Chapter 23 Using R functions

In order to use R functions in Rcpp, you can use Function and Environment.

23.1 Function

Using the Function class, you can call R functions from Rcpp. The argument given to the R function is determined based on position and name.

Use Named() or _[] to pass a value to an argument by specifying argument name. Name() can be used in two ways: Named("argument_name", value) or Named("argument_name") = value.

The code example below shows an example of calling the R function rnorm (n, mean, sd)from the function defined in Rcpp. However, when calling a package function using Function class, you have to add the package environment to the search path using the library() function in R in advance.

// [[Rcpp::export]]
NumericVector my_fun(){
    // calling rnorm()
    Function f("rnorm");   

    // Next code is interpreted as rnorm(n=5, mean=10, sd=2)
    return f(5, Named("sd")=2, _["mean"]=10);

Execution example

> my_fun()
[1]  8.014863 10.459980  7.741581  9.000762 11.465920

In the above example, the return type of R function called from Rcpp is assumed NumericVector. However, as in the example below, the return type of an R function called from Rcpp function is sometimes undefined. In such a case it would be better to assign the return value of the function into RObject or List element.

The code below shows an example of defining simplified R function lapply() with Rcpp.

// [[Rcpp::export]]
List rcpp_lapply(List input, Function f) {
    // Applies the Function f to each element of the List input and returns the result as List

    // Number of elements in the List input
    R_xlen_t n = input.length();

    // Creating a List for output
    List out(n);

    // Applying f() to each element of "input" and store it to "out".
    // The type of the return value of f() is unknown, but it can be assigned to the List element.
    for(R_xlen_t i = 0; i < n; ++i) {
        out[i] = f(input[i]);
    return out;

23.2 Environment

By using Environment class, you can retrieve objects (variables and functions) from packages and other environments.

The code below shows an example of calling Matrix() function in the Matrix package. When calling a package function in this way, it is not necessary to attach the package using library() function.

// [[Rcpp::export]]
S4 rcpp_package_function(NumericMatrix m){

    // Obtaining namespace of Matrix package
    Environment pkg = Environment::namespace_env("Matrix");

    // Picking up Matrix() function from Matrix package
    Function f = pkg["Matrix"];

    // Executing Matrix( m, sparse = TRIE )
    return f( m, Named("sparse", true));

Execution result

> m <- matrix(c(1,0,0,2), nrow = 2, ncol = 2)
> rcpp_package_function(m)
2 x 2 sparse Matrix of class "dsCMatrix"

[1,] 1 .
[2,] . 2