[datatable-help] Extract Single Column as Vector

[Eduard Antonyan]

Well, looking at the documentation:

j: A single column name, single expresson of column names, list() of
expressions of column names, an expression or function call that evaluates
to list (including data.frame and data.table which are lists, too), or *(when
with=FALSE) same as j in [.data.frame.*
...
with:* *By default with=TRUE and j is evaluated within the frame of x. The
column names can be used as variables. *When with=FALSE, j works as it does
in [.data.frame.*

*
*
The bolded out part of the documentation doesn't match the actual behavior.



On Fri, May 17, 2013 at 2:44 PM, Frank Erickson <FErickson at psu.edu> wrote:

> @Arun and eddi: This question has come up before.
>
> http://r.789695.n4.nabble.com/Better-hacks-getting-a-vector-AND-using-with-inserting-chunks-of-rows-tt4666592.html
> (And I'm sure there are other times, too.) I can't say I've heard anyone
> arguing about it, though. :)
>
> I guess it works that way because
> ...in dt[ ,a], j is an expression which evaluates to a vector
> ...in dt[,"a",with=FALSE] the option turns on the "you must want one or
> more columns" mode, translating j from "a" to list(a)
>
> It's unintuitive if you're expecting data frame behavior (you know,
> drop=TRUE, as Arun mentioned), but if you've already seen dt[,list(a)], it
> shouldn't be much of a surprise. Adding the drop option, and maybe
> defaulting it to TRUE when with=FALSE might satisfy eddi's concern...?
>
>
> On Fri, May 17, 2013 at 10:22 AM, Eduard Antonyan <
> eduard.antonyan at gmail.com> wrote:
>
>> 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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>>
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