160_s3_s4.Rmd

# S3・S4 class

## S3 class

The S3 class is actually a list whose attribute `class` has its own value. For that, see the section `List` and  section Attributes for creating S3 objects and accessing its elements.

In the code example below, as an example of handling objects of S3, we show the function that receives the return value of function `lm()` and computes RMSE (Root Mean Square Error) as an index of the prediction accuracy of the model in the learning data.

```cpp
//Receiving lm() model object and calculate RMSE
// [[Rcpp::export]]
double rcpp_rmse(List lm_model) {
    // Since S3 is a list, data type of the argument is specified as List.

    // If the object given to this function is not an lm() model object,
    // it outputs an error message and stops execution.
    if (! lm_model.inherits("lm")) stop("Input must be a lm() model object.");

    // Extracting residuals (i.e. actual - prediction) from the S3 object
    NumericVector resid  = lm_model["residuals"];

    // Number of elements of the residual vector
    R_xlen_t n = resid.length();

    // The sum of squares of the residual vector
    double rmse(0.0);
    for(double& r : resid){
        rmse += r*r;
    }

    // Divide the residual sum of squares by the number of elements and take the square root
    return(sqrt((1.0/n)*rmse));
}
```

As an example of execution, use R's sample data `mtcars` to calculate the RMSE of the model linearly regressing the fuel efficiency of the car.

```R
> mod <- lm(mpg ~ ., data = mtcars)
> rcpp_rmse(mod)
[1] 2.146905
```



## S4 class


### Accessing to slot

To access the slots of a S4 class object, use the `slot()` member function. Also, use the `hasSlot()` member function to check if the object have a slot with a specific name.

```cpp
x.slot("slot_name");
x.hasSlot("slot_name");
```

### Creating a new S4 class object

Rcpp alone can not define a new S4 class, but you can create an S4 class object defined in R.

The following code example shows how to define a new S4 class Person in R, then create an object of Person class with Rcpp.

We first define S4 class "Person". This class holds name and birthday of a person in slot `name` and `birth`.

```R
# R code
# Defining S4 class Person in R
setClass (
  # Class name
  "Person",

  # Defining slot type
  representation (
    name = "character",
    birth = "Date"
  ),

  # Initializing slots
  prototype = list(
    name = as.character(NULL),
    birth = as.Date(as.character(NULL))
  )
)

# Creating an object of Person class in R
person_01 <- new("Person",
                 name = "Ronald Fisher",
                 birth = as.Date("1890-02-17"))
```

The following code example creates an object of Person class in Rcpp and set values for the slots.

```cpp
// [[Rcpp::export]]
S4 rcpp_s4(){

    // Creating an object of Person class
    S4 x("Person");

    // Setting values to the slots
    x.slot("name")  = "Sewall Wright";
    x.slot("birth") = Date("1889-12-21");

    return(x);
}
```

Execution result

```
> rcpp_s4()
An object of class "Person"
Slot "name":
[1] "Sewall Wright"

Slot "birth":
[1] "1889-12-21"
```