[R] counting frequencies across two columns

[Jason Priem]

I've got a data frame describing comments on an electronic journal, 
wherein each row is a unique comment, like so:

  commentID  author articleID
1         1   smith         2
2         2   jones         3
3         3 andrews         2
4         4   jones         1
5         5 johnson         3
6         6   smith         2

I want know the number of unique authors per article.  I can get a table 
of article frequencies with table(articleID), but I can't figure out how 
to count frequencies in a different column.  I'm sure there's an easy 
way, but I guess I'm too new at this to find it.  Thanks for your help!

Jason Priem
PhD student, School of Information and Library Science, University of 
North Carolina-Chapel Hill