[Rd] S4 methods and polymorophism

From: Eric Lecoutre <lecoutre_at_stat.ucl.ac.be>
Date: Wed 22 Sep 2004 - 18:21:48 EST

Hello R Power Users,

There is a rather large introduction you may skip to go directly to my question about S4 classes.

I am working on some toy code to help me get in through S4 classes. Doing this homework, I have some questions about S4 classes. I have carefully read available help and some additional material such as Robert Gentleman's slides and special attention to "Programming with Data".

I am working with R 1.9 on Windows.

Final goal is the complete rewriting of my R2HTML package from scratch to provide to users a convenient way to describe their output and to allow different output format (provided by special functions - drivers, " la" Sweave - HTML, LaTeX, XML...).

As BillGates guys often have good ideas (or find or buy them), I take the structure of Word documents as a starting point.

A document will be a collection of Objects, which will be either a standard R object or a graph, in both cases with some Formatting Options (FO) added. Formatting Options (a class) is a list of named components like (font-size: 16, forecolor: darkred).

Here is what I have for the moment:

> setClassUnion("ROMA_ID", c("NULL","character"))
> setClass("Objects",representation("list",ID="ROMA_ID"))

ie: Objects is a list (there will be objects of class "Object" without "s") and ID provided to all objects.

Then, as I will manipulate several such collections, such as Documents, Graphs, and so on, I define the virtual class Collection. At the moment:

> setClassUnion("Collection",c("Objects","Documents","Graphs"))

Then, I follow Chambers's samples to add some functionality such as show() functions and manipulations. A "+" method applied on two "Object" creates a new Objects collection.

As you can see from this output, both the definition of the mehod for "+" and for "show" are correct:

ID: numeric

         <ROMA> Object
         Classe: numeric
         FO:  alignment

ID:  character
         <ROMA> Object
         Classe: character
         FO:  font;fontsize;fontcolor;alignment
---------- output ---------------

I have succedded adding a method to generic "+" operator, which is initially defined as:

> getMethod("+")


As I understand it, the fact that it is a .Primitive call ensures the default method for any extendion (that is: we can add methods).

Now, I try to add a method for length to handle "Collection" classes.

> getMethod("length")


I dont undertand the following error message when trying to define this method:

> setMethod("length","Collection",function(object){ length(object@ID)})
Error in conformMethod(signature, mnames, fnames, f) :

         In method for function "length": Formal arguments omitted in the method definition cannot be in the signature (x = "Collection")

How is it possible to extend length to my own class?

Be HappyR and HappyR again,


Eric Lecoutre
UCL / Institut de Statistique
Voie du Roman Pays, 20
1348 Louvain-la-Neuve

tel: (+32)(0)10473050


If the statistics are boring, then you've got the wrong numbers. -Edward Tufte

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https://stat.ethz.ch/mailman/listinfo/r-devel Received on Wed Sep 22 18:30:17 2004

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