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

Thanks - that does seem to work, but it doesn't perform as well as the pre-copying version.

Here's  a Gist so the conversation can be more concrete:

  https://gist.github.com/kenahoo/4991485

For me, the countSteps() version is about 10 times faster than countSteps2().

 -Ken

From: John Merrill [mailto:john.merrill at gmail.com]
Sent: Tuesday, February 19, 2013 5:46 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

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.)

From: John Merrill [mailto:john.merrill at gmail.com<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<mailto: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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