How to scale a NumericMatrix in-place with Rcpp? - c++

Question

This is what I'm doing now
library(Rcpp)
A <- diag(c(1.0, 2.0, 3.0))
rownames(A) <- c('X', 'Y', 'Z')
colnames(A) <- c('A', 'B', 'C')
cppFunction('
void scaleMatrix(NumericMatrix& A, double x) {
A = A * x;
}')
Unfortunately It doesn't work :(
> A
A B C
X 1 0 0
Y 0 2 0
Z 0 0 3
> scaleMatrix(A, 2)
> A
A B C
X 1 0 0
Y 0 2 0
Z 0 0 3
I learned from Rcpp FAQ, Question 5.1 that Rcpp should be able to change the object I passed by value. Stealing an example from Dirk's answer to my previous question:
> library(Rcpp)
> cppFunction("void inplaceMod(NumericVector x) { x = x * 2; }")
> x <- as.numeric(1:5)
> inplaceMod(x)
> x
[1] 2 4 6 8 10
I'm confused: it is possible to modify a NumericVector in-place, but not a NumericMatrix?

Answer 1

You can preserve the row and column names by using NumericVector instead of NumericMatrix, keeping in mind that a matrix in R is just a vector with attached dimensions. You can do this switch either when going from R to C++ (scaleVector below) or within C++ (scaleMatrix below taken from a now deleted answer by #Roland):
library(Rcpp)
cppFunction('
NumericVector scaleVector(NumericVector& A, double x) {
A = A * x;
return A;
}')
cppFunction('
NumericMatrix scaleMatrix(NumericMatrix& A, double x) {
NumericVector B = A;
B = B * x;
return A;
}')
If one applies these two function to your matrix, the row and column names are preserved. However, the matrix is not changed in place:
A <- diag(1:3)
rownames(A) <- c('X', 'Y', 'Z')
colnames(A) <- c('A', 'B', 'C')
scaleMatrix(A, 2)
#> A B C
#> X 2 0 0
#> Y 0 4 0
#> Z 0 0 6
scaleVector(A, 2)
#> A B C
#> X 2 0 0
#> Y 0 4 0
#> Z 0 0 6
A
#> A B C
#> X 1 0 0
#> Y 0 2 0
#> Z 0 0 3
The reason for that is that diag(1:3) is actually an integer matrix, so a copy is made when you transfer it to a numeric matrix (or vector):
is.integer(A)
#> [1] TRUE
If one uses a numeric matrix to begin with, modification is done in place:
A <- diag(c(1.0, 2.0, 3.0))
rownames(A) <- c('X', 'Y', 'Z')
colnames(A) <- c('A', 'B', 'C')
scaleMatrix(A, 2)
#> A B C
#> X 2 0 0
#> Y 0 4 0
#> Z 0 0 6
scaleVector(A, 2)
#> A B C
#> X 4 0 0
#> Y 0 8 0
#> Z 0 0 12
A
#> A B C
#> X 4 0 0
#> Y 0 8 0
#> Z 0 0 12