[datatable-help] Incrementing number in j-expression to get group index fails (bug?)

[Matthew Dowle]

Hi,

With groupInd set to 0 in the global environment, I get the same. The
first group (internally, and you don't normally need to know this) is
specially run first. data.table uses the result of the first group to
infer properies about the query and make some (very often, very good)
guesses. Then it races through the remaining groups in C.

So, just use <<-, to make it clear it's the variable in the global
environment you want to update. So, no bug :-)

> groupInd = 0
> dt[, list(z, groupInd <<- groupInd + 1), by=list(x,y)]
     x y z V2
[1,] 0 0 1  1
[2,] 0 1 2  2
[3,] 0 1 4  2
[4,] 1 0 3  3
[5,] 1 0 5  3
[6,] 1 1 6  4
>

Matthew


> Hi all,
>
> I have still another issue, but I promise this will be my last posting
> for today ;-)
>
> Suppose you have the following table:
>
>  > dt <- data.table(x=c(0,0,1,0,1,1), y=c(0,1,0,1,0,1), z=1:6)
>  > dt
>      x y z
> [1,] 0 0 1
> [2,] 0 1 2
> [3,] 1 0 3
> [4,] 0 1 4
> [5,] 1 0 5
> [6,] 1 1 6
>
> I want to group the table by columns x and y and get for each z (which
> is really just a sort of primary key) the index of the group it belongs
> to. Grouping gives:
>
>  > dt[,z,by=list(x,y)]
>      x y z
> [1,] 0 0 1
> [2,] 0 1 2
> [3,] 0 1 4
> [4,] 1 0 3
> [5,] 1 0 5
> [6,] 1 1 6
>
> The information I need looks like: z=1 assigned to group 1, z=2 assigned
> to group 2, z=4 assigned to group 2 etc.
>
> I tried this by incrementing a counter in the j expression, but the
> result lets me suspect that there is a bug somewhere:
>
>  > dt[, list(z, groupInd <- groupInd + 1), by=list(x,y)]
>      x y z V2
> [1,] 0 0 1  1
> [2,] 0 1 2  1
> [3,] 0 1 4  1
> [4,] 1 0 3  2
> [5,] 1 0 5  2
> [6,] 1 1 6  3
>
> Incrementing the group counter only works from the second group on. Any
> ideas?
>
> Best regards,
>
> Andreas
>
> --
> Andreas Borg
> Medizinische Informatik
>
> UNIVERSITÄTSMEDIZIN
> der Johannes Gutenberg-Universität
> Institut für Medizinische Biometrie, Epidemiologie und Informatik
> Obere Zahlbacher Straße 69, 55131 Mainz
> www.imbei.uni-mainz.de
>
> Telefon +49 (0) 6131 175062
> E-Mail: borg at imbei.uni-mainz.de
>
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