[R] Still trying to avoid loops

[Tom Wright]

Given a dataframe:
dat<-data.frame(S=factor(c(rep('a',2),rep('b',1),rep('c',3)),levels=c('b','a','c')),
		D=c(5,1,3,2,3,4))

where S is a subject identifier and D a visit (actually a date in my
real dataset). I would like to generate another column giving the visit
number

R=c(2,1,1,1,2,3)

My current solution uses nested loops and is slow and ugly. I've looked
at by() but can't see how to keep the order of R correct.

Thanks,
Tom