<div dir="ltr">I'm sorry, I was unclear. I wasn't referring to the use of sprintf, which is, as you say, a bug (albeit, in my own defense, one I was aware of and left in place for clarity. In production code, I'd have checked for the number of rows at the top of the function and failed if there were too many of them.) I was referring to the in parameters. For some reason, when large List parameters are sent in, the conversion from S Expression to List object in cppfunction fails with an out of buffer error. I assume that the buffer size is limited by design, since there's a simple work around.</div>
<div class="gmail_extra"><br><br><div class="gmail_quote">On Thu, Feb 7, 2013 at 12:28 PM, Davor Cubranic <span dir="ltr"><<a href="mailto:cubranic@stat.ubc.ca" target="_blank">cubranic@stat.ubc.ca</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word">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.<div>
<br></div><div>A more "C++ way" to do it, and most importantly safer, would be to use std::ostringstream:</div><div><br></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div>for (int i = 0; i < nrows; i++) {</div>
<div> std::ostringstream rowname;</div><div> rowname << i;</div><div> row_names(j) = rowname;</div><div>}</div><div>for (int j = 0; j < ncols; j++) {</div><div> std::ostringstream colname;</div><div> colname << < "X." << j;</div>
<div> col_names(j) = colname;</div><div>}</div></blockquote><span class="HOEnZb"><font color="#888888"><div><br></div><div>Davor</div></font></span><div><div class="h5"><div><br></div><div><br></div><div><div><div>On 2013-01-18, at 3:25 PM, John Merrill wrote:</div>
<br><blockquote type="cite"><span style="border-collapse:separate;font-family:Helvetica;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:-webkit-auto;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;font-size:medium">Sure. I'll write something up for the gallery, but here's the crude outline.<div>
<br></div><div>Here's the C++ code:</div><div><pre><font color="#000000">#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;
}</font></pre>There are some subtleties in this code:</div><div><br></div><div>* 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.</div>
<div>* 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.</div></span><br></blockquote></div><br>
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