data.frame - how to calculate the number of rows

Hello,

I had to give some more information to my posting:

as ouput there are more than one row, depending on the values of column A
(transformierung to distinct values, here 3 and 4, but there are much 
more)

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

Output:

       A            B                C                      num rows
1      3            6                 6.16                  3
2      4            5                 4.00                  3

B is mean and C variance.


thank you
Macki

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