Dear All,
I am not very into object-oriented programming, but I would like to
learn the ropes for some R applications.
Quoting from the online R language definition (paragraph 5.1)
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.
I am a bit confused: when calling e.g. xpos.rthetapoint, I understand
that x contains the polar representation of a point, so x=(r,theta),
but how do I exactly write it to pass it to xpos.rthetapoint? I have
made several attempts, but so far none of them works, so I may have
misunderstood something.
Many thanks
Lorenzo
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