CODEWITHSUNDEEP
rcpp
Shubham Gupta
Shubham Gupta

Reputation: 660

How to parse to rows first in Rcpp::NumericMatrix

Generally when we write a function in c++ to parse a 2D array, it passes through the first row then move to the second row.

for(int i = 0; i < ROW_SIZE; i++){
    for(int j = 0; j < COL_SIZE; j++){
        *((Mat+i*COL_SIZE) + j) = value;
    }
}

However, when I use Rcpp::NumericMatrix it parses through columns first. 

// [[Rcpp::export]]
NumericMatrix TestMatrixParsing(){
  NumericMatrix xx(4, 5);
  int xsize = xx.nrow() * xx.ncol();
  for (int i = 0; i < xsize; i++) {
    xx[i] = i+100;
  }
  return xx;
}

/*** R
TestMatrixParsing()
#      [,1] [,2] [,3] [,4] [,5]
# [1,]  100  104  108  112  116
# [2,]  101  105  109  113  117
# [3,]  102  106  110  114  118
# [4,]  103  107  111  115  119
*/

Is there any way to force it to parse through rows as my previous code was written to have matrix stored as consecutive rows so pointer doesn't have to jump equal to the COL_SIZE.

Upvotes: 0

Views: 275

Answers (1)

Ralf Stubner
Ralf Stubner

Reputation: 26823

Rcpp::NumericMatrix just follows the way R lays out its memory, which is in column major mode. A simple solution would be to transpose the matrix:

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericMatrix TestMatrixParsing(){
  Rcpp::NumericMatrix xx(4, 5);
  int xsize = xx.nrow() * xx.ncol();
  xx = Rcpp::transpose(xx);
  for (int i = 0; i < xsize; ++i) {
    xx[i] = i + 100;
  }
  xx = Rcpp::transpose(xx);
  return xx;
}

/*** R
TestMatrixParsing()
#      [,1] [,2] [,3] [,4] [,5]
# [1,]  100  101  102  103  104
# [2,]  105  106  107  108  109
# [3,]  110  111  112  113  114
# [4,]  115  116  117  118  119
*/

That might be to expensive for large matrices though. In that case it is probably best to adapt your algorithm.

Upvotes: 2

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