On 9 October 2014 at 14:07, Christian Authmann wrote:
| Hello,
|
| this sounds like a newbie question, but I haven't been able to find a
| good answer in Rcpp's documentation, source or testcases..
|
| Say I have an internal data type which I want to wrap/unwrap as a DataFrame.
|
| template<> SEXP wrap(const Foo &foo) {
| Rcpp::DataFrame dataframe;
| for (auto key : foo.keys()) {
| if (key.is_numeric()) {
| Rcpp::NumericVector vec;
| // Fill vector with numbers
| dataframe[ key ] = vec;
| }
| else if (key.is_string()) {
| Rcpp::StringVector vec;
| // Fill vector with strings
| dataframe[ key ] = vec;
| }
| // etc
| }
| }
|
| template<> Foo as(SEXP sexp) {
| Rcpp::DataFrame dataframe(sexp);
| // now what?
| }
|
|
| First, I need to iterate over all vectors in the dataframe. I could
| access a vector using
| dataframe["my_key"]
| but that would require me to have a list of all the keys/column names
| first. R has a colnames() function, but getting that into an
| Rcpp::Function and calling it seems overly complicated..
| I couldn't find anything in Rcpp's source or testcases.. but maybe
| there's a simpler way?
DataFrame objects are clunky, that's just the way it is. It is a glorified
list of vectors, possibly of different types, with the added constraint of
equal length. Not much more really.
| Problem number two, how do I type the vector once I have it? The
| dataframe can contain NumericVector or StringVector (or possibly
| something different). Should I just construct each type, try/catch the
| exceptions and see what fits, or is there a better way to determine the
| type of a SEXP?
In my usage I typically assume that I know the layout. So if the third column
is, say, a vector of strings I would assign it to that.
For free-form incoming data that is a little harder but I believe we have
several 'dispatching based on types' examples floating around here, on the
Rcpp Gallery and/or other places.
The joys of interfacing a dynamically-typed language with a statically-typed
one :)
Dirk
--
http://dirk.eddelbuettel.com | @eddelbuettel | e...@debian.org
_______________________________________________
Rcpp-devel mailing list
Rcpp-devel@lists.r-forge.r-project.org
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
