Apply a function to groups within a data.frame in R

Question

I am trying to get the cumulative sum of a variable (v) for groups (\"a\" and \"b\") within a dataframe. How can I get the result at the bottom -- whose rows are even number

Answer 1

split<- is a pretty weird beast

split(d$cs, d$g) <- lapply(split(d$v, d$g), cumsum)

leading to

> d
   g v cs
1  a 1  1
2  b 4  4
3  a 1  2
4  b 4  8
5  a 1  3
6  b 4 12
7  a 2  5
8  b 8 20
9  a 2  7
10 b 8 28
11 a 2  9
12 b 8 36