Rcpp Attributes

Greater Boston useR Group

Origins of S

Languages in R and CRAN

Native Code: Problems and Opportunities

Can We Have the Best of Both Worlds?

On May 5th, 1976, a group of users from the statistics research group at Bell Labs discussed creating a new system for statistical computing. Some of the requirements:

• Interoperability with their decade plus investment in Fortran code and libraries
• Readily understood and used by statisticians
• Interactive execution for iterative data exploration
• Extensibilty for research into statistical methods

Native Code

• Access to proven libraries and algorithms
• Extremely high performance

Interpeted Code

• Accessible high-level language
• Interactive workflow for data analysis
• Supports rapid prototyping, research, and experimentation

• When we directly use native code exposed by base R and packages our code runs fast
• If only that were enough!
  • We write tons of custom R code, often it's too slow
  • Many C/C++ libraries are available, but aren't conveniently wrapped by an R package
• What can be done? Should R users be writing more code in C/C++?
• Perhaps, but…

 #include <R.h>
 #include <Rinternals.h>

 SEXP reverse(SEXP x) {
   SEXP res;
   int i, r, P=0;
   PROTECT(res = Rf_allocVector(
                  REALSXP, Rf_length(x)));
   for(i=Rf_length(x), r=0; i>0; i--, r++)
     REAL(res)[r] = REAL(x)[i-1];
   UNPROTECT(1);
   return res;
}

R users need a system that:

• Allows us to write C++ code that looks and acts like R code
• Eliminates memory management concerns
• Requires no build system
• Has a fully interactive execution model

The Rcpp Package

Why “Attributes”?

Interactive C++ with sourceCpp

Some Examples

• Created by Dirk Eddelbuettel & Romain François (with contributions from Doug Bates, John Chambers, and JJ Allaire)
• Includes broad support for vectors, matrices, functions, environments, and more.
• High level “sugar” API provides vectorized C++ equivalents for many R functions
• Full support for STL algorithms
• Currently used by over 120 packages on CRAN
• More at: http://www.rcpp.org

// Matrix of 4 rows & 5 columns
NumericMatrix xx(4, 5);

// Fill with value
int xsize = xx.nrow() * xx.ncol();
for (int i = 0; i < xsize; i++) {
  xx[i] = 7;
}

// Assign to single element
xx(0,1) = 4;

NumericVector x = NumericVector::create(
                    -2.0, -1.0, 1.0, 2.0);

NumericVector xAbs = abs( x );
NumericVector xCeiling = ceiling( x );
NumericVector xFloor = floor( x );

bool b = all( x < 3.0 ).is_true();

// lookup rnorm function
Environment stats("package:stats");
Function rnorm = stats["rnorm"];

// call rnorm function
rnorm(100, 
      _["mean"] = 10.2, 
      _["sd"] = 3.2);

// raise an R error
if (failed)
  stop("an unexpected error occurred!");

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
double piCpp(int N) {
  NumericVector x = runif(N);
  NumericVector y = runif(N);
  NumericVector d = sqrt(x*x + y*y);
  return 4.0 * sum(d < 1.0) / N;
}

> sourceCpp("pi.cpp")
> piCpp(1000)
[1] 3.048

Attributes are annotations added to source code to express intent or automatically generate code (i.e. compiler extensions). They are part of the C++11 standard.

[[omp::parallel]]
void someFunction() {}

In R we use compilers that don't (yet) support C++11, so attributes are added as specially formatted comments:

// [[Rcpp::export]]

• sourceCpp is a function that sources C++ files just as the source function sources R files
• Functions within the source file decorated with the [[Rcpp::export]] attribute are made available to R
• Look Ma!
  • No build system
  • No memory management
  • No boilerplate R <-> C++ data marshaling code
  • Mix C++ and R in the same source file
  • Fully interactive workflow

Source:

http://gallery.rcpp.org/articles/run_sum-benchmark/

set.seed(42)
y <- rnorm(100000)

benchmark(runSumCpp(y, 4500), 
          runSumR(y, 4500),
          order = "relative")[,1:4]

                     reps    time     rel
1 runSumCpp(y, 4500)  100   0.082    1.00
2   runSumR(y, 4500)  100  33.717  411.18

Source:

http://gallery.rcpp.org/articles/transforming-a-matrix/

sourceCpp("matrix.cpp")
m <- matrix(c(1,2,3, 11,12,13), 
            nrow = 2, ncol=3)
matrixSqrt(m)

         [,1]     [,2]     [,3]
[1,] 1.000000 1.732051 3.464102
[2,] 1.414214 3.316625 3.605551

Source:

http://gallery.rcpp.org/articles/creating-xts-from-c++/

library(xts)
sourceCpp("xts.cpp")
foo <- createXts(1, 4) 
foo

           [,1]
1970-01-02    1
1970-01-03    2
1970-01-04    3
1970-01-05    4

• You can use the [[Rcpp::depends]] attribute to declare a dependency on an R package.

• The inclusion of the [[Rcpp::depends]] attribute causes sourceCpp to configure the build environment to compile and link against the package

• Dependencies are discovered both by scanning for package include directories and by invoking inline plugins if they are available for a package.

http://gallery.rcpp.org/articles/fast-linear-model-with-armadillo/

Source

http://gallery.rcpp.org/articles/using-bigmemory-with-rcpp/

• [[Rcpp::plugins]] attribute loads inline plugins
• Capable of custom build configuration and/or importing arbitrary external libraries
• Essentially a shareable “recipe” for build configuration
• Examples:
  • Enabling C++11 compiler features
  • Adding OpenMP switches to compiler and linker
  • Dynamically probing the system for a library

http://gallery.rcpp.org/articles/simple-lambda-func-c++11/

Code built with sourceCpp can be seamlessly moved into packages for sharing with a wider audience via the compileAttributes() function:

Rcpp::compileAttributes()

Scans all C++ files in the src directory of the package looking for [[Rcpp::export]] attributes and writes the following files with R and C++ bindings:

• R/RcppExports.R
• src/RcppExports.cpp

• Source file identical to what we'd use with sourceCpp

• Need to call compileAttributes right before building the package

• Tools that automatically call compileAttributes:

  • devtools
  • RStudio

• Also supports transposing roxygen2 comments

Two talks worth watching to get a flavor for what's happening in modern C++:

• Not Your Father's C++ Herb Sutter (Chair, ISO C++ Standards Committee) http://goo.gl/N9YbS

• Clang: Defending C++ from Murphy's Million Monkey's
  Chandler Carruth (Google)
  http://goo.gl/ixNGP

• Rcpp Website: http://www.rcpp.org/
• Rcpp Book: http://www.rcpp.org/book/
• Rcpp for Beginners: http://goo.gl/If5Oz
• Rcpp Attributes: http://goo.gl/PIRN9

http://gallery.rcpp.org

Contains a wealth of articles (all with full source code) demonstrating the use of Rcpp.

Thank You!

Slides for this talk are at:

http://rpubs.com/jjallaire/rcpp-attributes-boston-useR