Bizarre behavior of Rcpp NumericMatrix when multiplied by scalar on left/right - rcpp

Question

Can anyone explain the following behavior?
When declaring a new NumericMatrix, y, as the original matrix, x, multiplied by a scalar, c, the order of the scalar/matrix multiplication matters. If I multiply the scalar on the left and the matrix on the right (e.g. NumericMatrix y = c * x;), then I get bizarre behavior. The original matrix, x, is changed!
However if I put the original matrix to the left and multiply the scalar on the right (e.g. NumericMatrix y = x * c;), x remains unchanged.
This doesn't seem to affect other data types. I've tested with int and NumericVector.
Example: Problem appears when using NumericMatrix
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericMatrix testfun(NumericMatrix x) {
NumericMatrix y(x.rows(), x.cols());
y = x * 2;
std::cout << x; // x is unmodified
y = 2 * x;
std::cout << x; // x is now modified
return x;
}
/*** R
x <- matrix(2, nrow = 3, ncol = 3)
print(x)
y <- testfun(x = x)
print(y)
print(x)
*/
The output is as follows.
> x <- matrix(2, nrow = 3, ncol = 3)
> print(x)
[,1] [,2] [,3]
[1,] 2 2 2
[2,] 2 2 2
[3,] 2 2 2
> y <- testfun(x = x)
2.00000 2.00000 2.00000
2.00000 2.00000 2.00000
2.00000 2.00000 2.00000
4.00000 4.00000 4.00000
4.00000 4.00000 4.00000
4.00000 4.00000 4.00000
> print(y)
[,1] [,2] [,3]
[1,] 4 4 4
[2,] 4 4 4
[3,] 4 4 4
> print(x)
[,1] [,2] [,3]
[1,] 4 4 4
[2,] 4 4 4
[3,] 4 4 4
Here is my session info
> sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Catalina 10.15.1
Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] compiler_3.6.1 tools_3.6.1 RcppArmadillo_0.9.800.1.0
[4] Rcpp_1.0.2 RcppProgress_0.4.1 packrat_0.5.0
[7] RcppParallel_4.4.4
.

Answer 1

The question is way too long as posted and hides its point. We do not need the second and third examples. All we need is this code:
Code
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericMatrix testfun(NumericMatrix x) {
NumericMatrix y(x.rows(), x.cols());
y = x * 2;
std::cout << x; // x is unmodified
y = 2 * x; // contrast with x * 2
std::cout << x; // x is now modified
return x;
}
/*** R
print(x <- matrix(2, nrow = 2, ncol = 2))
print(y <- testfun(x = x))
print(x)
*/
Output
R> Rcpp::sourceCpp("~/git/stackoverflow/59515517/question.cpp")
R> print(x <- matrix(2, nrow = 2, ncol = 2))
[,1] [,2]
[1,] 2 2
[2,] 2 2
R> print(y <- testfun(x = x))
2.00000 2.00000
2.00000 2.00000
4.00000 4.00000
4.00000 4.00000
[,1] [,2]
[1,] 4 4
[2,] 4 4
R> print(x)
[,1] [,2]
[1,] 4 4
[2,] 4 4
R>
Issue
Behaviour of x * 2 is different from 2 * x. The latter has a side effect. This is likely a bug,
Bigger issue
There is not really a point to do matrix algebra with Rcpp. The implementation is rudimentary and incomplete. If you want to do "math", use RcppArmadillo or RcppEigen.
Arma implementation
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
arma::mat testfun(arma::mat x) {
arma::mat y(x.n_rows, x.n_cols);
y = x * 2;
std::cout << x; // x is unmodified
y = 2 * x; // contrast with x * 2
std::cout << x; // x is now modified
return x;
}
/*** R
print(x <- matrix(2, nrow = 2, ncol = 2))
print(y <- testfun(x = x))
print(x)
*/
Arma output
R> Rcpp::sourceCpp("~/git/stackoverflow/59515517/answer.cpp")
R> print(x <- matrix(2, nrow = 2, ncol = 2))
[,1] [,2]
[1,] 2 2
[2,] 2 2
R> print(y <- testfun(x = x))
2.0000 2.0000
2.0000 2.0000
2.0000 2.0000
2.0000 2.0000
[,1] [,2]
[1,] 2 2
[2,] 2 2
R> print(x)
[,1] [,2]
[1,] 2 2
[2,] 2 2
R>
I'll see if I can get to the Rcpp bug you found here.
Edit: It is a bug, and I don't quite understand it yet, but I added something to the issue where the code originated: https://github.com/RcppCore/Rcpp/issues/365
Edit 2: The fix is now in master. Thanks to KK for the PR, to everbody else hinting in the comments what may be behind this, and especially to Ralf for getting us all to try the fix.