Re: [Rcpp-devel] Rcpp "version" of R's match function

Romain Francois Wed, 14 Nov 2012 23:55:03 -0800

Definitely.

I'm doing some polishing of the code and making it more general so thatit works with arbitrary sugar expressions too.


But yes. it will go in at some point today.

Romain

Le 15/11/12 08:36, Søren Højsgaard a écrit :

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
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--
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

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Re: [Rcpp-devel] Rcpp "version" of R's match function

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