[datatable-help] Bug report #5100 reg.

[Arunkumar Srinivasan]

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 (mailto: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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> 

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