[datatable-help] Unexpected behavior in setnames()

[Eduard Antonyan]

Because it's very useful for e.g. data presentation purposes.


On Fri, Nov 1, 2013 at 6:02 PM, Arunkumar Srinivasan
<aragorn168b at gmail.com>wrote:

>  Yes, it chooses the first. But we won't be able to perform any operation
> as intended. So why allow duplicate names (ex: in `setnames` as Alexandre
> asks)?
>
> Arun
>
> On Friday, November 1, 2013 at 11:57 PM, Eduard Antonyan wrote:
>
> I think currently it chooses the first "x", but it's definitely a good
> idea to add a warning there.
>
>
> On Fri, Nov 1, 2013 at 5:51 PM, Arunkumar Srinivasan <
> aragorn168b at gmail.com> wrote:
>
>  Ricardo added a bug report here on this topic:
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5008&group_id=240&atid=975
> But I don't think having duplicate names is an easy-to-implement concept.
> For ex:
>
> dt <- data.table(x=1:3, x=4:6, y=c(1,1,2))
> dt[, print(.SD), by=y]
>    x
> 1: 1
> 2: 2
>    x
> 1: 3
>
> .SD loses the second "x". Also, some other questions become difficult to
> handle. Ex:
>
> dt <- data.table(x=c(1,1,2,2), y=c(1,2,3,4), x=c(2,2,1,1))
> dt[, list(x=x/x[1], y=y), by=x]
>
> Which "x" should be choose for which operation?
>
> Arun
>
> On Friday, November 1, 2013 at 10:59 PM, Eduard Antonyan wrote:
>
> Having duplicate names is allowed and not that unusual in data.table
> framework, so there is no need to signal anything here.
>
> A different question is whether there should be a warning here:
>
>   dt = data.table(a = 1, a = 2)
>   dt[, a]
>
> and I think that'd be a pretty good FR to have.
>
>
> On Fri, Nov 1, 2013 at 4:49 PM, Alexandre Sieira <
> alexandre.sieira at gmail.com> wrote:
>
> I found this behavior during a debugging session:
>
> > d = data.table(a=1, b=2, c=3)
> > setnames(d, "a", "b")
> > d
>    b b c
> 1: 1 2 3
>
> Shouldn’t setnames() check if the new column names already exist before
> renaming, and signal an error or at least a warning if they do?
>
> --
> Alexandre Sieira
> CISA, CISSP, ISO 27001 Lead Auditor
>
> "The truth is rarely pure and never simple."
> Oscar Wilde, The Importance of Being Earnest, 1895, Act I
>
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