R devel - Jan 2023 - Recycling in arithmetic operations involving zero-length vectors

On 16/01/2023 6:55 a.m., David Winsemius wrote:
> 
> 
> Sent from my iPhone
> 
>> On Jan 16, 2023, at 6:11 PM, Duncan Murdoch <murdoch.duncan at
gmail.com> wrote:
>>
>> ?On 16/01/2023 5:23 a.m., Roland Fu? wrote:
>>> Dear R-core,
>>> The language definition is very clear:
>>> "As from R 1.4.0, any arithmetic operation involving a
zero-length
>>> vector has a zero-length result."
>>> Thus, `1 + numeric()` returns `numeric(0)`. However, I don't
find this
>>> very intuitive because usually the shorter vector is recycled to
the
>>> length of the longer vector. Would it be possible to throw at least
a
>>> warning for such cases? I don't expect them to be intended by
most users.
>>> Best regards,
>>
>> The previous paragraph says "If the length of the longer vector is
not a multiple of the shorter one, a warning is given."  Since 1 is not a
multiple of 0, that implies a warning should be given here.
>>
>> However, R 1.4.0 was released more than 20 years ago, so I would guess
there are lots of packages intentionally using this.  For example, it's a
way to propagate bad inputs through a long calculation that allows a single test
at the end.
>>
>> And even unintentional uses are unlikely to lead to problematic
results:  numeric(0) is usually a pretty clear signal that something is wrong.
>>
>> So I'd suggest a documentation change: "As from R 1.4.0, any
arithmetic operation involving a zero-length vector has a zero-length result
*without a warning*."
> 
> I doubt that a documentation change will help very much. Roland is
responding here with his and my surprise at the lack of a warning after
witnessing my answer to an R newb Q where the impression at seeing ?numeric(0)
was understood as the value 0. I suggested that many interpretations were
possible and that a warning was given for NA generation. I stand with Roland in
thinking a warning is appropriate.
I didn't see this exchange, but I don't understand "a warning was
given
for NA generation".  We don't get a warning for 1 + NA.  Do you mean 
you'd like to get one?

In any case, I think your anecdote illustrates a different problem: 
printing numeric() as numeric(0) confused a beginning user.  I've also 
seen people get confused by that.

Perhaps the change should be to the way numeric(0) is printed, but  that 
would also have consequences, since some people test the way output is 
printed.

Or perhaps we should just recognize that it's in the nature of being a 
beginning user to be confused sometimes, and just help them to grow out 
of that stage.

Before a change like one of these is made, someone should make it in a 
local copy, then run R CMD check on every package on CRAN to see how 
disruptive it is.  Maybe adding a warning() will uncover so few 
intentional uses that fixing them is worthwhile.

The trouble is, running checks across CRAN is a very resource-intensive 
exercise, and analyzing the results is a very developer-intensive 
exercise.  I'm sure the doc change is easier.

Duncan Murdoch

On Mon, Jan 16, 2023 at 7:28 PM Duncan Murdoch <murdoch.duncan at
gmail.com> wrote:
>
> On 16/01/2023 6:55 a.m., David Winsemius wrote:
> >
> >
> > Sent from my iPhone
> >
> >> On Jan 16, 2023, at 6:11 PM, Duncan Murdoch <murdoch.duncan at
gmail.com> wrote:
> >>
> >> ?On 16/01/2023 5:23 a.m., Roland Fu? wrote:
> >>> Dear R-core,
> >>> The language definition is very clear:
> >>> "As from R 1.4.0, any arithmetic operation involving a
zero-length
> >>> vector has a zero-length result."
> >>> Thus, `1 + numeric()` returns `numeric(0)`. However, I
don't find this
> >>> very intuitive because usually the shorter vector is recycled
to the
> >>> length of the longer vector. Would it be possible to throw at
least a
> >>> warning for such cases? I don't expect them to be intended
by most users.
> >>> Best regards,
> >>
> >> The previous paragraph says "If the length of the longer
vector is not a multiple of the shorter one, a warning is given."  Since 1
is not a multiple of 0, that implies a warning should be given here.
> >>
> >> However, R 1.4.0 was released more than 20 years ago, so I would
guess there are lots of packages intentionally using this.  For example,
it's a way to propagate bad inputs through a long calculation that allows a
single test at the end.
> >>
> >> And even unintentional uses are unlikely to lead to problematic
results:  numeric(0) is usually a pretty clear signal that something is wrong.
> >>
> >> So I'd suggest a documentation change: "As from R 1.4.0,
any arithmetic operation involving a zero-length vector has a zero-length result
*without a warning*."
> >
> > I doubt that a documentation change will help very much. Roland is
responding here with his and my surprise at the lack of a warning after
witnessing my answer to an R newb Q where the impression at seeing ?numeric(0)
was understood as the value 0. I suggested that many interpretations were
possible and that a warning was given for NA generation. I stand with Roland in
thinking a warning is appropriate.
>
> I didn't see this exchange, but I don't understand "a warning
was given
> for NA generation".  We don't get a warning for 1 + NA.  Do you
mean
> you'd like to get one?
>
> In any case, I think your anecdote illustrates a different problem:
> printing numeric() as numeric(0) confused a beginning user.  I've also
> seen people get confused by that.
>
> Perhaps the change should be to the way numeric(0) is printed, but  that
> would also have consequences, since some people test the way output is
> printed.
>
> Or perhaps we should just recognize that it's in the nature of being a
> beginning user to be confused sometimes, and just help them to grow out
> of that stage.
>
> Before a change like one of these is made, someone should make it in a
> local copy, then run R CMD check on every package on CRAN to see how
> disruptive it is.  Maybe adding a warning() will uncover so few
> intentional uses that fixing them is worthwhile.
To even do that, we would have to first decide which "cases" should
produce a warning.

Let's say `1 + x` should give a warning when x = numeric(0). Then
should `x^2` also produce a warning? Should `x^0.5`? Should `sqrt(x)`?
Should `log(x)`?

-Deepayan
> The trouble is, running checks across CRAN is a very resource-intensive
> exercise, and analyzing the results is a very developer-intensive
> exercise.  I'm sure the doc change is easier.
>
> Duncan Murdoch
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel