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

Christian Gunning xian at unm.edu
Tue Sep 21 13:20:54 CEST 2010

```On Tue, Sep 21, 2010 at 2:56 AM, Romain Francois
<romain at r-enthusiasts.com> wrote:
>
>>> 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 ) ;

Maybe solving the just the 3D case instead of n-D solve the majority
of use cases?

>> 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.

Since an R-style mat(2,) is illegal, perhaps a simple solution is
using booleans in the index position to indicate "all",   This solves
a decent range of use cases, and should be accessible to the "native
R" crowd. Here's a simple one:

NumericMatrix ret(inmat);
double lambda = as<double>(inlambda);
int nr = ret.nrows();
int nc = ret.ncols();
int i;
for (i=0; i<nr; i++) {
ret(i,true) = rpois(nc, lambda);
};

The next logical extension of matrix/array indexing beyond boolean
would be to allow NumericVectors in each of the index positions.  At
this point, though, there's an explosion of over-loaded functions - a
3D Num myArray(x, y, z) gives 27 separate functions for x,y,z chosen
from {bool, int, NumericVector}, plus NumericMatrix and NumericVector.
Here's where an indexer class starts to make sense - unified
constructors, along with clear dimensionality of the indexer and
simple checks of dimensional conformance between indexer and indexed.

As a side-note, I just spent some time with arma, and was sad to find
that arma_mat.insert_rows(atrow, rowvec) extends arma_mat, with no
apparent way to do row/col-level in-place replacement. So, at least
we're not whipping a dead horse.

Can you point me to the relevant NumericMatrix(i,j) indexer code?

thanks much,
Christian
--
A man, a plan, a cat, a ham, a yak, a yam, a hat, a canal – Panama!
```