Question

I know it is a very silly question but I could not sort it out that is why asking…
How can I extract the rows from a large data set by common IDs and take the means of these rows and make a column having these IDs as rownames.
e.g.

IDs Var2    
Ae4 2       
Ae4 4
Ae4 6
Bc3 3   
Bc3 5   
Ad2 8
Ad2 7

OutPut
Var(x)
Ae4 4
Bc3 4
Ad2 7.5

Answer 1

This kinds of things can easily be done using the plyr function ddply:

dat = data.frame(ID = rep(LETTERS[1:5], each = 20), value = runif(100))
> head(dat)
  ID      value
1  A 0.45800889
2  A 0.11221072
3  A 0.58833532
4  A 0.70056704
5  A 0.08337996
6  A 0.05195357

ddply(dat, .(ID), summarize, mn = mean(value))
  ID        mn
1  A 0.4960083
2  B 0.5809681
3  C 0.4512388
4  D 0.5079790
5  E 0.5397708

If your dataset is big, and/or the number of unique ID‘s is big, you could use data.table. See this paper for more detail about plyr.