[BioC] selecting probe set from expression matrix

[Srinivas Iyyer]

dear list,

I generated expressions for a study in the following
way.

rm.data <- justRMA()
ex.data <- exprs(rm.data)

Now I want to select 217523_at, 212063_at,
212014_x_at. 

How can I select these probesets. 

> dim(data)
[1] 22283   158


2 question: how do I know data is which type of
object. 

thank you
srini