如何在每个组中创建一个滞后变量?

I have a data.table:

我有一个data.table:

set.seed(1)
data <- data.table(time = c(1:3, 1:4),
                   groups = c(rep(c("b", "a"), c(3, 4))),
                   value = rnorm(7))

data
#    groups time      value
# 1:      b    1 -0.6264538
# 2:      b    2  0.1836433
# 3:      b    3 -0.8356286
# 4:      a    1  1.5952808
# 5:      a    2  0.3295078
# 6:      a    3 -0.8204684
# 7:      a    4  0.4874291
set.se