split dataframe cumulatively by variable level - r

Question

With a df like this:
x=data.frame(id=c(1,1,1,2,2,2,3,3,3), val=c(1,2,3,2,3,4,1,3,0))
I want to get output like this:
[[1]]
id val
1 1 1
2 1 2
3 1 3
[[2]]
id val
1 1 1
2 1 2
3 1 3
4 2 2
5 2 3
6 2 4
[[3]]
id val
1 1 1
2 1 2
3 1 3
4 2 2
5 2 3
6 2 4
7 3 1
8 3 3
9 3 0
where the df is split into a list of as many dataframes as there are levels of the splitting variable, i.e. id. Each dataframe should start at the first level and include all rows up to each successive level.
I can do this with a loop:
out<-NULL
for(i in 1:3){
out[[i]] <- x[x$id<=i,]
}
out
However, is there a simpler method using e.g. split that I am overlooking? Ideally a one liner.

Answer 1

You can do this in base R with split and Reduce using the accumulate=TRUE argument. split is used to split the data.frame into a list of data.frames by by ID. Reduce is applies rbind to each list element and adding the accumulate=TRUE successively combines the data.frames in the list.
Reduce(rbind, split(x, x$id), accumulate=TRUE)
[[1]]
id val
1 1 1
2 1 2
3 1 3
[[2]]
id val
1 1 1
2 1 2
3 1 3
4 2 2
5 2 3
6 2 4
[[3]]
id val
1 1 1
2 1 2
3 1 3
4 2 2
5 2 3
6 2 4
7 3 1
8 3 3
9 3 0