[R] data.frame - how to calculate the number of rows

[Pfeiffer & Koberstein Immobilien GmbH - Ralf Pfeiffer]

Hello,

it seems to be a simple problem, but  I couldn't find an answer in the 
archiv. (I think, it must has something to do with the group-select, like in 
php)

I've the following data.frame:

        A            B                C
1      3             6                 5
2      4             4                 20
3      5             8                 2

I want to get the number of the rows in the 4th column, like:

        A            B                C                      num rows
1      3            6                 6.16                  3


(B is mean, C is variance by using summaryBy(B+C~ A,  data=daten, 
FUN=c(mean,var))  from the doBy-package)

But, how can I add the column 'numrow' and count the number of the rows?

Thanks!

Macki