Assign group id start from 0 and end with 1 in R

You can simply use ave in base R:

with(DT, ave(v1, c(0, cumsum(v1)[-length(v1)]), FUN = seq_along))

#[1] 1 2 3 4 1 2 3

Or if you want in dplyr also you can use the lag() and with that just create groups and you can assign row numbers in each and every group:

library(dplyr)

DT %>% group_by(gr = lag(cumsum(v1), default = 0)) %>% mutate(ID = row_number())