[datatable-help] [data.table] Aggregate as vector

[Frank Erickson]

Oh, sorry, didn't notice you were making a new column, in that case:
dt[,V3:=list(list(V1)),by=V2]


On Thu, Jun 25, 2015 at 3:48 PM, Frank Erickson <fperickson at wisc.edu> wrote:

> Hi,
>
> I think you want
>
> dt[,.(list(V1)),by=V2]
> #    V2  V1
> # 1:  a 1,4
> # 2:  b 2,3
>
>
> Using the .() notation makes it easier to keep track of what columns are
> being made, I find.
>
> --Frank
>
> On Thu, Jun 25, 2015 at 3:46 PM, Nicolas Paris <niparisco at gmail.com>
> wrote:
>
>> Hi,
>>
>> Here an example what I want to get :
>>
>> > dt <- data.table(c(1,2,3,4),c("a","b","b","a"))
>> > dt
>>    V1 V2
>> 1:  1  a
>> 2:  2  b
>> 3:  3  b
>> 4:  4  a
>>
>> What I want to get is a V3 column, containing an aggregated vector :
>> > dt
>>    V1 V2 V3
>> 1:  1  a   c(1,4)
>> 2:  2  b   c(2,3)
>>
>> That does not work :
>>
>> dt[,V3 := as.list(c(V1)),by=V2]
>>
>> Is there a way to do so ?
>>
>> Thanks
>>
>>
>>
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>>
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>>
>
>
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