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

[Nicolas Paris]

Thanks a lot guys, I was sure that was possible
​

> ​
> The first list is for the syntax - (cols) := list(values)​


​Ok, I forgot this point​


2015-06-25 15:53 GMT-04:00 Arunkumar Srinivasan <aragorn168b at gmail.com>:

> To add to Frank’s reply, I think `dt[, V3 := .(list(V1)), by=V2]` doesn’t
> work with `1.9.4`, but has been fixed in 1.9.5.
> The first list is for the syntax - (cols) := list(values). Cases like
> `DT[, A := 1:5]` and is internally wrapped by `list()` for convenience, as
> they are very common use cases.
> --
> Arun
>
> On 25 Jun 2015 at 21:51:09, Frank Erickson (fperickson at wisc.edu) wrote:
>
> 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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