Is this a bug in `[`?

FYI, this behavior is documented in Section 3.4.1 'Indexing by
vectors' of 'R Language Definition' (accessible for instance via
help.start()):

"*Integer* [...] A special case is the zero index, which has null
effects: x[0] is an empty vector and otherwise including zeros among
positive or negative indices has the same effect as if they were
omitted."

The rest of that section is very useful and well written. I used it as
the go-to reference to implement support for all those indexing
alternatives in matrixStats.

/Henrik
On Sun, Aug 5, 2018 at 3:42 AM I?aki ?car <i.ucar86 at gmail.com> wrote:

El dom., 5 ago. 2018 a las 6:27, Kenny Bell (<kmbell56 at gmail.com>) escribi?:

This should more clearly illustrate the issue:

c(1, 2, 3, 4)[-seq_len(4)]
#> numeric(0)
c(1, 2, 3, 4)[-seq_len(3)]
#> [1] 4
c(1, 2, 3, 4)[-seq_len(2)]
#> [1] 3 4
c(1, 2, 3, 4)[-seq_len(1)]
#> [1] 2 3 4
c(1, 2, 3, 4)[-seq_len(0)]
#> numeric(0)
Created on 2018-08-05 by the reprex package (v0.2.0.9000).

IMO, the problem is that you are reading it sequentially: "-" remove
"seq_" a sequence "len(0)" of length zero. But that's not how R works
(how programming languages work in general). Instead, the sequence is
evaluated in the first place, and then the sign may apply as long as
you provided something that can hold a sign. And an empty element has
no sign, so the sign is lost.

I?aki

On Sun, Aug 5, 2018 at 3:58 AM Rui Barradas <ruipbarradas at sapo.pt> wrote:

?s 15:51 de 04/08/2018, I?aki ?car escreveu:

El s?b., 4 ago. 2018 a las 15:32, Rui Barradas
(<ruipbarradas at sapo.pt>) escribi?:

Hello,

Maybe I am not understanding how negative indexing works but

1) This is right.

(1:10)[-1]
#[1]  2  3  4  5  6  7  8  9 10

2) Are these right? They are at least surprising to me.

(1:10)[-0]
#integer(0)

(1:10)[-seq_len(0)]
#integer(0)


It was the last example that made me ask, seq_len(0) whould avoid an
if/else or something similar.

I think it's ok, because there is no negative zero integer, so -0 is 0.

Ok, this makes sense, I should have thought about that.

1.0/-0L # Inf
1.0/-0.0 # - Inf

And the same can be said for integer(0), which is the result of
seq_len(0): there is no negative empty integer.

I'm not completely convinced about this one, though.
I would expect -seq_len(n) to remove the first n elements from the
vector, therefore, when n == 0, it would remove none.

And integer(0) is not the same as 0.

(1:10)[-0] == (1:10)[0] == integer(0) # empty

(1:10)[-seq_len(0)] == (1:10)[-integer(0)]


And I have just reminded myself to run

identical(-integer(0), integer(0))

It returns TRUE so my intuition is wrong, R is right.
End of story.

Thanks for the help,

Rui Barradas

I?aki

Thanks in advance,

Rui Barradas