Rcpp: Syntactic sugar for * produces unexpected results when dealing with NumericMatrix

Question

A recently asked question has lead me to believe the syntactic sugar for * by Rcpp does not work as intended. In the linked question, the user is t

Answer 1

As I stated in previous answers, when I need to do actual math on matrices, I use Armadillo objects:

R> cppFunction('arma::mat scott(arma::mat x, double z) { 
+                 return(x*z); }', 
+              depends="RcppArmadillo")
R> scott(matrix(1:4,2), 2)
     [,1] [,2]
[1,]    2    6
[2,]    4    8
R> 

Sugar operations are nice, but not complete. We will certainly take patches, though.

And as we said a few times before: rcpp-devel is the proper support channel.

Edit (Oct 2016 or 2 1/2 years later): Searching for something else just got me back here. In the Rcpp 0.12.* series, some work when into operations between matrix and vector so the basic 'matrix times scalar' now works as you'd expect:

R> cppFunction("NumericMatrix testmat(NumericMatrix m, double multme) { 
+               NumericMatrix n = m * multme; 
+               return n; }") 
R> testmat(matrix(1:4,2), 1)
     [,1] [,2]
[1,]    1    3
[2,]    2    4
R> testmat(matrix(1:4,2), 3)
     [,1] [,2]
[1,]    3    9
[2,]    6   12
R> 

I'd probably still use RcppArmadillo for math on matrices though.

Answer 2

This is an unfortunate consequence of a bad design decision, namely making Rcpp matrices derive from Rcpp vectors.

I'm likely to revert this decision in Rcpp implementations I now maintain: Rcpp11 and Rcpp98. I don't think anymore that there any benefit of having Matrix derive from Vector and it gets in the way of CRTP that is used at the end of this file.