[datatable-help] Odd behavior with [.data.table and J

[Christian Hudon]

On Mon, Aug 6, 2012 at 1:31 PM, Matthew Dowle <mdowle at mdowle.plus.com>wrote:

> > Hello,
> >
> > I'm running into an odd behavior with data.table. Given the following
> > variables:
> >
> >> tbl = data.table(foo=c(1,2,3), bar=c(1.1, 2.2, 3.3))
> >> setkey(tbl, foo)
> >> i = data.frame(foo=1)
>
> And
>    i = data.table(foo=1)
> gives the same results below, so it doesn't seem related to whether i is a
> data.table -vs- a data.frame.
>
> >
> > ... I would expect the following three ways of indexing "tbl" using "i"
> to
> > give the same result, but they don't:
> >
> >> tbl[i]
> >    foo bar
> > 1:   1 1.1
> >> tbl[J(i)]
> > Error in `[.data.table`(tbl, J(i)) :
> >   typeof x.foo (double) != typeof i.V1 (list)
> >> tbl[data.table(i)]
> >    foo bar
> > 1:   1 1.1
> >
> > Anything I'm missing on why tbl[J(i)] wouldn't work like the other two?
> Or
> > have I hit a bug? I'm running R 2.15.1 64bit on Windows 7, with
> data.table
> > 1.8.2.
>
> J inside [] is an alias for list(), not data.table(). I don't think this
> changed in 1.8.2 but might be wrong. data.table() is much heavier than
> list() checking argument types up front and recycling vectors to ensure
> each item of the data.table has the same length, for instance.
> data.table() also unpacks data.frame and data.table arguments like a cbind
> would. Whereas list() treats data.table and data.frame arguments as though
> they are list columns.
>
> So it's doing something sensible and is correct behaviour on first glance.
> Please confirm this makes sense, and if so I'll add as as FR to improve
> error message and documentation.
>

Okay, this makes sense. But pretty much the whole of the documentation on
that topic is wrong, then. Hence my confusion.

As a single example (there are others), in the "Introduction to the
data.table package in R" document, page 6:

[...] Since we do this a lot, there is an alias for data.table called J(),
> short for join.
>

Thanks for your help,

  Christian
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