[Rcpp-devel] Filling NumericMatrix with NumericVectors with apply by row/column?

[Romain Francois]

Le 21/09/10 11:46, Christian Gunning a écrit :
>> You can always attach dimensions later:
>>
>> ret.attr( "dim" ) = Dimension( 2, 2, 2) ;
>
> Ah, yes.  Thanks.

I'd rather encapsulate this a bit rather than let people touch the dim 
attribute, or perhaps mimic what R is doing :

 > x <- 1:24
 > attr( x, "dim" ) <- c(2,12)
 > x
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
[1,]    1    3    5    7    9   11   13   15   17    19    21    23
[2,]    2    4    6    8   10   12   14   16   18    20    22    24
 > attr( x, "dim" ) <- c(2,17)
Erreur dans attr(x, "dim") <- c(2, 17) :
   dims [produit 34] ne correspond pas à la longueur de l'objet [24]

and gatekeep inside the attr() function

>> You can however Rcpp::wrap a cube. It is safer because the memory gets
>> copied, but less efficient, because the memory is copied.
>
> Got it.  I erroneously assumed an implicit wrap somewhere.

There was no implicit wrap in the code you shown.

>> 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 ) ;
>
> Yes, I've been thinking similarly about an Indexer class.  Accounting
> for a variable number of dimensions looks unpleasantly messy.

Yes. Feel free to share thoughts on that.

>>> * 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
>
> Oh, I was thinking more along the lines of "by what mechanism is
> dimension lost" than "why is grass green" :)
>
> -Christian

Because we make a new vector and don't keep the attributes of the old 
one, including the "dim" attribute.

-- 
Romain Francois
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