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        1        1      12   
1234        1        2      14
2143        2        1      19
3456        1        1      12 
3456        1        2      14 
3456        1        3      13   

etc       etc    etc       etc

This is a large dataset and it is not always obvious how many unique subjects contribute to each condition, etc.

I have this in a data.frame.

What I want is something like

cond   ofSs 
1       122 
2        98

Where for each "condition" I get a count of the number of unique Ss contributing data to that condition. Seems like this should be painfully simple.

Answer 1

Use the ddply function from the plyr package:

require(plyr)
df <- data.frame(subjectid = sample(1:3,7,T), 
                 cond = sample(1:2,7,T), obser = sample(1:7))

> ddply(df, .(cond), summarize, NumSubs = length(unique(subjectid)))
  cond NumSubs
1    1       1
2    2       2

The ddply function "splits" the data-frame by the cond variable, and produces a summary column NumSubs for each sub-data-frame.