Rcpp.Rmd

---
title: "Rcpp"
date: 2018-02-04T12:03:14+01:00
draft: false
image: "Rcpp.jpg"
categories: ["R"]
tags: ["R", "C++"]
---

## 1. What is Rcpp and why would you use it?

* `Rcpp` is a R library which let's you embed C++ code inside your R program.

* Useful when you have a bottleneck in your code which makes the execution last forever. In that case you can rewrite in into super-fast C++.

Or you can switch to Python. Or data.table. Or write it in [C](http://tomis9.com/cinr).

## 2. A "Hello World" example

Example taken from Avanced R by Hadley Wickam [(link)](http://adv-r.had.co.nz/Rcpp.html).

```{r}
library(Rcpp)
```

So far so good. This is how we create a C++ function `add`:

```{r}
cppFunction('int add(int x, int y, int z) {
  int sum = x + y + z;
  return sum;
}')
```

and invoke it:
```{r}
add(1, 2, 3)
```

You can check that this actually is a C++ function simply by:
```{r}
add
```

## 3. Using R's numeric vector as input and output

A slighlty more complicated function, which calculates a distance between a value and a vector of values:

```{r}
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;
}')
```

And let's run this function:

```{r}
pdistC(10, c(11, -12, 30))
```