Can I modify a row in-place with Rcpp? - c++

Question

Say, I would like to increase the first row of a matrix by one. The obvious approach is A.row(0) = A.row(0) + 1;, but it creates a new row instead of modifying the existing one, and may cause some performance issue when the matrix is large.
From a previous answer, I learned that I can do a point aliasing, but it only works for the whole matrix, not for individual rows.
library(Rcpp)
cppFunction('
void increaseFirstRow(NumericMatrix& A) {
NumericVector B = A.row(0);
B = B + 1;
}')
A <- diag(c(1.0, 2.0, 3.0))
increaseFirstRow(A)
The output is shown below. Note that the first row is not changed.
> A
[,1] [,2] [,3]
[1,] 1 0 0
[2,] 0 2 0
[3,] 0 0 3

Answer 1

Under the formulation above, I think you wanted to obtain a reference to specific parts of the matrix. The following work across matrix types:
*Matrix::Row = x( 0 , Rcpp::_); // first row
*Matrix::Column = x( Rcpp::_ , 0); // first column
*Matrix::Sub = x( Rcpp::Range(0, 1) , Rcpp::Range(2, 3)); // first 2 rows and 3 -4th column.
In your case, that would be:
#include <Rcpp.h>
// [[Rcpp::export]]
void row_ref(Rcpp::NumericMatrix M) {
// Create a reference to the 1st row in M.
Rcpp::NumericMatrix::Row x = M.row(0);
// Increase the first row in M.
x = x + 10;
}
Example:
(A <- diag(c(1.0, 2.0, 3.0)))
# [,1] [,2] [,3]
# [1,] 1 0 0
# [2,] 0 2 0
# [3,] 0 0 3
row_ref(A)
A
# [,1] [,2] [,3]
# [1,] 11 10 10
# [2,] 0 2 0
# [3,] 0 0 3

Answer 2

Here is a simple solution in RcppArmadillo, and, following an edit, in Rcpp itself:
Code with Example
#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
void incRow(arma::mat& M) {
M.row(0) = M.row(0) + 1;
}
// [[Rcpp::export]]
void incRow2(Rcpp::NumericMatrix M) {
M(0,Rcpp::_) = M(0,Rcpp::_) + 1;
}
/*** R
A <- diag(c(1.0, 2.0, 3.0))
incRow(A)
A
incRow2(A)
A
*/
Output
R> Rcpp::sourceCpp("/tmp/armarow.cpp")
R> A <- diag(c(1.0, 2.0, 3.0))
R> incRow(A)
R> A
[,1] [,2] [,3]
[1,] 2 1 1
[2,] 0 2 0
[3,] 0 0 3
R> incRow2(A)
R> A
[,1] [,2] [,3]
[1,] 3 2 2
[2,] 0 2 0
[3,] 0 0 3
R>