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 ([email protected]) 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 <[email protected]> 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 <[email protected]> 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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