c++ - In Rcpp (and RcppArmadillo), how to check if a vec contains complex numbers?

Question

In R, we can check whether a vector (e.g., vec1=c(1+1i,2)) contains complex numbers using is.complex (e.g., is.complex(vec1)). I wonder what the equivalent function is in RcppArmadillo?

And how to extract the real part of each element in the vector in RcppArmadillo, like Re(vec1) in R?

Answer 1

For extracting the real and imaginary parts, you can use the arma::real() and arma::imag() functions. Alternatively you can use the sugar function Rcpp::Re() and Rcpp::Im():

// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>

// [[Rcpp::export]]
arma::vec getRe(arma::cx_vec x) {
  return arma::real(x);
}

// [[Rcpp::export]]
Rcpp::NumericVector getIm(Rcpp::ComplexVector x) {
  return Rcpp::Im(x);
}

/*** R
set.seed(42)
N <- 5
vec <- complex(5, rnorm(5), rnorm(5))

t(getRe(vec))
#>            [,1]        [,2]      [,3]       [,4]       [,5]
#> [1,] -0.9390771 -0.04167943 0.8294135 -0.4393582 -0.3140354
Re(vec)
#> [1] -0.93907708 -0.04167943  0.82941349 -0.43935820 -0.31403543

getIm(vec)
#> [1] -2.1290236  2.5069224 -1.1273128  0.1660827  0.5767232
Im(vec)
#> [1] -2.1290236  2.5069224 -1.1273128  0.1660827  0.5767232
*/

If you were to use getRe(arma::vec x) above, you would get:

Warning message:
In getRe(vec) : imaginary parts discarded in coercion

You just cannot put complex numbers in an object that is not meant to store them. This is a consequence of C++ being a strongly typed language. So there is no need for an analogue to is.complex().

See the Armadillo documentation for further reference.