[Rd] Is this a bug in `[`?
Henrik Bengtsson
henrik@bengt@@on @ending from gm@il@com
Wed Aug 29 05:17:46 CEST 2018
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 Iñaki Úcar <i.ucar86 using gmail.com> wrote:
>
> El dom., 5 ago. 2018 a las 6:27, Kenny Bell (<kmbell56 using 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 <ruipbarradas using sapo.pt> wrote:
> >>
> >>
> >>
> >> Às 15:51 de 04/08/2018, Iñaki Úcar escreveu:
> >> > El sáb., 4 ago. 2018 a las 15:32, Rui Barradas
> >> > (<ruipbarradas using 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 using r-project.org mailing list
> >> >> https://stat.ethz.ch/mailman/listinfo/r-devel
> >>
> >> ______________________________________________
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>
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