Why does empty logical vector pass the stopifnot() check?

user_1953718 0
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Today I found that some of my stopifnot() tests are failing because the passed arguments evaluate to empty logical vectors.

Here is an example:

stopifnot(iris$nosuchcolumn == 2)  # passes without error

This is very unintuitive and seems to contradict a few other behaviours. Consider:

isTRUE(logical())
> FALSE

stopifnot(logical())
# passes

So stopifnot() passes even when this argument is not TRUE.
But furthermore, the behaviour of the above is different with different types of empty vectors.

isTRUE(numeric())
> FALSE

stopifnot(numeric())
# Error: numeric() are not all TRUE

Is there some logic to the above, or should this be considered a bug?

is-empty error-handling r boolean
2 Answers

The comments by akrun and r2evans are spot on.

However, to give details on why specifically this happens and why you're confused vs. isTRUE() behavior, note that stopifnot() checks for three things; the check is (where r is the result of the expression you pass):

if (!(is.logical(r) && !anyNA(r) && all(r)))

So, let's take a look:

is.logical(logical())
# [1] TRUE
!anyNA(logical())
# [1] TRUE
all(logical())
# [1] TRUE

is.logical(numeric())
# [1] FALSE
!anyNA(numeric())
# [1] TRUE
all(numeric())
# [1] TRUE

So, the only reason why logical() passes while numeric() fails is because numeric() is not "logical," as suggested by akrun. For this reason, you should avoid checks that may result in logical vectors of length 0, as suggested by r2evans.

user_83861400

Other answers cover the practical reasons why stopifnot behaves the way it does; but I agree with Karolis that the thread linked by Henrik adds the real explanation of why this is the case:

As author stopifnot(), I do agree with [OP]'s "gut feeling" [...] that stopifnot(dim(x) == c(3,4)) [...][should] stop in the case where x is a simple vector instead of a matrix/data.frame/... with dimensions c(3,4) ... but [...] the gut feeling is wrong because of the fundamental lemma of logic: [...]

"All statements about elements of the empty set are true"

Martin Maechler, ETH Zurich

Also, [...], any() is to "|" what sum() is to "+" and what all() is to "&" and prod() is to "*". All the operators have an identity element, namely FALSE, 0, TRUE, and 1 respectively, and the generic convention is that for an empty vector, we return the identity element, for the reason given above.

Peter D.

user_58510180
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