seattle, WA 98109