Creating and Using Objects in R

> Consider the following simple example. A point in two-dimensional
> Euclidean space can be specified by its Cartesian (x-y) or polar
> (r-theta) coordinates. Hence, to store information about the location
> of the point, we could define two classes, |"xypoint"| and
> |"rthetapoint"|. All the `xypoint' data structures are lists with an
> x-component and a y-component. All `rthetapoint' objects are lists
> with an r-component and a theta-component.
>
> Now, suppose we want to get the x-position from either type of object.
> This can easily be achieved through generic functions. We define the
> generic function |xpos| as follows.
>
>      xpos <- function(x, ...)
>          UseMethod("xpos")
>  
>
> Now we can define methods:
>
>      xpos.xypoint <- function(x) x$x
>      xpos.rthetapoint <- function(x) x$r * cos(x$theta)
>  
>
> The user simply calls the function |xpos| with either representation
> as the argument. The internal dispatching method finds the class of
> the object and calls the appropriate methods.
>