[datatable-help] BUG: droplevels mangles subsetted data.table

[Matthew Dowle]

I see the problem too but (just) adding droplevels.data.table might miss
the root cause.

> because the way the
> droplevels.data.frame method works isn't compatible with data.table
> indexing.

But it's intended to be. I can see the switch at the top of [.data.table
is detecting the caller isn't data.table aware, and it is then dispatching
to `[.data.frame` but why it then isn't working I'm not sure. Something to
do with the missing j or missing drop not being passed through correctly,
perhaps.

I have heard it said (once or twice) that data.table is "almost"
compatible with non-data.table-aware packages, but never had an example
before. I wonder if this is it!

A (fast) droplevels.data.table using := would be good anyway, though.

Matthew



> Hi,
>
> I see what the problem is -- we need to provide a
> droplevels.data.table S3 method, because the way the
> droplevels.data.frame method works isn't compatible with data.table
> indexing.
>
> Will fix:
>
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=1841&group_id=240&atid=975
>
> Thanks for raising the flag.
>
> Cheers,
> -steve
>
> On Tue, Feb 21, 2012 at 12:38 PM, pchalasani <pchalasani at gmail.com> wrote:
>>  Surprising that this wasn't noticed before, or perhaps I'm not
>> following
>> some recommended idiom to drop levels when using  data.table. The
>> following
>> code illustrates the bug clearly: The bug remains regardless of whether
>> I
>> use "subset" or simply use dt1 = dt[ name != 'a' ].
>>
>>
>>
>>    d <- data.table(name = c('a','b','c'), value = 1:3)
>>    dt <- data.table(d)
>>    setkey(dt,'name')
>>    dt1 <- subset(dt,name != 'a')  # or dt1 <- dt[ name != 'a' ]
>>    > dt1
>>          name value
>>     [1,]    b     2
>>     [2,]    c     3
>>
>>    > droplevels(dt1)
>>          name value
>>     [1,]    b     1
>>     [2,]    c     3
>>
>>
>>
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/BUG-droplevels-mangles-subsetted-data-table-tp4407694p4407694.html
>> Sent from the datatable-help mailing list archive at Nabble.com.
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>
>
>
> --
> Steve Lianoglou
> Graduate Student: Computational Systems Biology
>  | Memorial Sloan-Kettering Cancer Center
>  | Weill Medical College of Cornell University
> Contact Info: http://cbio.mskcc.org/~lianos/contact
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