Sure. I'll write something up for the gallery, but here's the crude
outline.
Here's the C++ code:
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
List BuildCheapDataFrame(List a) {
List returned_frame = clone(a);
GenericVector sample_row = returned_frame(1);
StringVector row_names(sample_row.length());
for (int i = 0; i < sample_row.length(); ++i) {
char name[5];
sprintf(&(name[0]), "%d", i);
row_names(i) = name;
}
returned_frame.attr("row.names") = row_names;
StringVector col_names(returned_frame.length());
for (int j = 0; j < returned_frame.length(); ++j) {
char name[6];
sprintf(&(name[0]), "X.%d", j);
col_names(j) = name;
}
returned_frame.attr("names") = col_names;
returned_frame.attr("class") = "data.frame";
return returned_frame;
}
There are some subtleties in this code:
* It turns out that one can't send super-large data frames to it because of
possible buffer overflows. I've never seen that problem when I've written
Rcpp functions which exchanged SEXPs with R, but this one uses Rcpp:export
in order to use sourceCpp.
* Notice the invocation of clone() in the first line of the code. If you
don't do that, you wind up side-effecting the parameter, which is not what
most people would expect.
Here's the timing, as measured on an AWS node:
> sourceCpp('/tmp/test_adf.cc')
> a <- replicate(250, 1:100, simplify=FALSE)
> system.time(replicate( { as.data.frame(a) ; NULL }, n=100))
user system elapsed
3.890 0.000 3.892
> system.time(replicate( { BuildCheapDataFrame(a) ; NULL }, n=100))
user system elapsed
0.020 0.000 0.022
Yes, that really is a factor of 200 speedup.
On Fri, Jan 18, 2013 at 8:16 AM, Paul Johnson <pauljoh...@gmail.com> wrote:
> On Thu, Jan 17, 2013 at 9:54 PM, John Merrill <john.merr...@gmail.com>
> wrote:
> > As of 2.15.1, data.frame appears to no longer be O(n^2) in the number of
> > columns in the frame. That's certainly an improvement, yes.
> >
> > However, by eliminating calls to data.frame and replacing them with
> direct
> > class modifications, I can take a routine which takes minutes and reduce
> it
> > to a routine which takes seconds. So, pragmatically, in Rcpp, I can get
> a
> > rough factor of sixty, it appears.
> >
> >
> Wow.
>
> When you have this written out, will you post links to it? I can
> learn from your examples, I think.
>
> pj
>
>
>
> > On Thu, Jan 17, 2013 at 7:46 PM, Paul Johnson <pauljoh...@gmail.com>
> wrote:
> >>
> >> On Tue, Jan 15, 2013 at 9:20 AM, John Merrill <john.merr...@gmail.com>
> >> wrote:
> >> > It appears that DataFrame::create is a thin layer on top of the R
> >> > data.frame
> >> > call. The guarantee correctness, but also means the performance of an
> >> > Rcpp
> >> > routine which returns a large data frame is limited by the performance
> >> > of
> >> > data.frame -- which is utterly horrible.
> >>
> >> Are you certain that this claim is still true?
> >>
> >> I was shocked/surprised by the package "dataframe" and the commentary
> >> about it. The author said that data.frame was slow because "This
> >> contains versions of standard data frame functions in R, modified to
> >> avoid making extra copies of inputs. This is faster, particularly for
> >> large data."
> >>
> >> it was repeatedly copying some objects and he proved a substantially
> >> faster approach.
> >>
> >> In the release notes for R-2.15.1, I recall seeing a note that R Core
> >> had responded by integrating several of those changes. But still
> >> data.frame is not fast for you?
> >>
> >> If they didn't make the core data.frame as fast, would you care to
> >> enlighten us by installing the dataframe package and letting us know
> >> if it is still faster?
> >>
> >> Or perhaps you are way ahead of me and you've already imitated
> >> Hesterberg's algorithms in your C++ design?
> >>
> >> pj
> >>
> >> --
> >> Paul E. Johnson
> >> Professor, Political Science Assoc. Director
> >> 1541 Lilac Lane, Room 504 Center for Research Methods
> >> University of Kansas University of Kansas
> >> http://pj.freefaculty.org http://quant.ku.edu
> >
> >
>
>
>
> --
> Paul E. Johnson
> Professor, Political Science Assoc. Director
> 1541 Lilac Lane, Room 504 Center for Research Methods
> University of Kansas University of Kansas
> http://pj.freefaculty.org http://quant.ku.edu
>
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