[R] index and by groups statement

[Robert Waters]

Dear R-users

Im working with a dataset that contains information
for 8 groups of data and I need to select a sample of
certain size (100 cubic feet by group) from this
database for each of these 8 groups. To clarify, here
is the starting code Im working with:

k<-nrow(dataset)
ix<-sort(runif(k),index.return=TRUE)$ix
M<-max(which(cumsum(dataset$volume[ix])<100))+1 
test<-dataset[ix[1:M],]

However, I don't know how to specify in this code the
instruction: "by groups"

Does anyone have an idea how to do this?

Thanks in advance

RW