[R] data.table query

[Gabor Grothendieck]

On Fri, Dec 3, 2010 at 6:38 PM, Santosh Srinivas
<santosh.srinivas at gmail.com> wrote:
> Hello Group,
>
> I need a modification in the data.table example to get my intended
> result shown below ... is there a more simple way!
>
> dt <- data.table(A = rep(1:3, each=4), B = rep(1:4, each=3), C = rep(1:2, 6))
> dt[, transform(.SD,D=mean(A)), by="B"]
>
>
> The result I want is below ... which is probably long winded!
>
> data.table(unique(as.data.frame(dt[, transform(.SD,D=mean(A)),
> by="B"][,list(B,D)])))
>     B        D
> [1,] 1 1.000000
> [2,] 2 1.666667
> [3,] 3 2.333333
> [4,] 4 3.000000
>

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