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

[Patrick Burns]

This is Circle 8..1.13 of the R Inferno.


On 05/08/2018 06:57, Rui Barradas wrote:
> Thanks.
> This is exactly the doubt I had.
> 
> Rui Barradas
> 
> Às 05:26 de 05/08/2018, Kenny Bell escreveu:
>> 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).
>>
>> On Sun, Aug 5, 2018 at 3:58 AM Rui Barradas <ruipbarradas using sapo.pt 
>> <mailto: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 <mailto: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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>>
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-- 
Patrick Burns
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