[R] data management question

[Deepankar Basu]

Hi all,

I have a data management question. I am using an panel dataset read into 
R as a dataframe, call it "ex". The variables in "ex" are: id  year  x

id: a character string which identifies the unit
year: identifies the time period
x: the variable of interest (which might contain NAs).

Here is an example:
 > id <- rep(c("A","B","C"),2)
 > year <- c(rep(1970,3),rep(1980,3))
 > x <- c(20,30,40,25,35,45)
 > ex <- data.frame(id=id,year=year,x=x)
 > ex
  id year  x
1  A 1970 20
2  B 1970 30
3  C 1970 40
4  A 1980 25
5  B 1980 35
6  C 1980 45


I want to draw a subset of "ex" by selecting only the A and B units:

 > ex1 <- subset(ex[which(ex$id=="A"|ex$id=="B"),])

Now I want to do some computations on x for each unit:

 > tapply(ex1$x, ex1$id, mean)
   A    B    C
22.5 32.5   NA

But this gives me an NA value for the unit C, which I thought I had 
already left out. How do I ensure that the computation (in the last 
step) is limited to only the units I have selected in the first step?

Deepankar