[Rd] dim<-() changed in R-devel; no longer removing "dimnames" when doing dim(x) <- dim(x)

[Hervé Pagès]

Hi Martin, Henrik,

I actually like this change.

Makes a lot of sense IMO that dim(x) <- dim(x) be a no-op, or, more 
generally, that foo(x) <- foo(x) be a no-op for any setter/getter combo.

FWIW S4Arrays::set_dim() does that too. It also preserves the dimnames 
if the right value is only adding or dropping outermost (ineffective) 
dimensions:

     > x <- array(1:6, dim=c(2,3,1), dimnames=list(c("A", "B"), 
c("x","y", "z"), "T"))
     > S4Arrays:::set_dim(x, 2:3)
       x y z
     A 1 3 5
     B 2 4 6

Note that this is consistent with drop().

Best,

H.

On 10/30/23 03:53, Martin Maechler wrote:
>>>>>> Henrik Bengtsson
>>>>>>      on Sun, 29 Oct 2023 10:42:19 -0700 writes:
>      > Hello,
>
>      > the fix of PR18612
>      > (https://bugs.r-project.org/show_bug.cgi?id=18612) in
>      > r85380
>      > (https://github.com/wch/r-source/commit/2653cc6203fce4c48874111c75bbccac3ac4e803)
>      > caused a change in `dim<-()`.  Specifically, in the past,
>      > any `dim<-()` assignment would _always_ remove "dimnames"
>      > and "names" attributes per help("dim"):
>
>
>      >     The replacement method changes the "dim" attribute
>      > (provided the new value is compatible) and removes any
>      > "dimnames" and "names" attributes.
>
>      > In the new version, assigning the same "dim" as before
>      > will no longer remove "dimnames".  I'm reporting here to
>      > check whether this change was intended, or if it was an
>      > unintended side effect of the bug fix.
>
>      > For example, in R Under development (unstable) (2023-10-21
>      > r85379), we would get:
>
>      >> x <- array(1:2, dim=c(1,2), dimnames=list("A",
>      >> c("a","b"))) str(dimnames(x))
>      > List of 2 $ : chr "A" $ : chr [1:2] "a" "b"
>
>      >> dim(x) <- dim(x) ## Removes "dimnames" no matter what
>      >> str(dimnames(x))
>      >  NULL
>
>
>      > whereas in R Under development (unstable) (2023-10-21
>      > r85380) and beyond, we now get:
>
>      >> x <- array(1:2, dim=c(1,2), dimnames=list("A",
>      >> c("a","b"))) str(dimnames(x))
>      > List of 2 $ : chr "A" $ : chr [1:2] "a" "b"
>
>      >> dim(x) <- dim(x) ## No longer removes "dimnames"
>      >> str(dimnames(x))
>      > List of 2 $ : chr "A" $ : chr [1:2] "a" "b"
>
>      >> dim(x) <- rev(dim(x)) ## Still removes "dimnames"
>      >> str(dimnames(x))
>      >  NULL
>
>      > /Henrik
>
> Thank you, Henrik.
>
> This is "funny" (in an unusal sense):
> indeed, the change was *in*advertent, by me (svn rev 85380).
>
> I had experimentally {i.e., only in my own private version of R-devel!}
> modified the behavior of `dim<-` somewhat
> such it does *not* unnecessarily drop dimnames,
> e.g., in your   `dim(x) <- dim(x)` case above,
> one could really argue that it's a "true loss" if x loses
> dimnames "unnecessarily" ...
>
> OTOH, I knew in the mean time that  `dim<-` has always been
> documented to drop dimnames in all cases,  and even more
> importantly, I got a strong recommendation to *not* go further
> with this idea -- not only for back compatibility reasons, but
> also for internal logical consistency.
>
> Most probably, we will just revert this inadvertent change,
> but before that ... since it has been out in the wild anyway,
> we could quickly consider if it *did* break code.
>
> I assume it did, or you would not have noticed ?
>
> Martin
>
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-- 
Hervé Pagès

Bioconductor Core Team
hpages.on.github using gmail.com

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