r - Counting unique items in data frame -

I want a simple number of the number of topics in each condition of a study. The data looks something like this:

  subject conductor observer variable 1234 1 1 12 1234 1 2 14 2143 2 1 9 3456 1 1 12 3456 1 2 14 3456 1 3 13 etc. etc.   
 This is a large dataset and it is not always clear how many unique topics contribute in each condition, etc.  
 I have this data in the frame.  
 What do I want  
  cond of ss 1 122 2 98   
 Where for each "status" I get a count In that situation, unique SS's contribution is the number of data it seems that painful should be simple.  plyr  to use the  ddply  function   
 
 
 Package:  
  Required (plyr) df & lt; - data.frame (subjectid = sample (1: 3,7, t), cond = sample (1: 2,7, T), audience = sample (1: 7)) gt; Ddply (df. (Cond), abbreviation, Salt / Length (unique (subjectid)) Capacity 1 1 2 2 2   
  ddply  Function  cond  variable, and produces the summary column  NumSubs  for each sub-data frame.