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

[Agrarimmobilien]

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
>
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