[Rd] Is this a bug in `[`?

[Iñaki Úcar]

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
>> >>
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