[Rcpp-devel] Struggling with Rcpp sugar

[Romain Francois]

Hi,

While there, consider this version based on sapply:


class Distance {
public:
     typedef double result_type ;
     Distance( double x_ ) : x(x_){}

     inline double operator()(double y) const { return pow( (y-x), 2.0 ) ; }

private:
     double x;
} ;

// [[Rcpp::export]]
NumericVector pdist5(double x, NumericVector ys) {
   return sapply( ys, Distance(x) ) ;
}


which here gives me quite good performance:

Unit: microseconds
              expr     min       lq   median       uq     max
1 pdist1(0.5, ys)  24.542  26.4825  27.8405  28.2305 326.597
2 pdist2(0.5, ys)  30.628  31.0030  31.1695  31.8715 608.207
3 pdist3(0.5, ys) 262.371 262.6280 262.9140 263.7840 563.796
4 pdist4(0.5, ys) 264.667 265.0150 265.2025 266.1770 580.343
5 pdist5(0.5, ys)  15.715  16.1375  16.3385  17.2195 318.412

Romain


Le 17/11/12 14:42, Hadley Wickham a écrit :
> Hi all,
>
> I've included what seems to be a simple application of Rcpp sugar
> below, but I'm getting some very strange results.  Any help would be
> much appreciate!
>
> Thanks,
>
> Hadley
>
> library(Rcpp)
> library(microbenchmark)
>
> # Compute distance between single point and vector of points
> pdist1 <- function(x, ys) {
>    (x - ys) ^ 2
> }
>
> cppFunction('
>    NumericVector pdist2(double x, NumericVector ys) {
>      int n = ys.size();
>      NumericVector out(n);
>
>      for(int i = 0; i < n; ++i) {
>        out[i] = pow(ys[i] - x, 2);
>      }
>      return out;
>    }
> ')
>
> ys <- runif(1e4)
> all.equal(pdist1(0.5, ys), pdist2(0.5, ys))
>
> library(microbenchmark)
> microbenchmark(
>    pdist1(0.5, ys),
>    pdist2(0.5, ys)
> )
> # C++ version about twice as fast, presumably because it avoids a
> # complete vector allocation.
>
>
> # Sugar version:
> cppFunction('
>    NumericVector pdist3(double x, NumericVector ys) {
>      return pow((x - ys), 2);
>    }
> ')
> all.equal(pdist1(0.5, ys), pdist3(0.5, ys))
>
> microbenchmark(
>    pdist1(0.5, ys),
>    pdist2(0.5, ys),
>    pdist3(0.5, ys)
> )
> # 10-fold slower??  Maybe it's because I'm using a double instead of
> # a numeric vector?
>
> cppFunction('
>    NumericVector pdist4(NumericVector x, NumericVector ys) {
>      return pow((x - ys), 2);
>    }
> ')
> all.equal(pdist1(0.5, ys), pdist4(0.5, ys))
>
> # Is this a bug in sugar? Should recycle to length of longest vector.
> # Let's try flipping the order of operations:
>
> cppFunction('
>    NumericVector pdist5(NumericVector x, NumericVector ys) {
>      return pow((ys - x), 2);
>    }
> ')
> all.equal(pdist1(0.5, ys), pdist5(0.5, ys))
> # Where are the missing values coming from??
>
>


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Romain Francois
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