R Language Rcpp Attributes


Example

Rcpp Attributes makes the process of working with R and C++ straightforward. The form of attributes take:

// [[Rcpp::attribute]]

The use of attributes is typically associated with:

// [[Rcpp::export]]

that is placed directly above a declared function header when reading in a C++ file via sourceCpp().

Below is an example of an external C++ file that uses attributes.

// Add code below into C++ file Rcpp_example.cpp

#include <Rcpp.h>
using namespace Rcpp;

// Place the export tag right above function declaration.
// [[Rcpp::export]]
double muRcpp(NumericVector x){

    int n = x.size(); // Size of vector
    double sum = 0; // Sum value

    // For loop, note cpp index shift to 0
    for(int i = 0; i < n; i++){
        // Shorthand for sum = sum + x[i]
        sum += x[i];
    }

    return sum/n; // Obtain and return the Mean
}

// Place dependent functions above call or
// declare the function definition with:
double muRcpp(NumericVector x);

// [[Rcpp::export]]
double varRcpp(NumericVector x, bool bias = true){

    // Calculate the mean using C++ function
    double mean = muRcpp(x);
    double sum = 0;

    int n = x.size();

    for(int i = 0; i < n; i++){
        sum += pow(x[i] - mean, 2.0); // Square
    }

    return sum/(n-bias); // Return variance
}

To use this external C++ file within R, we do the following:

require(Rcpp)

# Compile File
sourceCpp("path/to/file/Rcpp_example.cpp")

# Make some sample data
x = 1:5

all.equal(muRcpp(x), mean(x))
## TRUE

all.equal(varRcpp(x), var(x))
## TRUE