[Rcpp-devel] Named vectors

Dirk Eddelbuettel edd at debian.org
Thu Sep 9 17:30:45 CEST 2010

Hi Andrew,

On 9 September 2010 at 08:52, Andrew Redd wrote:
| What is the appropriate way to use/convert named vectors in C++ with
| Rcpp.  Basically I have a named vector of parameters that pass into
| the function.  I cannot be certain of their order, so would like to
| extract them by name.  For those initialized in C++ a
| std:map<std::string, double> should work, but how do declare one from
| a SEXP pointer that is passed in?  For example:
| draw <- cxxfunction(signature(A="numeric",t="numeric", params="numeric"),body="
|     SEXP draw( SEXP A, SEXP t, SEXP params){
|       Rcpp::NumericVector Admissions(A);
| 			Rcpp::NumericVector T(t);
| 			std::map<std::string, double> Params(params);
| 			std::vector<double> draws;
| 			for(int i=0;i<params[\"n\"];i++){
| 				draws.pushback(rbinom(Admissions[T[i]+1,2], 1/(1+exp(-params[\"nu\"])))));
| 			}
| 			return wrap(draws);
|     }
| 	",plugin='Rcpp')
| This of course does not compile because of std::map<std::string,
| double> Params(params); is not valid.  But is shows more of less what
| I'm trying to do.

There is no explicit 'unrolling' of named vectors into maps.  OTOH as<> works
over atomistic types that can be iterated, and we do talk about the opposite
way (using Rcpp::wrap) in the intro vignette.  There may be something close
in the 700+ unit tests.  Maybe we already can -- I just haven't tried.

I tend to pass named lists down from R and pick paraeters one-by-one out of
the Rcpp::List once in C++.  Here is a working example from something Shane
and I working on:

R side:

    val <- .Call("SVMregression",
                 X=x, y=y,

C++ side:

RcppExport SEXP SVMregression(SEXP Xs, SEXP Ys, SEXP svmParameters) {

    try {
        Rcpp::NumericMatrix xR = Rcpp::NumericMatrix(Xs);
        Rcpp::NumericVector yR = Rcpp::NumericVector(Ys);
        Rcpp::List rparam(svmParameters);
        double C = Rcpp::as<double>(rparam["C"]);
        double epsilon = Rcpp::as<double>(rparam["epsilon"]);
        double gamma = Rcpp::as<double>(rparam["gamma"]);
        double sigma = Rcpp::as<double>(rparam["sigma"]);
        string type = Rcpp::as<string>(rparam["type"]);
        string kernel = Rcpp::as<string>(rparam["kernel"]);

I am happy with this as the lists typically contain different types (double,
string, bool, ...) anyway.  

Does that help?


Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com

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