[Rcpp-devel] row iterators in RcppArmadillo

[Sokol Serguei]

Hi Jonathan,

Jonathan Fintzi has written at  Wed, 12 Oct 2016 13:43:20 -0700
> Hello,
>
> I am trying to use the std::copy() function to copy elements from an 
> Rcpp::NumericVector into rows of armadillo matrices. A simplified 
> version of my code is as follows:
>
> cppFunction('void copyinto(Rcpp::NumericVector& v, arma::mat& d1, 
> arma::mat& d2) {
>         std::copy(v.begin(), v.begin() + 3, d1.begin_row(0));
>         std::copy(v.begin() + 3, v.end(), d2.begin_row(0));
> }', depends = "RcppArmadillo")
>
> v <- runif(6)
> d1 <- matrix(0.0, 3,3)
> d2 <- matrix(0.0, 3,3)
>
> copyinto(v, d1, d2)
>
> However, when I attempt to compile the code, I receive a list of 
> compilation errors including "no type named 'XXXXXX' in 'class 
> arma::Mat<double>row_iterator", where XXXXXX is iterator_category, 
> value_type, difference_type, pointer, and reference.
>
> If I change the iterators to column iterators (e.g. d1.begin_col(0)) 
> everything compiles and works just fine. I feel that I must be missing 
> something quite basic here, but in searching around I haven't been 
> able to identify my mistake.
May be the problem is that you are trying to use std::copy for writing
to non contiguous memory when you use a row pointer and the class writers
were smart enough to prevent such situation.

The following code works as expected, may be it can do the job for you:

cppFunction('void copy2(arma::rowvec& v, arma::mat& d1, arma::mat& d2) {
         d1.row(0)=v(arma::span(0,2));
         d2.row(0)=v(arma::span(3,5));
}', depends = "RcppArmadillo")

v <- runif(6)
d1 <- matrix(0.0, 3,3)
d2 <- matrix(0.0, 3,3)
copy2(v, d1, d2)

>
> Thank you for your help!
> Jon
>
>
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