Thanks a lot; that seems elegant. Could one imagine that this goes into the next version of Rcpp? That would be nice.
Regards Søren -----Original Message----- From: Romain Francois [mailto:rom...@r-enthusiasts.com] Sent: 15. november 2012 00:50 To: Søren Højsgaard Cc: rcpp-devel@lists.r-forge.r-project.org Subject: Re: [Rcpp-devel] Rcpp "version" of R's match function Ah. Things are particularly interesting if you want to deal with strings. The code below implements a c++ version of match for character vectors. beware it uses trickery related to tricking the write barrier (but not an issue as we are not "writing"). require(Rcpp) sourceCpp( code = ' #include <Rcpp.h> using namespace Rcpp ; // [[Rcpp::export]] IntegerVector match_( const CharacterVector& x, const CharacterVector& table){ typedef std::tr1::unordered_map<SEXP,int> MAP ; typedef MAP::value_type VALUE ; // populate the hash MAP hash ; int n = table.size() ; SEXP* ptr = get_string_ptr(table) ; for( int i=0 ; i<n; i++){ hash.insert( std::make_pair<SEXP,int>(ptr[i], i + 1) ) ; } n = x.size() ; IntegerVector result(n) ; ptr = get_string_ptr(x) ; MAP::const_iterator end=hash.end() ; for( int i=0; i<n; i++){ MAP::const_iterator it = hash.find(ptr[i]) ; if( it == end ){ // no match result[i] = NA_INTEGER ; } else { result[i] = it->second ; } } return result ; } ' ) The key of the trick is to use the pointer of the CHARSXP SEXP as the key of the hash map. (strings are cached in R, see the CHARSXP cache in R internals: http://cran.r-project.org/doc/manuals/R-ints.html#The-CHARSXP-cache ) Then: > match_( c( "b", "k") , letters ) [1] 2 11 But also (fasten your seatbelt): > xx <- sample( letters, 1000000, replace = TRUE ) > system.time( match( xx, > letters ) ) utilisateur système écoulé 0.063 0.002 0.065 > system.time( match_( xx, letters ) ) utilisateur système écoulé 0.015 0.000 0.015 Romain Le 15/11/12 00:08, Søren Højsgaard a écrit : > Thanks! What I need to do is this > >> match(c("b","k"), letters) > [1] 2 11 > > Regards > Søren > > > -----Original Message----- > From: rcpp-devel-boun...@lists.r-forge.r-project.org > [mailto:rcpp-devel-boun...@lists.r-forge.r-project.org] On Behalf Of > Romain Francois > Sent: 15. november 2012 00:01 > To: rcpp-devel@lists.r-forge.r-project.org > Subject: Re: [Rcpp-devel] Rcpp "version" of R's match function > > Le 14/11/12 23:48, Søren Højsgaard a écrit : >> Dear all, >> >> I need to call R's match function from a c++ program so I can do >> Rcpp::Function R_match("match"); >> Rcpp::NumericVector out = R_match(x_, table_); >> >> However, I wonder if there is such a "match" function in C++ that I can call >> (to avoid the extra overhead)? >> >> Any ideas would be welcome >> >> Regards >> Søren > > We don't have one in Rcpp. It would not be too hard I guess to use some > algorithms from the STL, e.g. std::find. > http://www.cplusplus.com/reference/algorithm/find/ > > Not easy to say more withou seeing exactly what you are trying to do. > For example, do you need to store "out" or is it enough to work one value at > a time ... > > > Also, R's match returns an integer vector, so you probably want: > > Rcpp::IntegerVector out = R_match(x_, table_); > > otherwise you also pay for making the coercion to numeric on top of > the price you pay for calling back into R. > > -- Romain Francois Professional R Enthusiast +33(0) 6 28 91 30 30 R Graph Gallery: http://gallery.r-enthusiasts.com `- http://bit.ly/SweN1Z : SuperStorm Sandy blog: http://romainfrancois.blog.free.fr |- http://bit.ly/RE6sYH : OOP with Rcpp modules `- http://bit.ly/Thw7IK : Rcpp modules more flexible _______________________________________________ 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
