On 31 January 2013 at 17:41, Kevin Ushey wrote:
| I was wondering - is there a way to translate between Vectors and Matrices?
Eg,
| reinterpreting a 100 length NumericVector as a 10x10 NumericMatrix by setting
| the dimensions explicitly, or something of that form; preferably with a
minimal
| amount of copying.
I fear that just setting the dim argument won't be enough as we're strongly
typed in C++.
| Alternatively, is there a preferred method for constructing a NumericMatrix
| for output from a std::vector?
Yes, there are constructors that that rows, cols and a pointer to the
contiguous memory. That is what I would use. There should be examples
ploting. But as a sketch:
Rcpp::NumericMatrix M(10, 10, X.begin() );
Actually, as it is so cheap to test:
-------------------------------------------------------------------------------
#include <Rcpp.h>
// [[Rcpp::export]]
Rcpp::NumericMatrix reshapePlusPlus(Rcpp::NumericVector X) {
return Rcpp::NumericMatrix(sqrt(X.length()), sqrt(X.length()), X.begin());
}
-------------------------------------------------------------------------------
which pans out:
R> sourceCpp("/tmp/kevin.cpp")
R>
R> reshapePlusPlus(1:9)
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
R>
| The motivation is primarily for reshaping of R objects through Rcpp. I wasn't
| able to find anything specific in the vignettes.
Not sure if this faster than just setting dim in R though....
Dirk
--
Dirk Eddelbuettel | e...@debian.org | http://dirk.eddelbuettel.com
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