Scott Ritchie Scott Ritchie - 1 month ago 24
R Question

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

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 trying to multiply a matrix by a scalar.

R code

Here's what we're trying to achieve in
Rcpp
, but for now in plain
R
:

> m <- matrix(0:3, 2, 2)
> m * 3
[,1] [,2]
[1,] 0 6
[2,] 3 9


Rcpp Code

I've created some minimal examples demonstrating both the problem above, and also some unexpected behaviour along the way. First note that I'm consistently using
List
as a return type, because it removes the need for me to declare the appropriate type in advance:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]

List FooMat() {
// Create a fill a 2x2 matrix
NumericMatrix tmp(2,2);
for (int i = 0; i < 4; i++) {
tmp[i] = i;
}

return List::create(tmp);
}

// [[Rcpp::export]]
List FooMat2() {
// Create a fill a 2x2 matrix
NumericMatrix tmp(2,2);
for (int i = 0; i < 4; i++) {
tmp[i] = i;
}

NumericVector x(1);
x[1] = 3;

return List::create(tmp * x);
}

// [[Rcpp::export]]

List FooMat3() {
// Create a fill a 2x2 matrix
NumericMatrix tmp(2,2);
for (int i = 0; i < 4; i++) {
tmp[i] = i;
}

NumericVector x(1);
x[1] = 3;

return List::create(tmp * x[1]);
}

// [[Rcpp::export]]

List FooMat4() {
// Create a fill a 2x2 matrix
NumericMatrix tmp(2,2);
for (int i = 0; i < 4; i++) {
tmp[i] = i;
}

return List::create(tmp * 3);
}


Now if we source the file we get some odd behaviour:

# Proof that we can return a NumericMatrix in a List:
> FooMat()
[[1]]
[,1] [,2]
[1,] 0 2
[2,] 1 3

# Multiply the whole NumericMatrix by a whole NumericVector
# whose size is 1. Unsafe behaviour?
> FooMat2()
[[1]]
[1] 0.000000e+00 3.000000e+00 1.388988e-309 2.083483e-309

# Multiply the whole NumericMatrix by the first element of
# The NumericVector. Results are correct, but `*` converts
# the answer to a NumericVector instead of a NumericMatrix
> FooMat3()
[[1]]
[1] 0 3 6 9

# Same as FooMat3() except now we just multiply the NumericMatrix
# by an integer
> FooMat4()
[[1]]
[1] 0 3 6 9


One, the syntactic sugar for
*
provided by
Rcpp
does not seem to correctly handle multiplication of Matrices with scalars. Two, multiplying by a whole
NumericVector
, as in
FooMat2()
leads to unsafe behaviour.

Answer

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.