# Deepen your R experience with Rcpp

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by Joseph Rickert

It is

very likely that even a very casual observer what is happening in the world of

R these past few months would have come across some mention of Rcpp, the R

package that greatly facilitates R and C++ integration. Rcpp is hot! Over 130 R

packages now depend on Rcpp and it is likely to keep growing. The following plot

built using code Tal Galili posted to examine the log files from RStudio’s CRAN

mirror shows the number of downloads of Rcpp around the time R 3.0.0 was

released.

The intense activity over the first three days and relatively slow tapering

off is noteworthy, especially for what might be considered an “advanced”

package that takes some expertise to use. So, it is not surprising that here have been quite a few conference

presentations this year about some aspect or another of Rcpp, and Dirk

Eddelbuettel, Romain Francois and other experts seem to be hard pressed to keep

up with the demand for training. I had the opportunity

last week at the useR 2013 conference in Spain to attend the tutorial on Rcpp given

by Hadley Wickham and Romain. And, at roughly the same time that on the other side of the world, Dirk gave a similar tutorial to the Sydney Users of R Forum (SURF).

Romain and Hadley's tutorial was geared to people with

some R skills, but not necessarily any C++ experience. It was very well done;

exceptionally well done. Hadley and Romain are two experienced trainers who are so

good at what they do that they can quickly get a diverse group to a comfortable place where they can begin dealing with the material. The class

was positive, challenging and very motivating.

While I was sitting

there, probably hallucinating in the Albacete heat, I had the thought the Rcpp

phenomenon probably says something about the future of R. No, I don’t mean that

Rcpp or C++ is the future. It occurred to me though that I was seening the results of how a small but committed group of R experts cooperated to deal with a potential threat to R’s continued success. To my way of thinking, this kind of

sustained, creative effort and the willingness of R developers to

connect R to the rest of the computational world indicates that R is likely to

be the platform of choice for statistical computing some time to come.

So what is the threat? It is not big not big news that R can be slow. The following code from Hadley and Romain's tutorial

shows a straightforward C++ function to compute a simple weighted mean, a naïve

implementation of this same function in R, and the built in weighted.mean()

function from the base stats package.

# Script to compare C++ and R library(Rcpp) # C++ Function in Rcpp wrapper cppFunction(' double wmean(NumericVector x, NumericVector w) { int n = x.size(); double total = 0, total_w = 0; for(int i = 0; i < n; ++i) { total += x[i] * w[i]; total_w += w[i]; } return total / total_w; } ') # Naive R function wmeanR <- function(x, w) { total <- 0 total_w <- 0 for (i in seq_along(x)) { total <- total + x[i] * w[i] total_w <- total_w + w[i] } total / total_w } x <- rnorm(100000000) w <- rnorm(100000000) system.time(wmean(x,w)) system.time(wmeanR(x,w)) # The proper way to compute a simple weighted mean in R # using a built in function from the base stats package system.time(weighted.mean(x,w))

On my laptop, the naïve R function took

229.47 seconds to run, the built in R function ran in 4.52 seconds, and the C++

function took only 0.28 seconds to execute. Yes, C++ is a lot faster. But, this

is a somewhat contrived example and it is not unreasonable to expect that a

statistician could spend her entire career running weighted.mean() on vectors

of reasonable size and never even consider that R might be slower that

something else. (For vectors of length 1,000,000, weighted.mean() took 0.06

seconds to run on my PC). Speed of execution needs to be evaluated in context. I can't imagine any statistician interuppting the flow of an R session to save a few seconds on a once-in-a-while calculation. However, it is nice to know that there is a reasonable way to proceed in R if the calculation needs to be done 100,000 times.

My three main take-aways from my tutorial were;

- For garden variety

programming (no objects or classes) C++ is not only accessible, but might also

be fun. - Rcpp along with RTools does

an incredible amount of “heavy lifting”, hiding the details of working with a

compiled language from the R user and providing a big league environment for

writing high performance, R based code. - Even if you have some

considerable experience with R, it may turn out that R is even richer than you

thought.

It was a delightful

surprise to realize that gaining some experience C++ might enhance one’s

motivation to learn even more about R. Yes, it is important to know that one can

attempt serious work in R that might have critical execution time constraints and that there are tools such as Rcpp available to help one power through

bottlenecks. However, the richer experience of the tutorial was to consider the

rewards of learning more about the structure of R and all R it has to offer.

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