How can I delete certain rows according to two columns which have symmetricl values in data.table in R?

Question

For example, I have a table as follows:

DT <- data.table(
  A = c(1,1,1,2,2,2,3,3,3), 
  B = c(1,2,3,1,2,3,1,2,3),
  key = \"A\"
)

I wand to

Answer 1

Another option:

DT[, g := paste(B, A, sep="_")][A < B, g := paste(A, B, sep="_")][!duplicated(g), !"g"]

   A B
1: 1 1
2: 1 2
3: 1 3
4: 2 2
5: 2 3
6: 3 3

So ...

1. make a grouping variable as A + B,
2. flip the order to B + A on subset A < B or A > B
3. dedupe on the grouping variable

The last step could alternately be unique(DT, by="g").