[Rd] By default, `names<-` alters S4 objects

[John Chambers]

You set up a names slot in a non-vector.  Maybe that should be allowed, 
maybe not.  But in any case I would not expect the names() primitive to 
find it, because your object has a non-vector type ("S4").  You could do 
a at names if you thought that made sense:


 > setClass("A", representation(names="character"))
[1] "A"
 > a <- new("A")
 > a at names <- "xx"
 > a at names
[1] "xx"
 > names(a)
NULL


If you wanted something sensible, it's more like:

 > setClass("B", representation(names = "character"), contains = "integer")
[1] "B"
 > b <- new("B", 1:5)
 > names(b) <- letters[1:5]
 > b
An object of class "B"
[1] 1 2 3 4 5
Slot "names":
[1] "a" "b" "c" "d" "e"

 > names(b)
[1] "a" "b" "c" "d" "e"

This allows both the S4 and the primitive code to deal with a 
well-defined object.

John


On 5/15/11 3:02 PM, Hervé Pagès wrote:
> On 11-05-15 11:33 AM, John Chambers wrote:
>> This is basically a case of a user error that is not being caught:
>
> Sure!
>
> https://stat.ethz.ch/pipermail/r-devel/2009-March/052386.html
......

>
> Ah, that's interesting. I didn't know I could put a names slot in my
> class. Last time I tried was at least 3 years ago and that was causing
> problems (don't remember the exact details) so I ended up using NAMES
> instead. Trying again with R-2.14:
>
>  > setClass("A", representation(names="character"))
>
>  > a <- new("A")
>
>  > attributes(a)
> $names
> character(0)
>
> $class
> [1] "A"
> attr(,"package")
> [1] ".GlobalEnv"
>
>  > names(a)
> NULL
>
>  > names(a) <- "K"
>
>  > attributes(a)
> $names
> [1] "K"
>
> $class
> [1] "A"
> attr(,"package")
> [1] ".GlobalEnv"
>
>  > names(a)
> NULL
>
> Surprise! But that's another story...
>
>>
>> The modification that would make sense would be to give you an error in
>> the above code. Not a bad idea, but it's likely to generate more
>> complaints in other contexts, particularly where people don't
>> distinguish the "list" class from lists with names (the "namedList"
>> class).
>>
>> A plausible strategy:
>> 1. If the class has a vector data slot and no names slot, assign the
>> names but with a warning.
>>
>> 2. Otherwise, throw an error.
>>
>> (I.e., I would prefer an error throughout, but discretion ....)
>
> Or, at a minimum (if no consensus can be reached about the above
> strategy), not add a "names" attribute set to NULL. My original
> post was more about keeping the internal representation of objects
> "normalized", in general, so identical() is more likely to be
> meaningful.
>
> Thanks,
> H.
>
>>
>> Comments?
>>
>> John
>>
>>
>>>
>>> Thanks,
>>> H.
>>>
>>>
>>
>> ______________________________________________
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>
>