Sunday, July 31, 2011

Rcpp: embedding C++ in R scripts

Rcpp is an R package to aid in making R extensions written in C++. It also includes a side-project for embedding R code in a C++ program (including being able to use all the R packages), but most amazing of all is the integration with R's inline package. Let me show you a minimal (but complete) script:
src='Rcpp::List output(1);output[0]="Hello World";return output;'
Yes, it is the classic Hello World script, the output looks like:
[1] "Hello World"
What do I call this amazing? The contents of src is the body of a C++ function. The cxxfunction takes your C++ code, sends it to your C++ compiler, compiles it with the necessary headers, and finally embeds it in your R session as an R extension. It just works, there is no catch (*).

(I could have used a character object instead of a list. The Rcpp::List is just like an R list, meaning each element can be of a different type. If you've always wanted something like this in C++, also take a look at boost::any.)

The cxxfunction() call takes 1.9s to run on my machine, so the overhead to start-up the compiler and run the compilation process is not too bad.

Okay, not a motivating example, but the Rcpp project is not just technically amazing, it is also well documented. You can find some more interesting examples in the documentation, and if this has got your interest you will enjoy this video (1.5hrs) of a talk the Rcpp main authors did at Google.

By the way, if anyone knows of a PHP project for embedding C++ like this, please let me know!

*: You need to install the Rcpp and inline packages, and you need R version 2.12.0 or later. If you are using Ubuntu and your default version of R is too old, there are good instructions here on getting the latest R version as a deb package.

No comments: