[R] Object-oriented programming in R

[Gabor Grothendieck]

p.plot is an S3 object and you are attempting to define an S4 class
so you if you really want to do that then you would need this to let
S4 know about the ggplot class:

# S4 class with S3 slot
setClass("ggplot") # make ggplot visible to S4
 setClass("AClass", representation(mFirst = "numeric", mSecond = "ggplot"))
a <- new("AClass", mFirst = 1, mSecond = p.plot)

Probably you don't want to use S4 at all here but rather
define an S3 subclass, "A" of the "ggplot" class and
give it an attribute also called "A".

# S3 subclass
AClass <- function(mFirst, mSecond) {
   attr(mSecond, "A") <- mFirst
   class(mSecond) <- c("A", setdiff(class(mSecond), "A"))
   mSecond
}
a2 <- AClass(1, p.plot)

or alternately represent A as an S3 list holding with the two objects
as components:

# S3 class but not subclass
AClass2 <- function(mfirst, mSecond) {
    out <- list(mFirst = mFirst, mSecond = mSecond)
    class(out) <- "A"
    out
}
a3 <- AClass2(1, p.plot)

Also note that

1. ggplot uses S3 at the user level but uses the "proto"
package internally.  Depending on what you want to do using proto
objects might be preferable.  See home page at
http://r-proto.googlecode.com

2. R statements do not normally end in a semicolon.

On Wed, May 27, 2009 at 12:00 PM, Luc Villandre
<villandl at dms.umontreal.ca> wrote:
> Dear R-users,
>
> I have very recently started learning about object-oriented programming in
> R. I am far from being an expert in programming, although I do have an
> elementary C++ background.
>
> Please take a look at these lines of code.
>>
>> some.data = data.frame(V1 = 1:5, V2 = 6:10) ;
>> p.plot = ggplot(data=some.data,aes(x=V1, y=V2)) ;
>> class(p.plot) ;
>> [1] "ggplot"
>
> My understanding is that the object p.plot belongs to the "ggplot" class.
> However, a new class definition like
>>
>> setClass("AClass", representation(mFirst = "numeric", mSecond = "ggplot"))
>> ;
>
> yields the warning
>>
>> Warning message:
>> In .completeClassSlots(ClassDef, where) :
>>  undefined slot classes in definition of "AClass": mSecond(class "ggplot")
>
> The ggplot object is also a list :
>>
>> is.list(p.plot)
>> [1] TRUE
>
> So, I guess I could identify mSecond as being a list.
>
> However, I don't understand why "ggplot" is not considered a valid slot
> type. I thought setClass() was analogous to the class declaration in C++,
> but I guess I might be wrong. Would anyone care to provide additional
> explanations about this?
>
> I decided to explore object-oriented programming in R so that I could
> organize the output from my analysis in a more rigorous fashion and then
> define custom methods that would yield relevant output. However, I'm
> starting to wonder if this aspect is not better suited for package builders.
> R lists are already very powerful and convenient templates. Although it
> wouldn't be as elegant, I could define functions that would take lists
> outputted by the different steps of my analysis and do what I want with
> them. I'm wondering what the merits of both approaches in the context of R
> would be. If anyone has any thoughts about this, I'd be most glad to read
> them.
>
> Cheers,
> --
> *Luc Villandré*
> /Biostatistician
> McGill University Health Center -
> Montreal Children's Hospital Research Institute/
>
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