Le 21/09/10 10:57, Christian Gunning a écrit :
On Tue, Sep 7, 2010 at 11:40 PM, Romain Francois
<rom...@r-enthusiasts.com> wrote:
In the first case, could I conceivably turn an input 3D SEXP array
into, e.g., NumericVector( Dimension(2,3,4) ), create an arma::cube
out of it, process it and return?
I think so. I have not tried though ... have you ?
Here we go - it works under certain circumstances:
fx_flat<- cxxfunction( signature(x = "integer", y = "array" ) ,
'
NumericVector ret(Dimension(2,2,2));
ret = NumericVector(y);
ret = ret * as<int>(x); // this "squishes" ret back to 1D
return ret;
', plugin = "RcppArmadillo" )
Ah. We should implement NumericVector::operator*= etc ...
You can always attach dimensions later:
ret.attr( "dim" ) = Dimension( 2, 2, 2) ;
I'll try to think of some ways to keep the dimensions. Perhaps we do
need some sort of Rcpp::Array class
fx_flat( 2L, array(1:8, dim=c(2,2,2)) )
[1] 2 4 6 8 10 12 14 16
fx_cube<- cxxfunction( signature(x = "integer", y = "array" ) ,
'
// note - NumericVector ret(y) is flat
NumericVector ret(Dimension(2,2,2));
// use copy constructor to keep dimension
ret = NumericVector(y);
// copy_aux_mem = false is the only way to return retcube?
arma::cube retcube(ret.begin(), 2, 2, 2, false);
retcube = retcube * as<int>(x);
return ret;
', plugin = "RcppArmadillo" )
fx_cube( 2L, array(1:8, dim=c(2,2,2)) )
, , 1
[,1] [,2]
[1,] 2 6
[2,] 4 8
, , 2
[,1] [,2]
[1,] 10 14
[2,] 12 16
Yes. Armadillo has a separate class for cubes, which makes all this
work. That's why I think we need Rcpp::Array too.
A few points stand out to me -
* If i'm not doing pointer-magic and/or only using Rcpp/Armadillo
operators, is copy_aux_mem = false a reasonable way to return retcube
after processing? I don't see any other way to get an SEXP out of an
arma::cube, but i'm skeptical of "can be dangerous unless you know",
since i don't.
I think this is fine. You essentially tell armadillo that the memory is
yours and no to mess with it, it won't.
You can however Rcpp::wrap a cube. It is safer because the memory gets
copied, but less efficient, because the memory is copied.
* It seems to me that row/column/slice indexer semantics, rather than
linearArray[ thisrow + thiscol*(ncol-1) ] semantics (which i always
seem to mess up) is one reason these mailing list questions about
arrays keep popping up.
Sure. I need to think a little bit more about how to do Rcpp::Array, etc
... because an Array could have an arbitrary number of dimensions.
Perhaps embedding information into the class would work.
// make an array of 3 dimensions, etc ...
Rcpp::Array<3> ret( y ) ;
I have very little time to devote to this right now.
* What about "ret = ret * as<int>(x);" causes ret to lose it's dimension?
Omission, lack of time, I'm not using arrays all that much personally,
etc ...
best,
Christian
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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