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

Romain Francois Wed, 14 Nov 2012 15:50:01 -0800

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 notan 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 thekey of the hash map. (strings are cached in R, see the CHARSXP cache inR 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.



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

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