By Bob Rudis (@hrbrmstr)
Wed 14 May 2014 | tags: rstats, r, rcpp, -- (permalink)

In our book we provide examples of how to convert IPv4 addresses to integer format (and back). We held ourselves to using only basic R functionality since the book had to be at an introductory level. On a fairly modern box, the ip2long function takes (roughly) 0.1s to convert 4,000 IPv4 address to integers (I just happened to have a file with 4K of IPv4 addresses lying around). For raw R code, that’s not too shabby, but we can incorporate some of the Rcpp techinques we showed in previous posts to crank that time down significantly. Don’t worry, this post will be much shorter than the previous one since we’re not building a whole package, just showing you a quick way to smooth out bottlenecks by (briefly) dropping into C++ and taking advantage of the Boost libraries.

For those unfamiliar with C++, Boost is a collection of robust and rigorously developed/peer-reviewed C++ libraries that are very compatible with Rcpp. We’re going to use the ip::address_v4 class to replace the functionality of two of the book’s IPv4 conversion functions (ip2long and long2ip). Put the following code into a file called iputils.cpp

#include <Rcpp.h> 
#include <boost/asio/ip/address_v4.hpp>

using namespace Rcpp;

# we're modeling these sample routine names off of 
# the C inet_ntop and inet_pton functions

#' Convert IP in dotted (char) notation to integer
// [[Rcpp::export]]
unsigned long rinet_pton (CharacterVector ip) { 
  return(boost::asio::ip::address_v4::from_string(ip[0]).to_ulong());
}

#' Convert an IP in integer foramt to dotted (char) notation
// [[Rcpp::export]]
CharacterVector rinet_ntop (unsigned long addr) {
  return(boost::asio::ip::address_v4(addr).to_string());
}

Now, either in another R file or in the R console, do the following:

# these make the Rcpp magic happen
library(Rcpp)
library(inline)

# this compiles our code and makes the
# two functions available to our session
sourceCpp("iputils.cpp")

# test convert an IPv4 string to integer
rinet_pton("10.0.0.0")
[1] 167772160

# test conversion back
rinet_ntop(167772160)
[1] "10.0.0.0"

The iputils.cpp file will need to be in the working directory for that bit of code to work (which is why packages are usually a better route). The call to sourceCpp does most of the heavy lifting for us (with some help from the [[Rcpp::export]] hint in the code which tells sourceCpp to do quite a bit of work for you under the covers). The sourceCpp function takes care of ensuring that proper memory allocation & garbage collection protection is performed and also handles all return value wrapping (conversion). As you can see in the code snippet, the Boost asio library provides two methods that make it super-easy to use native versions of the IP address conversion functions and also highlights the object compatibilty between Rcpp and C++.

Performing the same 4,000 IPv4 conversion exercise now takes 0.01s (remember, the pure R version took 0.1s). For a few thousand IP addresses, the difference is negligible, but if you’re working with millions or billions of IP addresses, this speedup can help dramatically and keep your processing in R vs potentially splitting up you workflow between R and, say, Python.

Exercise for the reader!

Try modifying the functions to handle both IPv4 and IPv6 addresses. You can start by writing two similar functions just to get your feet wet and then work on the logic necessary to combine the four into two. If you do the exercise, drop us a note here, on Twitter or over at github and we’ll feature you in an upcoming post and podcast!

If the world of Rcpp seems intriguing, you’d do well to pick up a copy of Dirk Eddelbuttel’s Seamless R & C++ Integration with Rcpp. He goes into great detail with tons of examples that should make it much easier take advantage of the functionality that Rcpp provides.