Fastest way to cross-tabulate two massive logical vectors in R

Question

For two logical vectors, x and y, of length > 1E8, what is the fastest way to calculate the 2x2 cross tabulations?

I suspect the answer is to w

Answer 1

Here's an answer with Rcpp, tabulating only those entries that are not both 0. I suspect there must be several ways to improve this, as this is unusually slow; it's also my first attempt with Rcpp, so there may be some obvious inefficiencies associated with moving the data around. I wrote an example that is purposefully plain vanilla, which should let others demonstrate how this can be improved.

library(Rcpp)
library(inline)
doCrossTab <- cxxfunction(signature(x="integer", y = "integer"), body='
  Rcpp::IntegerVector Vx(x);
  Rcpp::IntegerVector Vy(y);
  Rcpp::IntegerVector V(3);
  for(int i = 0; i < Vx.length(); i++) {
    if( (Vx(i) == 1) & ( Vy(i) == 1) ){ V[0]++; } 
    else if( (Vx(i) == 1) & ( Vy(i) == 0) ){ V[1]++; } 
    else if( (Vx(i) == 0) & ( Vy(i) == 1) ){ V[2]++; } 
 }
  return( wrap(V));
  ', plugin="Rcpp")

Timing results for N = 3E8:

   user  system elapsed 
 10.930   1.620  12.586 

This takes more than 6X as long as func_find01B in my 2nd answer.