[Rd] faster base::sequence

[Romain Francois]

Hello,

Based on yesterday's R-help thread (help: program efficiency), and 
following Bill's suggestions, it appeared that sequence:

 > sequence
function (nvec)
unlist(lapply(nvec, seq_len))
<environment: namespace:base>

could benefit from being written in C to avoid unnecessary memory 
allocations.

I made this version using inline:

require( inline )
sequence_c <- local( {
     fx <- cfunction( signature( x = "integer"), '
         int n = length(x) ;
         int* px = INTEGER(x) ;
         int x_i, s = 0 ;
         /* error checking */
         for( int i=0; i<n; i++){
             x_i = px[i] ;
             /* this includes the check for NA */
             if( x_i <= 0 ) error( "needs non negative integer" ) ;
             s += x_i ;
         }

         SEXP res = PROTECT( allocVector( INTSXP, s ) ) ;
         int * p_res = INTEGER(res) ;
         for( int i=0; i<n; i++){
             x_i = px[i] ;
             for( int j=0; j<x_i; j++, p_res++)
                 *p_res = j+1 ;
         }
         UNPROTECT(1) ;
         return res ;
     ' )
     function( nvec ){
         fx( as.integer(nvec) )
     }
})


And here are some timings:

 > x <- 1:10000
 > system.time( a <- sequence(x ) )
utilisateur     système      écoulé
       0.191       0.108       0.298
 > system.time( b <- sequence_c(x ) )
utilisateur     système      écoulé
       0.060       0.063       0.122
 > identical( a, b )
[1] TRUE



 > system.time( for( i in 1:10000) sequence(1:10) )
utilisateur     système      écoulé
       0.119       0.000       0.119
 >
 > system.time( for( i in 1:10000) sequence_c(1:10) )
utilisateur     système      écoulé
       0.019       0.000       0.019


I would write a proper patch if someone from R-core is willing to push it.

Romain

-- 
Romain Francois
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http://romainfrancois.blog.free.fr
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