[Rd] ROBUSTNESS: x || y and x && y to give warning/error if length(x) != 1 or length(y) != 1

[Hadley Wickham]

I think this is an excellent idea as it eliminates a situation which
is almost certainly user error. Making it an error would break a small
amount of existing code (even if for the better), so perhaps it should
start as a warning, but be optionally upgraded to an error. It would
be nice to have a fixed date (R version) in the future when the
default will change to error.

In an ideal world, I think the following four cases should all return
the same error:

if (logical()) 1
#> Error in if (logical()) 1: argument is of length zero
if (c(TRUE, TRUE)) 1
#> Warning in if (c(TRUE, TRUE)) 1: the condition has length > 1 and only the
#> first element will be used
#> [1] 1

logical() || TRUE
#> [1] TRUE
c(TRUE, TRUE) || TRUE
#> [1] TRUE

i.e. I think that `if`, `&&`, and `||` should all check that their
input is a logical (or numeric) vector of length 1.

Hadley

On Tue, Aug 28, 2018 at 10:03 PM Henrik Bengtsson
<henrik.bengtsson using gmail.com> wrote:
>
> # Issue
>
> 'x || y' performs 'x[1] || y' for length(x) > 1.  For instance (here
> using R 3.5.1),
>
> > c(TRUE, TRUE) || FALSE
> [1] TRUE
> > c(TRUE, FALSE) || FALSE
> [1] TRUE
> > c(TRUE, NA) || FALSE
> [1] TRUE
> > c(FALSE, TRUE) || FALSE
> [1] FALSE
>
> This property is symmetric in LHS and RHS (i.e. 'y || x' behaves the
> same) and it also applies to 'x && y'.
>
> Note also how the above truncation of 'x' is completely silent -
> there's neither an error nor a warning being produced.
>
>
> # Discussion/Suggestion
>
> Using 'x || y' and 'x && y' with a non-scalar 'x' or 'y' is likely a
> mistake.  Either the code is written assuming 'x' and 'y' are scalars,
> or there is a coding error and vectorized versions 'x | y' and 'x & y'
> were intended.  Should 'x || y' always be considered an mistake if
> 'length(x) != 1' or 'length(y) != 1'?  If so, should it be a warning
> or an error?  For instance,
> '''r
> > x <- c(TRUE, TRUE)
> > y <- FALSE
> > x || y
>
> Error in x || y : applying scalar operator || to non-scalar elements
> Execution halted
>
> What about the case where 'length(x) == 0' or 'length(y) == 0'?  Today
> 'x || y' returns 'NA' in such cases, e.g.
>
> > logical(0) || c(FALSE, NA)
> [1] NA
> > logical(0) || logical(0)
> [1] NA
> > logical(0) && logical(0)
> [1] NA
>
> I don't know the background for this behavior, but I'm sure there is
> an argument behind that one.  Maybe it's simply that '||' and '&&'
> should always return a scalar logical and neither TRUE nor FALSE can
> be returned.
>
> /Henrik
>
> PS. This is in the same vein as
> https://mailman.stat.ethz.ch/pipermail/r-devel/2017-March/073817.html
> - in R (>=3.4.0) we now get that if (1:2 == 1) ... is an error if
> _R_CHECK_LENGTH_1_CONDITION_=true
>
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