[R] data frame

[Rhett Eckstein]

Dear R users:

> s4 <- seq(length=10, from=1, by=5)
> s<-data.frame(s4,s4,s4)
I would like to do some modification to s.
And I want the form like the following,if it is possible, how should I do?
The last column is the sum of previous three column.
   s4 s4.1 s4.2    sum
1   1                    1
2    6                   6
3   11    1            12
4   16    6            22
5   21   11    1     33
6   26   16    6     48
7   31    21   11   63
8   36    26   16   78
9   41    31   21   93
10 46    36   26   108


Thanks for any help !!