[Rcpp-devel] Using pointers with Numeric Vectors

[Alon Honig]

My current RCPP program is only 4 times faster than its R byte code
compiled equivalent. I am trying to speed up my code and from what I
understand using pointers prevents an external function from copying the
entire object when executing its methods.   I am having difficulty finding
salient examples that use points and NumericVectors. Please tell me how in
this same code I could get the functions "get_sum" and "get_var" to point
to the NumericVector object "v" rather than copy the whole thing.

library(inline)
library(Rcpp)
 a=1:1000000
Rcpp.var = cxxfunction(signature(input="numeric"), plugin="Rcpp",
body="

 NumericVector v = input;

 int n = v.size();

 double v_mean =  get_sum(v,n)/n;

 double v_var = get_var(v,v_mean,n);

 return wrap(v_var);

",includes="

 double get_var(NumericVector v,double m,int l)

 {double a = 0;

     for (int i = 0; i <l;i++)

{a += (v[i]-m)*(v[i]-m);}

return(a/l);

 }


 double get_sum(NumericVector v,int l)

 { double s = 0;

   for (int i = 0; i <l;i++)

{s += v[i];}

return(s);

 }

 ")

 b=system.time(for (i in 1:100)Rcpp.var (a))
 c= system.time(for (i in 1:100)var (a))



Thank you Alon.

P.S. I am aware that the "get_var" function provides the population
variance and not the sample variance.
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