How to element-wise divide 2 sparse matrices in Armadillo?

Question

I am new to C++ but I am working with R and RcppArmadillo and my goal is to divide element-wise 2 sparse matrices. I have read in the documentation of Armadillo that the operator is / but when I source my code with sourceCpp I am getting this error:

no match for 'operator/' (operand types are 'arma::sp_mat' {aka 'arma::SpMat'} and 'arma::sp_mat' {aka 'arma::SpMat'})

I have write a small example of code. Please note that when I remove the division function from the script, the multiplication function works fine.

// [[Rcpp::depends(RcppArmadillo)]]
#include <RcppArmadillo.h>

using namespace Rcpp;

//' Sparse Matrix element-wise multiplication
//'
//' @param X sp_mat 
// [[Rcpp::export]]
arma::sp_mat sp_mat_mul(arma::sp_mat X){
  
  arma::sp_mat Y = X % X;
  return Y;
  
}


//' Sparse Matrix element-wise division
//'
//' @param X sp_mat 
// [[Rcpp::export]]
arma::sp_mat sp_mat_div(arma::sp_mat X){
  
  arma::sp_mat Y = X / X;
  return Y;
  
}

Thank you for your time, I really appreciate it!

Answer 1

Use transform/foreach to calculate the inverse then multiply.

//' Sparse Matrix element-wise division
//'
//' @param X sp_mat 
// [[Rcpp::export]]
arma::sp_mat sp_mat_div(arma::sp_mat X){
  arma::sp_mat x_inverse(X);
  x_inverse.transform([](double val) {return (1.0/val);});
  arma::sp_mat Y = X % x_inverse;
  return Y;
}

R:

i <- c(1,3:8)
j <- c(2,9,6:10)
x <- 7 * (1:7)
A <- sparseMatrix(i, j, x = x)

sp_mat_div(A)
[1,] . 1 . . . . . . . .
[2,] . . . . . . . . . .
[3,] . . . . . . . . 1 .
[4,] . . . . . 1 . . . .
[5,] . . . . . . 1 . . .
[6,] . . . . . . . 1 . .
[7,] . . . . . . . . 1 .
[8,] . . . . . . . . . 1

As an aside, are you sure this operation makes sense?

You are ignoring all the zeroes in the matrix (since it is sparse), and if this were a non-sparse matrix, you'd be dividing by zero a lot.