How to subset() from data frame using specific rows

You can use something like this:

testdata <- c("A1", "A2", "A3", "B1", "B2", "B3")
grep("^A", testdata)

[1] 1 2 3

grepl("^A", testdata)

[1]  TRUE  TRUE  TRUE FALSE FALSE FALSE

Sarah,

   I don't see how this gives me a data frame containing only those sites I
specify. I want to plot by sites-within-streams specifying which param
factor to use.

Thanks,

Rich