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

[peter dalgaard]

Actually, sum(), all(), any(), prod() all return a _carefully considered_ result in the case of an empty vector. The sum over a disjoint union of sets should be the sum of the sums over each set, so if a set is empty, its contribution to the sum must be zero. Similarly, the product over an empty set is 1. Logical aggregate all() should be stable for "&" and any() for "|", hence all(logical(0))==TRUE and any(logical(0))==FALSE. 

-pd

> On 16 Jan 2023, at 16:53 , GILLIBERT, Andre <Andre.Gillibert using chu-rouen.fr> wrote:
> 
> Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
> 
>> 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)`?
> 
> 
> The most probable errors would be in functions taking two arguments (e.g. `+`) and for which one argument has length >= 2 while the other has length 0.
> 
> In my experience, most code with accidental zero-length propagations (e.g. typo in data_frame$field) quickly lead to errors, that are easy to debug (except for beginners), and so, do not need a warning.
> 
> The only cases where zero-length propagation is really dangerous in my experience is in code using an aggregating function like sum(), all() or any(), because it silently returns a valid value for a zero-length argument. Emitting warnings for sum(numeric(0)) would probably have too many false positives but a (length >= 2) vs (length == 0) warning for common binary operators could sometimes catch the issue before it reaches the aggregating function.
> 
> -- 
> Sincerely
> André GILLIBERT
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-- 
Peter Dalgaard, Professor,
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