[R] cumsum and subsets of a data frame?

[Michael A. Miller]

I have a question about using cumsum on subsets of a data frame.
Suppose I have a frame that looks something like this

> tmp
       f x  y
1   left 1  0
2   left 2  0
3   left 3  9
4   left 4 10
5   left 5 23
6   left 6 45
7   left 7 13
8   left 8  2
9   left 9  6
10 right 1 10
11 right 2 26
12 right 3  9
13 right 4 50
14 right 5 78
15 right 6 20
16 right 7  7
17 right 8 20
18 right 9 19
 
I'm plotting things like this will lattice

> library(lattice)
> xyplot(y ~ x | f, data=tmp)

If I plot the cumsum with xyplot( cumsum(y) ~ x | f, data=tmp),
it is summed across the values of the factor f.  Can anyone
suggest a way to calculate the cumulative sum of y in this data
frame such that it is reset for each value of f?  The resulting
frame would look like this:

> tmp
       f x  y   s
1   left 1  0   0
2   left 2  0   0
3   left 3  9   9
4   left 4 10  19
5   left 5 23  42
6   left 6 45  87
7   left 7 13 100
8   left 8  2 102
9   left 9  6 108
10 right 1 10  10
11 right 2 26  36
12 right 3  9  45
13 right 4 50  95
14 right 5 78 173
15 right 6 20 193
16 right 7  7 200
17 right 8 20 220
18 right 9 19 239

I know how to calculate the pieces with, for example, 

> cumsum(tmp$y[tmp$f=='right'])

but I don't know how to get this piecewise into the data frame or
how to automate it.  Any suggestions?

Thanks, Mike

-- 
Michael A. Miller                               mmiller3 at iupui.edu
  Imaging Sciences, Department of Radiology, IU School of Medicine
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