Hi,

   As an example, how can I get the data such that field a of ab, ab["a"],
equals 3? I expect the answer to be the union of 2 and 4, as

   Thanks,

> a<-c(1,3,4,3,5,6,5)
> b<-c(2,4,6,7,3,1,2)
> ab<-data.frame(a,b)
> ab
  a b
1 1 2
2 3 4
3 4 6
4 3 7
5 5 3
6 6 1
7 5 2

> ab[a==3]
Error in `[.data.frame`(ab, a == 3) : undefined columns selected
> ab[ab["a"]==3]
[1] 3 3 4 7

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