[Rd] Problem with _new_ if class "lm" in object representatio n.
Witold Eryk Wolski
wolski at molgen.mpg.de
Thu Sep 30 14:35:43 CEST 2004
But "any" rises some other problems well known from S3. One has "any"
for "free" in S3. You don't need S4. . But I know how "if" polluted
functions look like in S3. They are hard to understand and to
maintain. Hence I am quite happy to use S4.
Type-checking is usefull if you program with data. Also if you are on
the "C side" of the object. A further problem which I have with "any"
is, that lm="any" in setClass tells you exactly ANYthing (and you are
back in S3). And thats not what I like.*
Hence, I do not want to use any in this case.
I think that allowing to assing NULL to a slot will solve some problems
which I have with S4 so far.
a) How to express that an a slot content was not assigned so far? J.C.
suggest to use character(0), lm(1~1) etc. The problem I have with
insterting lm(1~1) in slot mod in my class Ctest are that I have 100000
instances of object Ctest. At initialization everyone contains an object
lm(1~1). But please note the following.
Unfortunately S4 do not have references. Hence, I can not store a
reference to a single instance of lm(1~1) .
Exactly the same problems I have if I want to "delete" an object. For
example you like to have a slot which stores some data which is
important if the object is in one state. But if the state of the object
changes you do not need that data anymore. Hence, you would like to
delete it to get storage. You can only do it if you have like you
suggest "any", but the consequence is "if" polluted code, or using
But the documentation for slot states.
check: " You should never set this to 'FALSE' in normal
use, since the result can create invalid objects. "
b) The Problem which I have writing intitialization functions like
suggested by J.C is: Imagine you have an object with twenty slots. You
have to write and initialize function with approximately 20 "if"'s. One
of the contribution of S4, as mentioned above, is to help you to avoid
writing such "if" polluted functions. But as I sometimes forget any role
has it's exceptions. So I can live with one function looking ugly in S4
instead of having all functions "if" polluted like in case of S3. The
only consequence so far is that I try to avoid to write initialize
functions. I do it only in some rare very special cases.
The NULL exception in slot assignement will allow me object
initialization using just "new" without providing a "if" polluted
> setClass("foo", representation(x="numeric", y="bar"))
> new("foo", x=1)
Error in validObject(.Object) : Invalid "foo" object: Invalid object for
slot "y" in class "foo": got class "NULL", should be or extend class "bar"
Are there S4 inherent solutions to this questions?
*(I would rather compare "any" to the Object class then to null in java.
It has a different function.)
Pfaff, Bernhard wrote:
>>Thanks a lot for pointing me to setOldClass().
>>What is exactly the point against it, to allow NULL to be a
>>which can be assigned to any slot? I think in java one can
>>to any slot (I do not like java but not because of that).
>>I am asking because for some objects it may be hard to define
>>equivalent to numeric(0). And even lm(1~1) is not that what I
>>for. I would like to have the case
>>of an unassigned slot (Just think about storage?) even to the expense
>>checking every time if the slot is not null.
>how about "ANY" and checking where suitable/needed if the passed object for
>"mod" is of class "lm" at a later stage in your code, like in:
>test1 <- new("Ctest",mod=lm(1~1))
>is(test1 at mod, "lm")
>test2 <- new("Ctest",test="bla")
>is(test2 at mod, "lm")
>test3 <- new("Ctest",bla="brum")
>is(test3 at mod, "lm")
>test4 <- new("Ctest",test="bla",bla="brum")
>is(test4 at mod, "lm")
>>From there on you could use stop() or warning() or whatever you want to
>return to the user as information which class mod should belong to.
More information about the R-devel