Why we need C++ in R

• Sometimes R is not efficient, it is user friendly but not process friendly.
• R is extreme inefficient for loop, but C++ is very efficient.
• Problems that require advanced data structures and algorithms that R doesn’t provide. C++ has efficient implementations of many important data structures.

Don’t know C++? Not a problem.

• A working knowledge of C++ is helpful, but not essential.
• Many good tutorials and references are freely available, including:
  • http://www.learncpp.com/
  • http://www.cplusplus.com/
  • Book: Seamless R and C++ Integration with Rcpp, (Dirk Eddelbuettel)

Rcpp

Rcpp: for Seamless R and C++ Integration.

• The Rcpp package has become the most widely used language extension for R.
• As of September 2016, 778 packages on CRAN and a further 76 on BioConductor deploy Rcpp to extend R.

Prerequisites:

• Install the latest version of Rcpp from CRAN with install.packages(“Rcpp”).
• The package includes cppFunction() and sourceCpp(), which makes it very easy to connect C++ to R.
• You’ll also need a working C++ compiler. To get it:
  • On Windows, install Rtools.
  • On Mac, install Xcode from the app store.
  • On Linux, sudo apt-get install r-base-dev or similar.

Warm up example

cppFunction() allows you to write C++ functions in R.

## Getting started with C++
library(Rcpp)
cppFunction('int add(int x, int y) {
  int sum = x + y;
  return sum;
  }')
# add works like a regular R function
add

## function (x, y) 
## .Call(<pointer: 0x10aef6a10>, x, y)

add(3,2)

## [1] 5

Note on C++ code

• In C++, you have to declare types of input and output.
• (Rcpp specific) The scalar equivalents:
  • double: double
  • integer: int
  • character: string
  • logical: bool
• (Rcpp specific) The classes for the most common types of R vectors are:
  • NumericVector
  • IntegerVector
  • CharacterVector
  • LogicalVector.
• You must use an explicit return statement to return a value from a function.
• Every statement is terminated by a “;”.

More examples

1. No inputs, scalar output.
2. Scalar input, scalar output.
3. Vector input, scalar output.
4. Vector input, vector output.
5. Matrix input, vector output.

No inputs, scalar output (1).

• in R

one <- function() 1L

• in C

cppFunction('int oneC() {
              return 1;
            }')

• evaluation

one()

## [1] 1

oneC()

## [1] 1

Scalar input, scalar output (2).

• in R

signR <- function(x) {
  if (x > 0) {
    1
  } else if (x == 0) {
    0
  } else {
    -1
  }
}

• in C

cppFunction('int signC(int x) {
              if (x > 0) {
                return 1;
              } else if (x == 0) {
                return 0;
              } else {
                return -1;
              }
            }')

• evaluation

signR(-10)

## [1] -1

signC(-10)

## [1] -1

Vector input, scalar output (3).

• in R

sumR <- function(x) {
  total <- 0
  for (i in seq_along(x)) {
    total <- total + x[i]
  }
  total
}

• in C

cppFunction('double sumC(NumericVector x) {
              int n = x.size();
              double total = 0;
              for(int i = 0; i < n; ++i) {
                total += x[i];
              }
              return total;
            }')

Vector input, scalar output (3) - evaluation

x <- runif(1e7)
system.time(sum(x))

##    user  system elapsed 
##   0.009   0.000   0.009

system.time(sumC(x))

##    user  system elapsed 
##   0.009   0.000   0.010

system.time(sumR(x))

##    user  system elapsed 
##   0.292   0.001   0.293

Vector input, vector output (4).

• in R

pdistR <- function(x, ys) {
  sqrt((x - ys) ^ 2)
}

• in C

cppFunction('NumericVector pdistC(double x, NumericVector ys) {
              int n = ys.size();
              NumericVector out(n);
               
              for(int i = 0; i < n; ++i) {
                out[i] = sqrt(pow(ys[i] - x, 2.0));
              }
              return out;
            }')

• evaluation

pdistR(3,1:5)

## [1] 2 1 0 1 2

pdistC(3,1:5)

## [1] 2 1 0 1 2

Matrix input, vector output (5).

• in R

rowSums

• in C

cppFunction('NumericVector rowSumsC(NumericMatrix x) {
              int nrow = x.nrow(), ncol = x.ncol();
              NumericVector out(nrow);
            
              for (int i = 0; i < nrow; i++) {
                double total = 0;
                for (int j = 0; j < ncol; j++) {
                  total += x(i, j);
                }
                out[i] = total;
              }
              return out;
            }')

• evaluation

set.seed(1014)
x <- matrix(sample(100), 10)
rowSums(x)

##  [1] 446 514 480 514 352 627 525 586 572 434

rowSumsC(x)

##  [1] 446 514 480 514 352 627 525 586 572 434

More note on C++ code

• Syntax if, while, break are identical, but to skip one iteration you need to use continue instead of next.
• To find the length of the vector, we use the .size() method, which returns an integer. (C++ is objective oriented language, . is similar to slot in S4 in R).
• The for statement has a different syntax: for(init; check; increment).
• This loop is initialized by creating a new variable called i with value 0. Before each iteration we check that i < n, and terminate the loop if it’s not.
• In C++, vector indices start at 0. I’ll say this again because it’s so important: IN C++, VECTOR INDICES START AT 0!
• Use = for assignment, not <-.
• C++ provides operators that modify in-place:
  • total += x[i] is equivalent to total = total + x[i].
  • Similar in-place operators are -=, *=, and /=.

Using sourceCpp

• So far, we’ve used inline C++ with cppFunction().
• For real problems, it’s usually easier to use stand-alone C++ files and then source them into R using sourceCpp().
• Your stand-alone C++ file should have extension .cpp, and needs to start with:

#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]

• To compile the C++ code, use sourceCpp(“path/to/file.cpp”).
• See examples: meanC.cpp

Using sourceCpp

• meanC.cpp

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
double meanC(NumericVector x) {
  int n = x.size();
  double total = 0;

  for(int i = 0; i < n; ++i) {
    total += x[i];
  }
  return total/n;
}

• In R:

sourceCpp('meanC.cpp')
meanC(1:100)

list - mean percentage error

• mpe.cpp

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
double mpe(List mod) {
  if (!mod.inherits("lm")) stop("Input must be a linear model");

  NumericVector resid = as<NumericVector>(mod["residuals"]);
  NumericVector fitted = as<NumericVector>(mod["fitted.values"]);

  int n = resid.size();
  double err = 0;
  for(int i = 0; i < n; ++i) {
    err += resid[i] / (fitted[i] + resid[i]);
  }
  return err / n;
}

list - mean percentage error

sourceCpp('mpe.cpp') 
mod <- lm(mpg ~ wt, data = mtcars)
mpe(mod)

lapply

• lapply1.cpp

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
List lapply1(List input, Function f) {
  int n = input.size();
  List out(n);

  for(int i = 0; i < n; i++) {
    out[i] = f(input[i]);
  }

  return out;
}

sourceCpp('lapply1.cpp')

lapply(1:10,function(x) x^2)

lapply1(1:10,function(x) x^2)

Gibbs sampling example

• http://dirk.eddelbuettel.com/blog/2011/07/14/
  • Consider the same Gibbs sampler for a bivariate distribution: \[f(x,y) = k x^2 \exp( -x y^2 - y^2 + 2y - 4x)\]
  • conditional distributions are: \[f(x|y) = (x^2)* \exp(-x*(4+y*y)),\] which is a Gamma density. \[f(y|x) = \exp(-0.5*2*(x+1)*(y^2 - 2*y/(x+1)),\] which is a Gaussian density.

gibbs_r <- function(N, thin) {
  mat <- matrix(nrow = N, ncol = 2)
  x <- y <- 0

  for (i in 1:N) {
    for (j in 1:thin) {
      x <- rgamma(1, 3, y * y + 4)
      y <- rnorm(1, 1 / (x + 1), 1 / sqrt(2 * (x + 1)))
    }
    mat[i, ] <- c(x, y)
  }
  mat
}

Gibbs sampling evaluation

• gibbs_cpp.cpp

sourceCpp('gibbs_cpp.cpp')

system.time(gibbs_r(100, 10))
system.time(gibbs_cpp(100, 10))

Using Rcpp in a package (minimum steps)

Also refer to https://caleb-huo.github.io/teaching/2020FALL/lectures/week5_RPackage/Rpackage.html for regular steps

1. usethis::create_package() create an new R package
2. devtools::document()
3. usethis::use_rcpp()
4. Copy your R code with documentation in inside R folder
5. put cpp file in src
6. Rcpp::compileAttributes()
7. devtools::document() generate R documentation
8. devtools::install() install the package

example r file

If your package name is GatorCPP

• need to put the following somewhere in the .R file, following the instruction of Rcpp::compileAttributes()
  • ##’ @useDynLib GatorCPP
  • ##’ @importFrom Rcpp sourceCpp

##' mean calling c
##'
##' mean calling c balalal
##' @title mean calling c
##' @param x a vector
##' @return mean of the vector
##' @author Caleb
##' @export
##' @useDynLib GatorCPP
##' @importFrom Rcpp sourceCpp
##' @examples
##' meanRC(1:10)
meanRC <- function(x){
  meanC(x)
}

example c file

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
double meanC(NumericVector x) {
  int n = x.size();
  double total = 0;

  for(int i = 0; i < n; ++i) {
    total += x[i];
  }
  return total / n;
}

Wrap up the package

WD <- "~/Desktop"
setwd(WD)
usethis::create_package("GatorCPP") ## create the package
setwd(file.path(WD, "GatorCPP")) ## get into the package
devtools::document() ## need to do this, otherwise the next step use_rcpp won't work
usethis::use_rcpp() ## intialize rcpp
## put in R code in R and cpp code in src
Rcpp::compileAttributes() ## will generate a RcppExports.R
# devtools::load_all() ## this step seems not necessary this year
devtools::document() ## generate documentation
devtools::install() ## install the package
## use the package
library(GatorCPP)
#?meanRC
meanRC(1:10)

Reference

• http://adv-r.had.co.nz/Rcpp.html
• https://cran.r-project.org/web/packages/Rcpp/vignettes/Rcpp-package.pdf