Using mapply in Rcpp11

May 22, 2014
By

(This article was first published on R Enthusiast and R/C++ hero, and kindly contributed to R-bloggers)

mapply is a well known (or perhaps not) function in R. mapply applies a function to extracts from one or more vectors. For example in R:

> mapply( function(x,y, z) x + y + z, 1:4, 4:1, 2)
# [1] 7 7 7 7

Notice how the last argument is recycled as we would expect in R.

I've recently updated mapply in Rcpp11 to be as flexible as possible. It handles a variable number of parameters (which the Rcpp version did not do), checks at compile time that it makes sense to apply the function to elements of the input vectors. It also handles primitives instead of vectors, doing recycling.

Here is an example using both vectors and a primitive.

#include <Rcpp.h>
using namespace Rcpp ;

// [[Rcpp::export]]
NumericVector mapply_example(NumericVector x, NumericVector y, double z){

    auto fun = [](double a, double b, double c){ return a + b + c ;} ;
    return mapply( fun, x, y, z ) ;

}

In this example, the applied function is a lambda, nicely captured with auto because that's awesome, but we can use a named function previously declared.

double fun(double a, double b, double c){  
    return a + b + c ;
}

// [[Rcpp::export]]
NumericVector mapply_example(NumericVector x, NumericVector y, double z){  
    return mapply( fun, x, y, z ) ;
}

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