From pburn@ @ending from pburn@@@e@net@com  Sun Aug  5 09:46:06 2018
From: pburn@ @ending from pburn@@@e@net@com (Patrick Burns)
Date: Sun, 5 Aug 2018 08:46:06 +0100
Subject: [Rd] Is this a bug in `[`?
In-Reply-To: <bd6150f0-c23d-741f-13c0-cf0f31228b2b@sapo.pt>
References: <4cf085ac-8f7e-38b5-ea02-302df94ad7b5@sapo.pt>
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 <bd6150f0-c23d-741f-13c0-cf0f31228b2b@sapo.pt>
Message-ID: <90106d5c-4af6-b752-89f6-ca67030dfa46@pburns.seanet.com>

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
> 
> ?s 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 at sapo.pt 
>> <mailto: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 <mailto: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
>> ???? >>
>> ???? >> ______________________________________________
>> ???? >> R-devel at r-project.org <mailto:R-devel at r-project.org> mailing list
>> ???? >> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>> ??? ______________________________________________
>> ??? R-devel at r-project.org <mailto:R-devel at r-project.org> mailing list
>> ??? https://stat.ethz.ch/mailman/listinfo/r-devel
>>
> 
> ______________________________________________
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> 

-- 
Patrick Burns
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[Rd] Is this a bug in `[`?
Patrick Burns