[datatable-help] Extract/Subset a data.table with multiple indices

[Sterkens, Patrick [JRDBE]]

R.experts,
The following may be asked & answered but I was not able to find the
answer. So here goes my first R-question.
I am using data.table v1.8.1 to benefit from the :=  with by=.

(stringAsFactors=F)

Say I have 
>DT=data.table(expand.grid(ID=c("ID1","ID2","ID3"),Tx=c("Ctl","Low","Med
ium","High"),>Target=c("IS","G1","G2")))
>setkey(DT, Target,Tx,ID)
>set.seed(1)
>DT[,value:=rnorm(36)]


..and I want to subset the data.table with all "Ctl"  (i.e. from the
second key Target) with a binary search.
Doing this

> DT["Ctl"] 

Does not generate the  desired result, because it searches under the
first key for "Ctl"

  Target ID Tx value TX
1:    Ctl NA NA    NA NA


While vector search does
> DT[Tx=="Ctl"]
    ID  Tx Target      value
1: ID1 Ctl     IS  0.6107264
2: ID2 Ctl     IS -0.9340976
3: ID3 Ctl     IS -1.2536334
4: ID1 Ctl     G1  0.5939462
5: ID2 Ctl     G1  0.3329504
6: ID3 Ctl     G1  1.0630998
7: ID1 Ctl     G2 -1.2765922
8: ID2 Ctl     G2 -0.5732654
9: ID3 Ctl     G2 -1.2246126

Is it possible to do a binary search on one of the keas only ? if yes,
how do you tell  data.table that ?

Thanks
Patrick

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