Mapping Rcpp NumericVector to *static* or *fixed-size* Eigen Vectors?

Question

This thread provides an example of converting an Rcpp::NumericVector to an Eigen::VectorXd:

Rcpp::cppFunction("bool conv(NumericVector X) { \
      Eigen::Map<Eigen::VectorXd> \
      XS(Rcpp::as<Eigen::Map<Eigen::VectorXd> >(X)); return true; } ", 
            depends="RcppEigen")
conv(1:4)

but I can't manage to adapt this to creating statically-sized or fixed-size Eigen containers

Rcpp::cppFunction("bool conv(NumericVector X) { \
      Eigen::Map<Eigen::Matrix<double,4,1>> \
      XS(Rcpp::as<Eigen::Map<Eigen::Matrix<double,4,1>> >(X)); return true; } ", 
                  depends="RcppEigen")
conv(1:4)

How can this be done?

Answer 1

Here is a minimal brute-force approach of instantiating a fixed-size 4x1 matrix ... and then using memcpy() to copy the payload over. As my comment earlier mentioned we have no code in RcppEigen that transfer from a SEXP R object to such an Eigen type so there is not a lot more we can do.

Code

#include <RcppEigen.h>

// [[Rcpp::depends(RcppEigen)]]

// [[Rcpp::export]]
bool conv1(Rcpp::NumericVector X) {
    Eigen::Map<Eigen::VectorXd> XS(Rcpp::as<Eigen::Map<Eigen::VectorXd>>(X));
    return true;
}

// [[Rcpp::export]]
bool conv2(Rcpp::NumericVector X) {
    const int n = 4;
    if (X.size() != n) Rcpp::stop("Wrong size");
    Eigen::Matrix<double,4,1> m;
    std::memcpy(&m[0,0], &X[0], n*sizeof(double));
    Rcpp::Rcout << m << std::endl;
    return true;
}

/*** R
x <- as.numeric(1:4)
conv1(x)
conv2(x)
*/

Output

> Rcpp::sourceCpp("~/git/stackoverflow/76706728/answer.cpp")

> x <- as.numeric(1:4)

> conv1(x)
[1] TRUE

> conv2(x)
1
2
3
4
[1] TRUE
>