[Rd] ROBUSTNESS: x || y and x && y to give warning/error if length(x) != 1 or length(y) != 1
Joris Meys
jori@mey@ @ending from gm@il@com
Thu Aug 30 13:56:48 CEST 2018
I have to agree with Emil here. && and || are short circuited like in C and
C++. That means that
TRUE || c(TRUE, FALSE)
FALSE && c(TRUE, FALSE)
cannot give an error because the second part is never evaluated. Throwing a
warning or error for
c(TRUE, FALSE) || TRUE
would mean that the operator gives a different result depending on the
order of the objects, breaking the symmetry. Also that would be undesirable.
Regarding logical(0): per the documentation, it is indeed so that ||, &&
and isTRUE always return a length-one logical vector. Hence the NA.
On a sidenote: there is no such thing as a scalar in R. What you call
scalar, is really a length-one vector. That seems like a detail, but is
important in understanding why this admittedly confusing behaviour actually
makes sense within the framework of R imho. I do understand your objections
and suggestions, but it would boil down to removing short circuited
operators from R.
My 2 cents.
Cheers
Joris
On Wed, Aug 29, 2018 at 5:03 AM 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
>
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
--
Joris Meys
Statistical consultant
Department of Data Analysis and Mathematical Modelling
Ghent University
Coupure Links 653, B-9000 Gent (Belgium)
<https://maps.google.com/?q=Coupure+links+653,%C2%A0B-9000+Gent,%C2%A0Belgium&entry=gmail&source=g>
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