Hello,
Thanks for the pointer.
Inline.
On 29/08/2018 04:17, Henrik Bengtsson wrote:
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."
So I was in part right, the zero index is handled as a special case.
My use case was an operation in a function. I wasn't testing whether the
result was of length zero, I was just using seq_len(result) to avoid the
test. And found the error surprising.
Thanks again,
Rui Barradas
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 Iñaki Úcar <i.uca...@gmail.com> wrote:
El dom., 5 ago. 2018 a las 6:27, Kenny Bell (<kmbel...@gmail.com>) escribió:
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.
Iñaki
On Sun, Aug 5, 2018 at 3:58 AM Rui Barradas <ruipbarra...@sapo.pt> wrote:
Às 15:51 de 04/08/2018, Iñaki Úcar escreveu:
El sáb., 4 ago. 2018 a las 15:32, Rui Barradas
(<ruipbarra...@sapo.pt>) escribió:
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
Iñaki
Thanks in advance,
Rui Barradas
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
---
This email has been checked for viruses by AVG.
https://www.avg.com
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
