[Rd] ROBUSTNESS: x || y and x && y to give warning/error if length(x) != 1 or length(y) != 1
Hugh Parsonage
hugh@p@r@on@ge @ending from gm@il@com
Sat Sep 1 09:42:52 CEST 2018
I would add my support for the change.
>From a cursory survey of existing code, I'd say that usage like `class(x)
== 'y'` -- rather than inherits(x, 'y') or is.y -- is probably going to be
the major source of new warnings. So perhaps in the NEWS item it could be
noted as a clue for developers encountering nascent warnings.
Of course `if (class(x) == 'y')` already throws a warning, just not `if
(class(x) == 'y' && TRUE)`.
On Thu, 30 Aug 2018 at 21:09 Emil Bode <emil.bode using dans.knaw.nl> wrote:
> I have to disagree, I think one of the advantages of '||' (or &&) is the
> lazy evaluation, i.e. you can use the first condition to "not care" about
> the second (and stop errors from being thrown).
> So if I want to check if x is a length-one numeric with value a value
> between 0 and 1, I can do 'class(x)=='numeric' && length(x)==1 && x>0 &&
> x<1'.
> In your proposal, having x=c(1,2) would throw an error or multiple
> warnings.
> Also code that relies on the second argument not being evaluated would
> break, as we need to evaluate y in order to know length(y)
> There may be some benefit in checking for length(x) only, though that
> could also cause some false positives (e.g. 'x==-1 || length(x)==0' would
> be a bit ugly, but not necessarily wrong, same for someone too lazy to
> write x[1] instead of x).
>
> And I don’t really see the advantage. The casting to length one is (I
> think), a feature, not a bug. If I have/need a length one x, and a length
> one y, why not use '|' and '&'? I have to admit I only use them in
> if-statements, and if I need an error to be thrown when x and y are not
> length one, I can use the shorter versions and then the if throws a warning
> (or an error for a length-0 or NA result).
>
> I get it that for someone just starting in R, the differences between |
> and || can be confusing, but I guess that's just the price to pay for
> having a vectorized language.
>
> Best regards,
> Emil Bode
>
> Data-analyst
>
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>
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> On 29/08/2018, 05:03, "R-devel on behalf of Henrik Bengtsson" <
> r-devel-bounces using r-project.org on behalf of 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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