[datatable-help] Return Select/Join that does NOT match?

[Branson Owen]

Thank you very much for a quick answer to [1] and [2], Matthew! Here
is an example for [3]

using:

R-2.11.0 x32
data.table1.4

> DT = data.table(A = c("o", "x"), B = 1:10, key = "A")

      A  B
 [1,] o  1
 [2,] o  3
 [3,] o  5
 [4,] o  7
 [5,] o  9
 [6,] x  2
 [7,] x  4
 [8,] x  6
 [9,] x  8
[10,] x 10

> DT[CJ(unique(A))]
Error hint: the i expression sees the column variables. Column names
(variables) will mask variables in the calling frame. Check for any
conflicts.
Error in `[.data.table`(DT, CJ(unique(A))) :
Error in unique(A) : object 'A' not found

> DT[  {  CJ(unique(A))  }  ]
     A B
[1,] o 1
[2,] x 2

Interestingly, I can replace CJ() with data.table(), then I don't need {}

> DT[data.table(unique(A))]
     A B
[1,] o 1
[2,] x 2

*** NOTE that I don't use {} like the example using CJ()

Finally, I can also use "by", but the FAQ says "i expression" is faster

 > DT[, B[1], by = A]
     A B
[1,] o 1
[2,] x 2


> [3] Might be a bug there. Please can you provide a small reproducible
> example and version information. I'm assuming by key1, key2 you really
> mean col1, col2. Thanks, Matthew

>> [3] I thought I can do this:
>>
>> DataTable[ CJ( FN(key1), FN(key2), FN(key3) ) ], but it complains
>> about column names.
>> *FN is a function
>>
>> Later I found I can do this, DataTable[ { CJ( FN(key1), FN(key2),
>> FN(key3) ) } ],
>> I just add { } outside CJ
>>
>> Don't understand why, but at least it works. I really wonder whether I
>> should do this or there is a more correct syntax?