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

I would use ave. If you look at the source of ave, you'll see it essentially wraps Martin Morgan's solution.

R> g <- factor(c("a","b","a","b","a","b","a","b","a","b","a","b"))
R> v <- c(1,4,1,4,1,4,2,8,2,8,2,8)
R> d <- data.frame(g,v)
R> d$cs <- ave(v, g, FUN=cumsum)
R> 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