[Rcpp-devel] Efficient DataFrame access by row & column

John Merrill john.merrill at gmail.com
Wed Feb 20 00:46:19 CET 2013

Well, here's a snippet from a much larger routine I used deep inside an
implementation of kd-trees:

  for (int i = 0; i < instances_df.size(); ++i) {
    const NumericVector& data_column = instances_df[i];
    for (int j = 0; j < training_instances.size(); ++j) {
      // Argument order changes here...
      instances[j][i] = data_column[training_instances[j]];

To set expectations, training_instances can be very large indeed (ca. 1M).
  The code is quite fast.

(And sorry, Dirk -- yes, I really do have an access of the form x[i][j].
 Mea culpa, etc.)

On Tue, Feb 19, 2013 at 3:26 PM, Ken Williams
<Ken.Williams at windlogics.com>wrote:

> > From: John Merrill [mailto:john.merrill at gmail.com]
> > Sent: Tuesday, February 19, 2013 5:24 PM
> > To: Ken Williams
> > Cc: Yan Zhou; Dirk Eddelbuettel; rcpp-devel at lists.r-forge.r-project.org
> > Subject: Re: [Rcpp-devel] Efficient DataFrame access by row & column
> >
> > I'm a little puzzled by your question.  Could you use a reference
> instead of instantiating a new copy?
> I would love to use a reference, but I don't know how.  That's in fact the
> essence of my question. =)
> Is there already some example code somewhere showing how to get reference
> to a DataFrame column without copying?  I must be just missing it.
>  -Ken
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