[datatable-help] basic data.table query

[Steve Lianoglou]

Hi,

On Fri, Dec 3, 2010 at 7:45 PM, Santosh Srinivas
<santosh.srinivas at gmail.com> wrote:
> Someone in r-help recommended me to post the question in this group … TIA
>
>
>
> 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

I'm not sure if I'm missing something, but this looks like what you want, no?

R> dt[, list(D=mean(A)), by='B']
     B        D
[1,] 1 1.000000
[2,] 2 1.666667
[3,] 3 2.333333
[4,] 4 3.000000

-- 
Steve Lianoglou
Graduate Student: Computational Systems Biology
 | Memorial Sloan-Kettering Cancer Center
 | Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact