rev with Rcpp and Rcpp11

README.md

Code from this stack overflow thread

results_Rcpp.txt

romain@naxos ~/git/dplyr $ RcppScript /tmp/rev.cpp

> library(microbenchmark)

> x = rep(c(1, 2, 3, 4), each = 1e+05)

> rev2 = function(x) {
+     nx = length(x)
+     y = vector("numeric", nx)
+     for (i in nx) y[i] = x[nx - i + 1]
+     y
+ }

> rev3 = function(x) {
+     x[length(x):1]
+ }

> rev4 = function(x) {
+     .subset(x, length(x):1)
+ }

> microbenchmark(rev(x), rev2(x), rev3(x), rev4(x),
+     Rcpprev2s(x), RcppRev3(x), CppRev(x))
Unit: microseconds
         expr      min        lq    median       uq      max neval
       rev(x) 2890.765 3166.3695 4067.1065 4367.688 31645.36   100
      rev2(x)  565.817  635.6205  978.3925 1974.910 29135.17   100
      rev3(x) 2874.174 3141.3745 4033.4480 4358.533 33939.75   100
      rev4(x) 2907.989 3209.4560 4143.6140 4343.261 31891.12   100
 Rcpprev2s(x)  540.032  667.2825  708.8470 1822.555 27664.31   100
  RcppRev3(x)  602.216  616.5715  632.3385 1242.201 30543.52   100
    CppRev(x) 1495.572 1722.6400 1835.0080 2833.515 30584.69   100

results_Rcpp11.txt

romain@naxos ~/git/dplyr $ Rcpp11Script /tmp/rev.cpp

> library(microbenchmark)

> x = rep(c(1, 2, 3, 4), each = 1e+05)

> rev2 = function(x) {
+     nx = length(x)
+     y = vector("numeric", nx)
+     for (i in nx) y[i] = x[nx - i + 1]
+     y
+ }

> rev3 = function(x) {
+     x[length(x):1]
+ }

> rev4 = function(x) {
+     .subset(x, length(x):1)
+ }

> microbenchmark(rev(x), rev2(x), rev3(x), rev4(x),
+     Rcpprev2s(x), RcppRev3(x), CppRev(x))
Unit: microseconds
         expr      min        lq    median       uq      max neval
       rev(x) 2963.545 3742.1755 4081.4955 4771.596 13583.02   100
      rev2(x)  493.937  675.3870 1444.4155 1798.793 10781.08   100
      rev3(x) 2901.671 3799.8440 4062.8965 4609.037 13660.12   100
      rev4(x) 2888.391 3177.7595 3951.7435 4371.901 21455.32   100
 Rcpprev2s(x)  454.979  550.4395  580.9995 1324.806 10015.25   100
  RcppRev3(x)  407.545  465.3735  489.7015 1006.441 10448.26   100
    CppRev(x) 1335.404 1621.1900 1889.8265 2336.011 11690.65   100

rev.cpp

#include <Rcpp.h>

using namespace Rcpp;

// [[Rcpp::export]]
IntegerVector Rcpprev2s(IntegerVector x){
    int nx = x.size();
    IntegerVector y(nx);
    for(int i = 0; i<nx; i++){
        y[i] = x[nx-i-1];
    }
    return(y);
}

// [[Rcpp::export]]
IntegerVector RcppRev3(IntegerVector x) { 
  return rev(x);
} 

// [[Rcpp::export]]
std::vector<int> CppRev(std::vector<int> & x) { 
  std::reverse(x.begin(), x.end());
  return x;
} 

/*** R

library(microbenchmark)
x = rep(c(1L,2L,3L,4L), each=1e5)
rev2 = function(x){
    nx = length(x)
    y = vector("numeric", nx)
    for(i in nx) y[i] = x[nx-i+1]
    y
}
rev3 = function(x){
    x[length(x):1]
}
rev4 = function(x){
    .subset(x, length(x):1)
}
microbenchmark(rev(x), rev2(x), rev3(x), rev4(x), 
               Rcpprev2s(x), RcppRev3(x), CppRev(x))
*/