> I'm coming late to the party but ... I was able to squeeze a couple > of milliseconds from the computation by expressing the counts as > integers and ensuring that I did not copy x by using an Eigen::Map. > It may well be that an Rcpp::NumericVector will not be a copy in this > case but I have found it difficult to determine exactly when an Rcpp > vector is going to be copied. With Eigen I can ensure that the > original storage in R will be used for the vector. > > count_eigen <- cxxfunction(signature(x="numeric", binwidth="numeric", > origin="numeric", nbins="integer"), ' > double binwidth_ = ::Rf_asReal(binwidth), origin_ = ::Rf_asReal(origin); > > Eigen::VectorXi counts(::Rf_asInteger(nbins)); > Eigen::Map<Eigen::VectorXd> x_(as<Eigen::Map<Eigen::VectorXd> >(x)); > > int n = x_.size(); > > counts.setZero(); > for (int i = 0; i < n; i++) counts[((x_[i] - origin_) / binwidth_)]++; > > return wrap(counts); > ', plugin="RcppEigen") > > > As you see, I use ::Rf_asReal() and ::Rf_asInteger() for conversion to > scalar doubles or scalar integers. Those functions are part of the R > API and are fast and general.
Thanks. Would you mind explaining a couple of the C++ idioms that you used? * What's going on with Eigen::Map<Eigen::VectorXd> x_(as<Eigen::Map<Eigen::VectorXd> >(x)) - you're creating a new Map (templated to contain VectorXds) called x_ and initialising with a converted x? * I find it hard to visually parse <Eigen::Map<Eigen::VectorXd> >(x). Is there a reason you don't write it as x_(as<Eigen::Map<Eigen::VectorXd>>(x)? Maybe because that would then form the >> operator? * What does ::Rf_asReal mean? How is it different to Rf_asReal? Thanks! Hadley -- Assistant Professor / Dobelman Family Junior Chair Department of Statistics / Rice University http://had.co.nz/ _______________________________________________ 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
