R help - Jan 2009 - "registering" a generic method for a class

Sorry in advance if this is too simple a question, but I'm stuck with
some odd behavior and I can't find the text to rid myself of this
(admittedly somewhat trivial) problem.  Note that I've done generic
programming with S3 "objects" in the past, but I've never really
dabbled in creating S4 objects until now.

So, I've created a new S4 class, call it "myclass", e.g. :
> setClass("myclass", representation(slot1 = "list"));
And I've also created an implementation of "print" for this class
via:
> setMethod("print", "myclass", function(x) {
print(paste("a myclass instance with a list of length", length(x at
slot1), "in slot1")); });
Now I can create a new object, say via:
> myobj <- new("myclass", slot1 = list());
The question now is the difference in output when I type the object
name alone on the command prompt vs when I type print(myobj) on the
command prompt, as seen below:
> myobj
An object of class ?myclass?
Slot "slot1":
list()
> print(myobj)
[1] "a myclass instance with a list of length 0 in slot1"

I thought that the "print" method is called when an object is called
from the command line, and to check this I checked the registered
implementations of "print" via:
> methods("print")
and in the long output I didn't see the "print.myclass" version. 
I
had thought that the setMethod method would essentially create this
function for me... but I must be missing a key step somewhere along
the line.  Can anyone help me figure out what I would need to do to
get the generic version of the method (in this toy example, "print")
to see my class' method as an implementation of the generic?  Thanks
much for any help!

In case of S4 objects "show" is called. Hence, you should implement
new
S4-methods for show ...

Matthias


m.u.r. wrote:
> Sorry in advance if this is too simple a question, but I'm stuck with
> some odd behavior and I can't find the text to rid myself of this
> (admittedly somewhat trivial) problem.  Note that I've done generic
> programming with S3 "objects" in the past, but I've never
really
> dabbled in creating S4 objects until now.
>
> So, I've created a new S4 class, call it "myclass", e.g. :
>
>   
>> setClass("myclass", representation(slot1 =
"list"));
>>     
>
> And I've also created an implementation of "print" for this
class via:
>
>   
>> setMethod("print", "myclass", function(x) {
print(paste("a myclass instance with a list of length", length(x at
slot1), "in slot1")); });
>>     
>
> Now I can create a new object, say via:
>
>   
>> myobj <- new("myclass", slot1 = list());
>>     
>
> The question now is the difference in output when I type the object
> name alone on the command prompt vs when I type print(myobj) on the
> command prompt, as seen below:
>
>   
>> myobj
>>     
> An object of class ?myclass?
> Slot "slot1":
> list()
>
>   
>> print(myobj)
>>     
> [1] "a myclass instance with a list of length 0 in slot1"
>
> I thought that the "print" method is called when an object is
called
> from the command line, and to check this I checked the registered
> implementations of "print" via:
>
>   
>> methods("print")
>>     
>
> and in the long output I didn't see the "print.myclass"
version.  I
> had thought that the setMethod method would essentially create this
> function for me... but I must be missing a key step somewhere along
> the line.  Can anyone help me figure out what I would need to do to
> get the generic version of the method (in this toy example,
"print")
> to see my class' method as an implementation of the generic?  Thanks
> much for any help!
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>   
-- 
Dr. Matthias Kohl
www.stamats.de