[Rcpp-devel] Pointer troubles

[Dirk Eddelbuettel]

On 18 August 2011 at 19:27, Christian Gunning wrote:
| > Comment: A concise example that uses an XPtr in a C++ function is
| > still lacking.
| 
| Sorry I missed Manuel's example when I last posted.  I guess I was
| thinking something like below, which is a direct analog of apply. This
| concisely mirrors the semantics in RcppDE (interchangable work
| function for a well-defined task).  Here, the user needs only change
| inc.  Only thing is, this example seems annoyingly long for inclusion
| in the quickref. 

Well do not have a publisher-mandated page limit, so I'd say the more the
merrier!

| Also, I'm not sure whether using inline here is more
| or less confusing to the overall idea...

I like it. Maybe add a comment or two more, but the basic idea comes across
nicely.

Dirk
 
| Any thoughts on documentation priorities?
| -xian
| 
| require(inline); require(Rcpp)
| 
| inc <- '
| // Define a simple function
| double myFun(SEXP xs) { // sum
|     Rcpp::NumericVector x(xs);
|     double sum = std::accumulate(x.begin(), x.end(), 0.0);
|     return(sum);
| }
| '
| 
| src <- '
| // Define a pointer to the above function
| typedef double (*funcPtr)(SEXP);
| return(XPtr<funcPtr>(new funcPtr(&myFun)));
| '
| 
| src1 <- '
| // Define a pointer to the above function
| typedef double (*funcPtr)(SEXP);
| XPtr<funcPtr> myptr(infun);
| // apply myptr to rows/columns of x
| NumericMatrix xx(x);
| int margin = as<int>(margin_);
| int nr = xx.nrow();
| int nc = xx.ncol();
| if ( margin == 1 ) {
|     NumericVector ret(nr);
|     for( int i = 0; i<nr; i++) {
|         ret[i] = (*myptr)( wrap(xx(i,_)) );
|     };
|     return(ret);
| };
| if ( margin == 2 ) {
|     NumericVector ret(nc);
|     for( int i = 0; i<nc; i++) {
|         ret[i] = (*myptr)( wrap(xx(_,i)) );
|     };
|     return(ret);
| };
| '
| 
| mksum <- cxxfunction(signature(), body = src, includes = inc, plugin="Rcpp")
| myapply <- cxxfunction(signature(x='numeric', margin_='integer', infun='XPtr'),
|                     body = src1, includes = inc, plugin="Rcpp")
| 
| aa = matrix(1:100, nrow=5)
| res1 = myapply(aa, 1, mksum() )
| res1R = apply(aa, 1, sum)
| res2 = myapply(aa, 2, mksum() )
| res2R = apply(aa, 2, sum)
| 
| -- 
| A man, a plan, a cat, a ham, a yak, a yam, a hat, a canal – Panama!

-- 
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