[Rcpp-devel] NA_REAL in RcppParallel

[Hadley Wickham]

NA_REAL is just a NaN with a special value in the payload, so it's fine to use.
Hadley

On Thu, Jun 2, 2016 at 7:07 AM, Jason Foster <jason.j.foster at gmail.com> wrote:
> Hi,
>
> My question is how can I return a thread safe NA_REAL using RcppParallel?
> I've read many posts about how it's not thread safe to call R or Rcpp APIs
> when using RcppParallel
> (http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2016-January/009061.html),
> so in the past I've played it safe by returning NAN in C++ instead (as it's
> my understanding NA_REAL is specific to R).
>
> The following example is a modified version of "Transforming a Matrix in
> Parallel using RcppParallel"
> (http://gallery.rcpp.org/articles/parallel-matrix-transform/) that checks
> whether the input value is NA before taking the sqrt, otherwise NA is
> returned (note: this is the desired behavior and for illustrative purposes
> only):
>
> // [[Rcpp::depends(RcppParallel)]]
> #include <Rcpp.h>
> #include <RcppParallel.h>
> using namespace Rcpp;
> using namespace RcppParallel;
>
> struct SquareRoot : public Worker
> {
>   // source vector
>   const RVector<double> input;
>
>   // destination vector
>   RVector<double> output;
>
>   // initialize with source and destination
>   SquareRoot(const NumericVector input, NumericVector output)
>     : input(input), output(output) {}
>
>   // take the square root of the range of elements requested
>   void operator()(std::size_t begin, std::size_t end) {
>     for (std::size_t i = begin; i < end; i++) {
>       if (std::isnan(input[i])) {
>         output[i] = NA_REAL; // this is the desired output (instead of NAN)
>       } else {
>         output[i] = sqrt(input[i]);
>       }
>     }
>   }
> };
>
> // [[Rcpp::export]]
> NumericVector parallelVectorSqrt(NumericVector x) {
>
>   // allocate the output vector
>   NumericVector output(x.size());
>
>   // SquareRoot functor (pass input and output vectors)
>   SquareRoot squareRoot(x, output);
>
>   // call parallelFor to do the work
>   parallelFor(0, x.size(), squareRoot);
>
>   // return the output vector
>   return output;
>
> }
>
> Any guidance or insight would be much appreciated. Thanks!
>
>
>
>
>
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