S4 class extends "data.frame", getDataPart sees "list"

Thank you. For "getDataPart" I was following (my interpretation of)
advice from the documentation for "Classes": The functions |getDataPart
<http://127.0.0.1:15455/library/methods/help/getDataPart>| and
|setDataPart <http://127.0.0.1:15455/library/methods/help/setDataPart>|
are a cleaner, but essentially equivalent way to deal with the data part.
I interpreted "cleaner" to mean "preferred." From your reply, John, it
sounds like I should go back to the object at .Data construct.

-Dan
On Tue, Jul 13, 2010 at 5:57 AM, John Chambers <jmc at r-project.org
<mailto:jmc at r-project.org>> wrote:

    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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