[datatable-help] Bug report #5100 reg.

[Eduard Antonyan]

I agree that it's inconsistent with data.frame, and imo that's a good
thing. We don't replicate the drop argument, so it wouldn't be possible to
return a data.table when with=FALSE and either way drop=TRUE by default is
a bad design choice in data.frame and matrix (that is unlikely to change
given R-core's attitude towards that type of a thing).

I'm always pro more and better documentation :)


On Thu, Nov 14, 2013 at 10:33 AM, Arunkumar Srinivasan <
aragorn168b at gmail.com> wrote:

>  Eddi, At the least, I think the documentation needs to be clearer on the
> use of "with=FALSE". It does feel inconsistent with the fact that "j" with
> a single column should return a vector. In data.frames, the type in "j"
> being column names, if it's just one column name, would return a vector,
> unless drop = FALSE. That is, DF[, "y"] will return a vector while DF[,
> c("x", "y")] will return a data.frame. So, it is inconsistent with
> data.frame here, I think.
>
>
> Arun
>
> On Thursday, November 14, 2013 at 5:25 PM, Eduard Antonyan wrote:
>
> DT[, y] returning a vector is I think the only correct behavior, given the
> understanding of j-expression as something evaluated in the DT environment.
> If they want a data.table they should simply use DT[, list(y)] or DT[,
> data.table(y)].
>
> I haven't thought about DT[, "y", with = FALSE] before as I pretty much
> never use that form, but I see an argument for it staying as is, because
> "y" and c("y") are the same and since we all presumably agree that DT[,
> c("y", "z"), with = FALSE] should return a data.table. If DT[, c("y"), with
> = FALSE] returned a different type that would mean inconsistent return
> types which makes life much harder for users (as evidenced by the periodic
> drop=FALSE questions that come up on SO).
>
> Going back to DT[, y], note that y and list(y) actually produce
> *different* results (in e.g. base_env), so there is no type consistency
> issue there between DT[, y] and DT[, list(y, z)].
>
>
> On Thu, Nov 14, 2013 at 6:09 AM, Arunkumar Srinivasan <
> aragorn168b at gmail.com> wrote:
>
>  Hi everybody,
>
> It'd be nice if you could weigh-in on the bug report filed by Bill here:
>
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=5100&group_id=240&atid=975
>
> The gist of it is:
>
> require(data.table)
> DT <- data.table(x=1:5, y=6:10, z=11:15)
> DT[, y] # returns a vector
> DT[, "y", with=FALSE] # returns a data.table
>
> The question from the bug report basically is: "why is that in the first
> case, 'j' has only one column and we get a vector, but in the second case,
> we get a data.table?"
>
> My question is: Is this behaviour okay or do you prefer that the first one
> returns a data.table as well or the second one (with "with=FALSE") returns
> a vector?
>
> Thank you,
> Arun
>
>
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