[Rd] S4 class extends "data.frame", getDataPart sees "list"

[John Chambers]

On 7/11/10 9:08 PM, Daniel Murphy wrote:
> R-Devel:
>
> When I get the data part of an S4 class that contains="data.frame", it gives
> me a list, even when the "data.frame" is the S4 version:
>
>> d<-data.frame(x=1:3)
>> isS4(d)
> [1] FALSE                           # of course
>> dS4<-new("data.frame",d)
>> isS4(dS4)
> [1] TRUE                            # ok
>> class(dS4)
> [1] "data.frame"                   # good
> attr(,"package")
> [1] "methods"
>> setClass("A", representation(label="character"), contains="data.frame")
> [1] "A"
>> a<-new("A",dS4, label="myFrame")
>> getDataPart(a)
> [[1]]                                  # oh?
> [1] 1 2 3
>
>> class(a at .Data)
> [1] "list"                           # hmm
>> names(a)
> [1] "x"                             # sure, that makes sense
>> a
> Object of class "A"
>    x
> 1 1
> 2 2
> 3 3
> Slot "label":
> [1] "myFrame"
>
>
> Was I wrong to have expected the "data part" of 'a' to be a "data.frame"?

Yes.  Also, there is a clue in the documentation for getDataPart: 
"rarely suitable to be called directly"
The data part, aka "data slot", generally does not have a class (S4 or S3).

You are probably looking for S3Part():

 > setClass("myFrame", contains = "data.frame")
[1] "myFrame"
 > z = new("myFrame", data.frame(x=1:3))
 > z
Object of class "myFrame"
   x
1 1
2 2
3 3
 > S3Part(z)
Object of class "data.frame"
   x
1 1
2 2
3 3
 > S3Part(z, strictS3 = TRUE)
   x
1 1
2 2
3 3



>
> Thanks.
>
> Dan Murphy
>
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>
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