Thanks Kevin,
This question came because when you do this
// [[Rcpp::export]]
DataFrame updateDFByValue(DataFrame df) {
int N = df.nrows();
NumericVector newCol(N,1.);
df["newCol"] = newCol;
return(df);
}
The DataFrame is returned to R as a list, and building back another
data.frame might
1. cost time
2. appear like a waste if what was intended was to update the data.frame
<http://cran.r-project.org/web/packages/data.table/index.html>
data.tableallows by-reference updates in R but there is no C api that
I know of, but
it is an enhanced data.frame so Rcpp deals with it as a data.frame, I
thought it was too bad to be able to update by reference in R and not in
C++ so I asked this genuine question.
Your way makes sense to me, I'll try to dig deeper.
Thanks
PS: Dirk answered me
here<http://stackoverflow.com/questions/15731106/passing-by-reference-a-data-frame-and-updating-it-with-rcpp>
2013/3/31 Kevin Ushey <kevinus...@gmail.com>
> I think the problem here is that the assignment df["newCol"] = newCol
> copies the dataframe. Note that something like this would work as expected:
>
>
> #include <Rcpp.h>
> using namespace Rcpp;
>
> // [[Rcpp::export]]
> void updateDFByRef(DataFrame& df) {
> int N = df.nrows();
> NumericVector newCol(N,1.);
> df[0] = newCol; // replace the 1st vector with the numeric vector of
> 1s, by ref
> return;
> }
>
> So, the reference to the original df is getting passed, the problem is
> figuring out how to assign a new vector to df without forcing a copy.
>
> I'm not sure if there's a ready-made solution, but I imagine the easiest
> way to do it would be:
>
> 1) Declare a new list of df.size()+1,
> 2) Copy the pointers to the new list (not sure the best way to do this in
> Rcpp),
> 3) Assign the vector you want to the new, last column,
> 4) Return that new list.
>
> This should work since internally, lists (VECSXP)s are just vectors of
> SEXPs (pointers) to other R vectors (REALSXPs, INTSXPs, and so on...)
>
> (Please correct me if I'm wrong on the above.)
>
> -Kevin
>
>
> On Sun, Mar 31, 2013 at 6:44 AM, stat quant <statqu...@outlook.com> wrote:
>
>> Hello list,
>> looking at Rcpp::DataFrame in the
>> gallery<http://gallery.rcpp.org/tags/dataframe/>I realized that I didn't
>> know how to modify a DataFrame by reference.
>> Googling a bit I found this post on
>> SO<http://stackoverflow.com/questions/13773529/passing-a-data-table-to-c-functions-using-rcpp-and-or-rcpparmadillo>and
>> this post
>> on the
>> archive<http://www.mail-archive.com/rcpp-devel@lists.r-forge.r-project.org/msg04919.html>
>> .
>> There is nothing obvious so I suspect I miss something big like "It is
>> already the case because" or "it does not make sense because".
>>
>> I tried the following which compiled but the data.frame object passed to
>> updateDFByRef in R stayed untouched
>>
>> #include <Rcpp.h>
>> using namespace Rcpp;
>>
>> // [[Rcpp::export]]
>> void updateDFByRef(DataFrame& df) {
>> int N = df.nrows();
>> NumericVector newCol(N,1.);
>> df["newCol"] = newCol;
>> return;
>> }
>>
>> Could somebody explain me what I am missing or kindly point me to a
>> document where I can find the explanation ?
>>
>> Cheers
>>
>> _______________________________________________
>> Rcpp-devel mailing list
>> Rcpp-devel@lists.r-forge.r-project.org
>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>>
>
>
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