[Rd] Recycling in arithmetic operations involving zero-length vectors

[Duncan Murdoch]

On 16/01/2023 5:23 a.m., Roland Fuß 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