Counting unique items in data frame

Question

I want a simple count of the number of subjects in each condition of a study. The data look something like this:

subjectid  cond   obser variable
1234                


        

Answer 1

Just to give you even more choice, you could also use tapply

tapply(a$subjectid, a$cond, function(x) length(unique(x)))
1 2 
2 1