I have a table with two columns: A 1 A 1 A 2 B 2 C 0 I would like to produce a third column that contains the counts of each unique combination of col1 and col2: A 1 2 A 1 2 A 2 1 B 2 1 C 0 1 How can I do this in R? Thanks in advance ... -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.