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[Rcpp-devel] Rcpp shortcut for R's as.matrix and as.vector

4 messages · Douglas Bates, Romain Francois

Original
Douglas Bates
# 
Most Lapack routines for solving linear systems from factorizations
allow the right hand side to be a matrix, thereby providing for
solving multiple systems in one  call.  I would like to preserve this
capability while also allowing for the most common case where the rhs
is a vector.  Also, I would like to return a vector solution rather
than a single matrix solution when it is a single column.

I can accomplish these "as.matrix" and "as.vector" translations by
calling the corresponding R functions.

static Rcpp::Function asMatrix("as.matrix")
static Rcpp::Function asVector("as.vector")

and using those within Rcpp.  Is there an alternative within Rcpp?  I
realize that I need to return another object so that it can have a
different class within C++ but I guess I would like

NumericMatrix mat = Rcpp::as<NumericMatrix>(myNumericVector);

or something similar.  It is not a big deal because the calls to the R
function are not a problem (or I could manipulate the SEXP internally)
but I'm wondering if I am missing something simple here.
Romain Francois
# 
Le 29/03/10 15:36, Douglas Bates a ?crit :
Not really. You can do :

x.attr( "dim" ) = R_NilValue ;

or

x.attr( "dim" ) = IntegerVector::create( 5, 5 ) ;


Here is a sketch for something we might want to add in the Matrix template:

SEXP as_vector(){
	SEXP x = ::Rf_duplicate( m_sexp ) ;
	::Rf_setAttrib( x, ::Rf_install( "dim" ), R_NilValue ) ;
	// perhaps also dimnames
	return x ;
}

or maybe this should return a VECTOR (which is a typedef to the 
appropriate Vector<RTYPE> in Matrix.

I don't really have time to play with this in the coming days, but if 
you try to add the function and some unit tests, I'll take time to 
review them. I could also sketch out Vector::as_matrix but I guess you 
also can.
I'm mot sure this will work (actually I'm almost sure this does not, but 
I did not test).

You would essentially be calling the NumericMatrix( SEXP ) constructor 
which would not be happy about the argument not being a matrix.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Douglas Bates
    

    

      
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On Tue, Mar 30, 2010 at 1:44 AM, Romain Francois

        
<romain at r-enthusiasts.com> wrote:

            

      
      
      
    

        
I'm asking all these questions because I haven't used templates in C++
and I keep thinking of ways that I would do things in other languages.
 With self-describing objects in R you can be rather flexible about
return types and for C code written against R you tend to have all
your objects expressed as SEXPs so again you have flexibility.  For
the time being I will simply use R's as.matrix in the R interface code
or as a Function object in Rcpp code.

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Romain Francois
    

    

      
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Le 30/03/10 15:46, Douglas Bates a ?crit :

            

      
      
      
    

        
Me too. templates are quite weird to start with, and then you can never 
look back.

            

      
      
      
    

        
Rcpp only gives the illusion of strong-typing. It can not afford actual 
strong typing because it is too dependent on R, and R uses loose typing. 
If you create a NumericVector and then assign a "dim" attribute to 
something of dimension 2, it is an R matrix, but it is not an 
Rcpp::NumericMatrix, as there is no way to change the c++ class of an 
object dynamically.

this is also why we have indexing operators that takes two ints defined 
in Vector and not Matrix.

The only reason for the Matrix template is that it can expose a 
different set of constructors, but what we need to make explicit is that 
it does not guarantee that the object will allways be a matrix. If you 
create a NumericMatrix and then you wipe the "dim" attribute, you still 
have a NumericMatrix, but this is no longer a matrix.

            

      
      
      
    

        
Sure. But would you say it is a good idea or a bad idea to have these 
helper functions in Rcpp ?

Matrix<RTYPE> Vector::as_matrix() ;
Vector<RTYPE> Matrix::as_vector() ;

or maybe they should both be in Vector, or maybe we should just have
Vector::dim().