[datatable-help] Skipping some Vi names

[Steve Lianoglou]

Hi Joseph,

On Thu, Jul 14, 2011 at 4:43 PM, Joseph Voelkel <jgvcqa at rit.edu> wrote:
> Continuing with the example below, here is another problem: names() returns
> NA’s for all Vi names except V1.
>
>> (dt2<-dt[,list("sum(A1),sum(A2),sum(A3)"),by="x,y"])
>
>      x y V1 V4 V5
> [1,] 1 1  1  7 13
> [2,] 1 2  4 10 16
> [3,] 2 1  2  8 14
> [4,] 2 3  5 11 17
> [5,] 3 2  3  9 15
> [6,] 3 3  6 12 18
>
>> names(dt2)
>
> [1] "x"  "y"  "V1" NA   NA

I'm curious why you put your expression in quotes? Did you see that in
the manual somewhere?

Not doing that fixes your problems:

R> (dt2<-dt[,list(sum(A1),sum(A2),sum(A3)),by="x,y"])
     x y V1 V2 V3
[1,] 1 1  1  7 13
[2,] 1 2  4 10 16
[3,] 2 1  2  8 14
[4,] 2 3  5 11 17
[5,] 3 2  3  9 15
[6,] 3 3  6 12 18

and

R> names(dt2)
[1] "x"  "y"  "V1" "V2" "V3"

All columns are named, and there are no "gaps" in the colnames ...

I guess Matthew can comment on why this happens, but just use normal
expressions/blocks for your `j` expression in the meantime :-)

If it's because you have a lot of column names, note that you can also do:

R> dt[,lapply(.SD, sum), by="x,y"]
dt[,lapply(.SD, sum), by="x,y"]
     x y A1 A2 A3
[1,] 1 1  1  7 13
[2,] 1 2  4 10 16
[3,] 2 1  2  8 14
[4,] 2 3  5 11 17
[5,] 3 2  3  9 15
[6,] 3 3  6 12 18

Note also that the column names are different here, too.

HTH,
-steve

-- 
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