Consider the following df,
考虑下面的df,
group = c('A','A','A','B','B','B','C','C')
time = c(-1,0,2,0,4,0,3,5)
value = seq(1:8)
df <- data.frame(group,time,value)
> df
group time value
1 A -1 1
2 A 0 2
3 A 2 3
4 B 0 4
5 B 4 5
6 B 0 6
7 C 3 7
8 C 5 8
group =
