[Rcpp-devel] matrix indexing

[Dirk Eddelbuettel]

On 23 January 2010 at 21:05, Romain Francois wrote:
| Hello,
| 
| Motivated by Christophe's question on R-help (cc'ed), I'd like to add 
| facilities for matrix (and maybe arrays) indexing into the new Rcpp api.
| 
| The classic api already ships matrix indexing through RcppMatrix and 
| RcppMatrixView classes. I'm not a big fan of using round brackets for 
| indexing, but apparently one cannot pass more than one parameter to [] 
| so ... I guess I'll have to sacrifice look and feel for a minute.
| 
| 
| Anyway I was thinking we can maybe add NumericVector::operator()( int, 
| int) (other vectors as well), so that we can do :
| 
| NumericVector m(x) ;
| double x11 = m(0,0) ;
| 
| 
| I was also thinking maybe making some sort of indexer class to take care 
| of conversion of 2 indices into 1d index; something like this pseudo code :
| 
| NumericVector m(x) ;
| NumericVector::MatrixIndexer indexer(m) ;
| double x11 = m[ indexer(0,0) ] ;
| 
| But this might be even more ugly than operator().
| 
| 
| Given than a REALSXP can change its mind about being a matrix, I'm not 
| sure I want to subclass NumericVector into NumericMatrix, etc. ...
| 
| Ideas ?

It's a really important topic, and I really don't know yet what I want. 

Recall another r-help questions of not too long ago when the almost-always-
complaining-yet-never-contributing Ivo Welch went on about needing a
faster lm().

I once wrote one using GNU gsl for the ordinary least squares part, but one
can do much_ better with proper C++ classes. As a grad student, I published a
review paper in the J of Applied Econometrics about Newmat (a C++ library I
used a lot at the time), and things have come a long way since.

Whit (also CC'ed) and I independently landed at using the clever armadillo
(templated C++) project at (IIRC) http://arma.sf.net, and there is also the
Eigen2 project (also templated C++, initially used only inside KDE).  

Whit essentially answered Ivo with a fast.lm project on github [ the boy is
lost in the wilderness and has yet not found the goodness that R-Forge is ;-)
-- see http://github.com/armstrtw/fast.lm ] 

Long story short: given that we pass these objects down to C++, we should
figure out a fast / light / convenient / elegant / [your term here] / ... way
to make use of them with other libraries.

Dirk

-- 
Three out of two people have difficulties with fractions.