[Rd] S4 class extending data.frame?

[Oleg Sklyar]

I had the same problem. Generally data.frame's behave like lists, but
while you can extend list, there are problems extending a data.frame
class. This comes down to the internal representation of the object I
guess. Vectors, including list, contain their information in a (hidden)
slot .Data (see the example below). data.frame's do not seem to follow
this convention.

Any idea how to go around?

The following example is exactly the same as Ben's for a data.frame, but
using a list. It works fine and one can see that the list structure is
stored in .Data

* ~: R
R version 2.6.1 (2007-11-26) 
> setClass("c3",representation(comment="character"),contains="list")
[1] "c3"
> l = list(1:3,2:4)
> z3 = new("c3",l,comment="hello")
> z3
An object of class “c3”
[[1]]
[1] 1 2 3

[[2]]
[1] 2 3 4

Slot "comment":
[1] "hello"

> z3 at .Data
[[1]]
[1] 1 2 3

[[2]]
[1] 2 3 4

Regards,
Oleg

On Thu, 2007-12-13 at 00:04 -0500, Ben Bolker wrote:
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> I would like to build an S4 class that extends
> a data frame, but includes several more slots.
> 
> Here's an example using integer as the base
> class instead:
> 
> setClass("c1",representation(comment="character"),contains="integer")
> z1 = new("c1",55,comment="hello")
> z1
> z1+10
> z1[1]
> z1 at comment
> 
>  -- in other words, it behaves exactly as an integer
> for access and operations but happens to have another slot.
> 
>  If I do this with a data frame instead, it doesn't seem to work
> at all.
> 
> setClass("c2",representation(comment="character"),contains="data.frame")
> d = data.frame(1:3,2:4)
> z2 = new("c2",d,comment="goodbye")
> z2  ## data all gone!!
> z2[,1]  ## Error ... object is not subsettable
> z2 at comment  ## still there
> 
>   I can achieve approximately the same effect by
> adding attributes, but I was hoping for the structure
> of S4 classes ...
> 
>   Programming with Data and the R Language Definition
> contain 2 references each to data frames, and neither of
> them has allowed me to figure out this behavior.
> 
>  (While I'm at it: it would be wonderful to have
> a "rich data frame" that could include as a column
> any object that had an appropriate length and
> [ method ... has anyone done anything in this direction?
> ?data.frame says the allowable types are
>  "(numeric, logical, factor and character and so on)",
>  but I'm having trouble sorting out what the limitations
> are ...)
> 
>   hoping for enlightenment (it would be lovely to be
> shown how to make this work, but a definitive statement
> that it is impossible would be useful too).
> 
>   cheers
>     Ben Bolker
> 
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-- 
Dr Oleg Sklyar * EBI-EMBL, Cambridge CB10 1SD, UK * +44-1223-494466