#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::export]]
arma::mat foo(arma::mat A, arma::uword block_start, arma::uword block_length) {
  arma::uword rows = A.n_rows;
  arma::mat B = arma::mat(A.begin_col(block_start), rows, block_length, false);
  B.fill(1.0);
  return A;
}

/*** R
A <- matrix(0, 4, 4)
foo(A, 0, 2)
> A <- matrix(0, 4, 4)
> foo(A, 0, 2)
     [,1] [,2] [,3] [,4]
[1,]    1    1    0    0
[2,]    1    1    0    0
[3,]    1    1    0    0
[4,]    1    1    0    0
*/