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

The most inefficient part I see is the creation of a new NumericVector inside the inner most loop. You copied each column n times, of which n-1 times are unnecessary.

Yan Zhou

On Feb 19, 2013, at 11:02 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

Ken,

On 19 February 2013 at 22:35, Ken Williams wrote:
| I have a need to loop through all the entries of a DataFrame by row, then
| column.  I know two different ways:

There have been prior discussions of this topic, as well as example posts --
even leading to a Rcpp Gallery article. Did you read any of these?  It wasn't
clear from your post.

| I?m also curious why it?s a syntax error in Case A to just write `df[j][i]` or

Eeeek.  I prefer the more C++-y way of writing df(j,i).  Square brackets only
work for vectors, and even then you may be better off with x(i) for
consistency.

Overall, your premise may be wrong too.  "We all know" that a data.frame is
not the fastest data structure in R, so by forcing ourselves to the same
access are we not handycapping ourselves.

Once you are in C++, you can use whatever C++ datatype you like.  A
data.frame really is just a list of vectors, each of the vectors has eg a
begin(0 iterator which you can (fairly costlessly) instantiate STL types.

And those give you performance guarantees.

Hope this helps,  Dirk

--
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com