[Rd] '==' operator: inconsistency in data.frame(...) == NULL

[Hilmar Berger]

Dear Martin,

On 11/09/2019 09:56, Martin Maechler wrote:
>
>      > I wonder if data.frame(<some non-empty data>) == NULL should also return
>      > a value instead of an error. R help reads:
>
>          > "At least one of |x| and |y| must be an atomic vector, but
> 	>  if the other is a list R attempts to coerce it to the
> 	>  type of the atomic vector: this will succeed if the list
> 	>  is made up of elements of length one that can be coerced
> 	>  to the correct type.
>
> 	>  If the two arguments are atomic vectors of different
> 	>  types, one is coerced to the type of the other, the
> 	>  (decreasing) order of precedence being character, complex,
> 	>  numeric, integer, logical and raw."
>
>      > It is not clear from the help what to expect for NULL or
>      > empty atomic vectors.
>
> Well, strictly speaking an error would be expected for NULL,
> as it is *not* an atomic vector, and your main issue
>
>   " data.frame(..) == NULL "
>
> would already be settled by the first half sentence from the
> doc, and strictly speaking, even  data.frame(NULL) == NULL
> "should" return an error ((Note: I'm not saying it really
>   should, but at least the reference does not say it should work at all))
Thanks, this explanation makes total sense to me. I did not consider 
that NULL might be non-atomic. Strangely, is.atomic(NULL) returns TRUE. 
On the other hand, I understand that one would not like to treat it like 
atomic in ==.

However, in this case one might expect that the error message would be 
more like that for S4 objects (which always seem to report an 
informative error message for ==):

 > Pos <- setClass("Pos", slots = c(latitude = "numeric", longitude = 
"numeric", altitude = "numeric"))
 > p = Pos()
 > p == NULL
Error in p == NULL :
   comparison (1) is possible only for atomic and list types
 > p == "FOO"
Error in p == "FOO" :
   comparison (1) is possible only for atomic and list types

In the data.frame()==NULL cases I have the impression that the fact that 
both sides are non-atomic is not properly detected and therefore R tries 
to go on with the == method for data.frames.

 From a cursory check in Ops.data.frame() and some debugging I have the 
impression that the case of the second argument being non-atomic or 
empty is not handled at all and the function progresses until the end, 
where it fails in the last step on an empty value:

matrix(unlist(value, recursive = FALSE, use.names = FALSE),
     nrow = nr, dimnames = list(rn, cn))

Best regards,
Hilmar

-- 
Dr. Hilmar Berger, MD
Max Planck Institute for Infection Biology
Charitéplatz 1
D-10117 Berlin
GERMANY

Phone:  + 49 30 28460 430
Fax:    + 49 30 28460 401
  
E-Mail: berger using mpiib-berlin.mpg.de
Web   : www.mpiib-berlin.mpg.de


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