[Rcpp-devel] Strange behaviro using Rcpp sugar with NumericMatrix

[Roland Denis]

Hello,

while comparing the performance between pure R and different other 
syntaxes that mix R and Rcpp, I found out a strange behavior when using 
Rcpp sugar on NumericMatrix. As far as I understand Rcpp (I'm a newbie 
in R and Rcpp), syntactic sugar is designed for NumericVector, but since 
NumericMatrix seems to be seen like an augmented NumericVector, I've 
supposed this works on NumericMatrix too (and, in fact, it compiles).

Here is a minimal example :

library(Rcpp)
cppFunction("
     NumericMatrix scaleSugar( NumericMatrix A, double k )
     {
         return k*A;
     }
")

A <- diag(3)
B = scaleSugar(A, 2)

Displaying A and B returns :
 > A
      [,1] [,2] [,3]
[1,]    2    0    0
[2,]    0    2    0
[3,]    0    0    2
 > B
      [,1] [,2] [,3]
[1,]    2    0    0
[2,]    0    2    0
[3,]    0    0    2

As you see, the matrix A seems to be multiplied in-place and returned.

Creating the returned matrix before the multiplication doesn't change 
the result :
library(Rcpp)
cppFunction("
     NumericMatrix scaleSugar( NumericMatrix A, double k )
     {
         NumericMatrix R( A.nrow(), A.ncol());
         R = k*A;
         return R;
     }
")

A <- diag(3)
B = scaleSugar(A, 2)

 > A
      [,1] [,2] [,3]
[1,]    2    0    0
[2,]    0    2    0
[3,]    0    0    2

However, copying A into R before the scaling works :
library(Rcpp)
cppFunction("
     NumericMatrix scaleSugar( NumericMatrix A, double k )
     {
         NumericMatrix R( A.nrow(), A.ncol(), A.begin());
         return k*R;
     }
")

A <- diag(3)
B = scaleSugar(A, 2)

 > A
      [,1] [,2] [,3]
[1,]    1    0    0
[2,]    0    1    0
[3,]    0    0    1


and, *more surprisingly*, reversing the multiplication operand's order 
works too :
library(Rcpp)
cppFunction("
     NumericMatrix scaleSugar( NumericMatrix A, double k )
     {
         return A*k;
     }
")

A <- diag(3)
B = scaleSugar(A, 2)

 > A
      [,1] [,2] [,3]
[1,]    1    0    0
[2,]    0    1    0
[3,]    0    0    1

Are Rcpp sugars supposed to work with NumericMatrix ?

Many thanks,

-- 
Roland