How to count unique combinations from a data table in R?

Question

I have a data table with three columns. The first two are a collection of the data points (categorical data that can be either A, B, or C). The third column is a concatenation o

Answer 1

You can also do something like this- Note- As mentioned by @chinsoon12, we can use pmin & pmax

 > setDT(dt1)[,list(Count=.N) ,paste(pmin(CAT1, CAT2), pmax(CAT1, CAT2), sep=' & ')]
   paste Count
1: a & a     1
2: b & b     2
3: c & c     2
4: a & b     2
5: a & c     3