Re: [Rd] Problem with _new_ if class "lm" in object representatio n.

From: Witold Eryk Wolski <>
Date: Thu 30 Sep 2004 - 22:35:43 EST


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.

  1. 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.

 > x<-lm(1~1)
 > object.size(x)
[1] 6388
 > object.size(NULL)
[1] 0

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 slot(obj,"mod",check=FALSE)<- NULL.

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 function initialize.

 > setClass("bar")
[1] "bar"
 > setClass("foo", representation(x="numeric", y="bar")) [1] "foo"
 > 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
>>special case
>>which can be assigned to any slot? I think in java one can
>>assing null
>>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
>>am looking
>>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.
>Hello Eryk,
>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:
> ,representation(
> test="character"
> ,bla="character"
> ,mod="ANY"
> )
> )
>test1 <- new("Ctest",mod=lm(1~1))
>is(test1@mod, "lm")
>test2 <- new("Ctest",test="bla")
>is(test2@mod, "lm")
>test3 <- new("Ctest",bla="brum")
>is(test3@mod, "lm")
>test4 <- new("Ctest",test="bla",bla="brum")
>is(test4@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.
> mailing list Received on Thu Sep 30 22:41:10 2004

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