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

> library(nlme)
> g <- factor(c("a","b","a","b","a","b","a","b","a","b","a","b"))
> v <- c(1,4,1,4,1,4,2,8,2,8,2,8)
> cs <- rep(0,12)
> d <- data.frame(g,v,cs)
> d <- d[order(d$g),]
> temp <- by(d$v,d$g,cumsum)
> d$cs <- do.call("c",temp)
> d
   g v cs
1  a 1  1
3  a 1  2
5  a 1  3
7  a 2  5
9  a 2  7
11 a 2  9
2  b 4  4
4  b 4  8
6  b 4 12
8  b 8 20
10 b 8 28
12 b 8 36

Another solution using the by function, but I had to order the data first