[datatable-help] Revisiting scoping rules in "j" (reviving Gabor's post)

[Eduard Antonyan]

I haven't checked yet what it does currently but what you wrote makes
perfect sense.
 On Nov 10, 2013 5:44 AM, "Arunkumar Srinivasan" <aragorn168b at gmail.com>
wrote:

>  Hi everyone,
>
> To revive the discussion Gabor started here:
> http://r.789695.n4.nabble.com/Problem-with-FAQ-2-8-tt4668878.html and the
> (related, but subtly different) FR mnel filed here:
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=2693&group_id=240&atid=978
>
> Suppose you have:
>
> require(data.table)
> d1 <- data.table(id1 = c(1L, 2L, 2L, 3L), val = 1:4, key = "id1")
> d2 <- data.table(id2 = c(1L, 2L, 4L), val2 = c(11, 12, 14),key = "id2")
>
> Then as Gabor points out: `d1[d2, id1]`  should *not* result in an error,
> because FAQ 2.8 states (copied from Gabor's post linked above):
>
> 1. The scope of X's subset; i.e., X's column names.
> 2. The scope of each row of Y; i.e., Y's column names (join inherited
> scope)
> …
>
> In this case, the desired output for `d1[d2, id1]` should then be:
>    id1 id1
> 1:   1   1
> 2:   2   2
> 3:   2   2
> 4:   4  NA
>
> That's what I at least understand from what the documentation intends.
>
> However, this recommends a subtle change to the current method of
> referring to columns, if we were to keep this idea. That is, consider the
> data.table "d3" as follows:
>
> d3 <- copy(d2)
> setnames(d3, names(d1))
>
> Now, what should `d1[d3, id1]` give? The answer, I believe, is same as
> `d1[d2, id1]`. Why? Because, X's (here d1's) column names should be looked
> up first (as per FAQ 2.8). Therefore, corresponding to d2=c(1,2,4), the
> values for "id1" are c(1, (2,2), NA). Now, if the old behaviour is to be
> intended - here comes the "subtle change", then one should do:
>
> d1[d3, i.d1] # referring to i's variables with the "i." notation.
>
> I've managed to implement the first part where X's columns are looked up
> so that `d1[d2, id1]` doesn't result in error. However, I'd like to ensure
> that my understanding of the FAQ is right (and that the FAQ makes sense -
> it does to me).
>
> Please let me know what you all think so that I can implement the second
> part and commit. This, I believe will let us get a step closer to the
> consistency in DT syntax.
>
> Arun
>
>
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