[datatable-help] Extract Single Column as Vector

[Eduard Antonyan]

I don't remember discussing this issue...? What is the conceptual
difference between dt[, a] and dt[, "a", with = F] and what does 'drop'
have to do with this??


On Fri, May 17, 2013 at 10:02 AM, Arunkumar Srinivasan <
aragorn168b at gmail.com> wrote:

>  Eduard, are we discussing the same thing again :)? Wasn't this somehow
> your question as well.. the discrepancy between:
>
> dt[, a] and dt[, "a", with=FALSE].
>
> There should be a drop=TRUE/FALSE option (as in the case of data.frame)
> that should be used when you use `with=FALSE`. Until then, the default
> option seems to be drop=FALSE, which results in a data.table.
>
> Alexandre, as of now, it could be done as Eduard points out.
>
> Arun
>
> On Friday, May 17, 2013 at 4:59 PM, Eduard Antonyan wrote:
>
> Use dt[[colname]], but this seems like a bug to me - I would've thought
> that dt[, a] and dt[, "a", with = F] should return the exact same thing.
>
>
> On Fri, May 17, 2013 at 9:42 AM, Alexandre Sieira <
> alexandre.sieira at gmail.com> wrote:
>
> Sorry if this is a basic question.
>
>
> I'm using R 3.0.0 and data.table 1.8.8. The documentation for 'j' states
> that "A single column or single expression returns that type, usually a
> vector."
>
>
> I am able to obtain this behavior if I know the column name in advance:
>
>
> > dt = data.table(a=c(1, 2, 3), b=c(4, 5, 6))
>
> > dt
>
>    a b
>
> 1: 1 4
>
> 2: 2 5
>
> 3: 3 6
>
> > str(dt[,a])
>
>  num [1:3] 1 2 3
>
>
> However, if I don't, no such luck:
>
> > colname="a"
> > str(dt[,colname,with=F])
> Classes ‘data.table’ and 'data.frame': 3 obs. of  1 variable:
>  $ a: num  1 2 3
>  - attr(*, ".internal.selfref")=<externalptr>
>
> If there a way to extract an entire column as a vector if I have the
> column name as a character scalar?
>
> Thank you!
>
> --
> Alexandre Sieira
> CISA, CISSP, ISO 27001 Lead Auditor
>
> "The truth is rarely pure and never simple."
> Oscar Wilde, The Importance of Being Earnest, 1895, Act I
>
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