[datatable-help] Extract Single Column as Vector

[Frank Erickson]

@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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