I come late to this discussion, but it should be pointed out that using
"sprintf" without ensuring that your buffer is long enough is not a "subtlety"
but a bug.
A more "C++ way" to do it, and most importantly safer, would be to use
std::ostringstream:
for (int i = 0; i < nrows; i++) {
std::ostringstream rowname;
rowname << i;
row_names(j) = rowname;
}
for (int j = 0; j < ncols; j++) {
std::ostringstream colname;
colname << < "X." << j;
col_names(j) = colname;
}
Davor
On 2013-01-18, at 3:25 PM, John Merrill wrote:
> 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.
>
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