nrow(rbind(character(), character())) returns 2 (as documented but very unintuitive, IMHO)

Thu, May 16, 2019 5:48 PM

Hi Herve,

Inline.

On Thu, May 16, 2019 at 4:45 PM Pages, Herve <hpages at fredhutch.org> wrote:

Sure, but

[1] 2

[1] 2

[1] 1

So even if I ultimately "lose"  this debate (which really wouldn't shock
me, even if R-core did agree with me there's backwards compatibility to
consider), you have to concede that the current behavior is more
complicated than the above is acknowledging.

By rights of the invariance that you and Hadley are advocating,  as far as
I understand it, the last should give 2 rows, one of which is all NAs,
rather than giving only one row as it currently does (and, I assume?,
always has).

So there are two different behavior patterns that could coherently (and
internally-consistently) be generalized to apply to the  rbind(character(),
character()) case, not just one. I'm making the case that the other one
(that length 0 vectors do not add rows because they don't contain data)
would be equally valid, and to N>1 people, at least equally intuitive.

Best,
~G

hmmm... not sure why ncol(cbind(aa=character(0), AA=character(0))) or
nrow(rbind(aa=character(0), AA=character(0))) should do anything
different from what they do.

In my experience, and more generally speaking, the desire to treat
0-length vectors as a special case that deviates from the
non-zero-length case has never been productive.

H.


On 5/16/19 13:17, Gabriel Becker wrote:

Hi all,

Apologies if this has been asked before (a quick google didn't  find it

for

me),and I know this is a case of behaving as documented but its so
unintuitive (to me at least) that I figured I'd bring it up here anyway.

figure its probably going to not be changed,  but I'm happy to submit a
patch if this is something R-core feels can/should change.

So I recently got bitten by the fact that

nrow(rbind(character(), character()))

[1] 2


I was checking whether the result of an rbind call had more than one row,
and that unexpected returned true, causing all sorts of shenanigans
downstream as I'm sure you can imagine.

Now I know that from ?rbind

For ?cbind? (?rbind?), vectors of zero length (including ?NULL?)

      are ignored unless the result would have zero rows (columns), for

      S compatibility.  (Zero-extent matrices do not occur in S3 and are

      not ignored in R.)

But there's a couple of things here. First, for the rowbind  case this
reads as "if there would be zero columns,  the vectors will not be
ignored". This wording implies to me that not ignoring the vectors is a
remedy to the "problem" of the potential for a zero-column return, but
thats not the case.  The result still has 0 columns, it just does not

also

have zero rows. So even if the behavior is not changed, perhaps this
wording can be massaged for clarity?

The other issue, which I admit is likely a problem with my intuition, but
which I don't think I'm alone in having, is that even if I can't have a

0x0

matrix (which is what I'd prefer) I would have expected/preferred a 1x0
matrix, the reasoning being that if we must avoid a 0x0 return value, we
would do the  minimum required to avoid, which is to not ignore the first
length 0 vector, to ensure a non-zero-extent matrix, but then ignore the
remaining ones as they contain information for 0 new rows.

Of course I can program around this now that I know the behavior, but
again, its so unintuitive (even for someone with a fairly well developed
intuition for R's sometimes "quirky" behavior) that I figured I'd bring

it

up.

Thoughts?

Best,
~G

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nrow(rbind(character(), character())) returns 2 (as documented but very unintuitive, IMHO)

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