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

My tool of choice for these things is the plyr package:

require(plyr)
> ddply(d,.(g),transform,cs = cumsum(v))
   g v cs
1  a 1  1
2  a 1  2
3  a 1  3
4  a 2  5
5  a 2  7
6  a 2  9
7  b 4  4
8  b 4  8
9  b 4 12
10 b 8 20
11 b 8 28
12 b 8 36