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

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*."

Duncan Murdoch

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.

David Winsemius
> 
> Duncan Murdoch
> 
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