030_basic_usage.Rmd

# Basic usage

You can use your Rcpp function in 3 steps.

1. Writing Rcpp source code
2. Compiling the code
3. Executing the function

## Writing your Rcpp code

The below code defines a function `rcpp_sum()` that calculates the sum of a vector. Save this content as a file named "sum.cpp".

**sum.cpp**

```c++
//sum.cpp
#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
double rcpp_sum(NumericVector v){
    double sum = 0;
    for(int i=0; i<v.length(); ++i){
        sum += v[i];
    }
    return(sum);
}
```

### Format for defining a function in Rcpp.

The following code shows the basic format for defining a Rcpp function.

```cpp
#include<Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
RETURN_TYPE FUNCTION_NAME(ARGUMENT_TYPE ARGUMENT){

    //do something

    return RETURN_VALUE;
}
```

* `#include<Rcpp.h>` : This sentence enables you to use classes and functions defined by the Rcpp package.

* `// [[Rcpp::export]]`：The function defined just below this sentence will be accessible from R. You need to attach this sentence to every function you want to use from R.

* `using namespace Rcpp;` : This sentence is optional. But if you did not write this sentence, you have to add the prefix `Rcpp::` to specify classes and functions defined by Rcpp. (For example, `Rcpp::NumericVector`)

* `RETURN_TYPE FUNCTION_NAME(ARGUMENT_TYPE ARGUMENT){}`：You need to specify data types of functions and arguments.

* `return RETURN_VALUE;`：`return` statement is mandatory if your function would return a value. However, if your function do not return a value (i.e. `RETURN_TYPE` is `void`), the return statement can be omitted.

 

## Compiling the code

The function `Rcpp::sourceCpp()` will compile your source code and load the defined function into R.

```R
library(Rcpp)
sourceCpp('sum.cpp')
```

## Executing the function

You can use your Rcpp functions as usual R functions.

```r
> rcpp_sum(1:10)
[1] 55

> sum(1:10)
[1] 55
```