Count comparison results without additional memory allocation

inplace_comparison.R

library(Rcpp)

`%count<%` <- cppFunction('
size_t count_less(NumericVector x, NumericVector y) {

  const size_t nx = x.size();
  const size_t ny = y.size();

  if (nx > 1 & ny > 1) stop("Only one parameter can be a vector!");

  size_t count = 0;

  if (nx == 1) {
    double c = x[0];
    for (int i = 0; i < ny; i++) count += c < y[i];
  } else {
    double c = y[0];
    for (int i = 0; i < nx; i++) count += x[i] < c;
  }

  return count;
}
')

set.seed(42)

N <- 100000000
v <- runif(N, 0, 10000)

system.time(sum(v < 5000))
system.time(v %count<% 5000)

Results:

> system.time(sum(v < 5000))
   user  system elapsed 
  0.488   0.115   0.605 
> system.time(v %count<% 5000)
   user  system elapsed 
  0.065   0.000   0.065