r - 在事件之前计算行数 - data.table

Question

我有一个数据集，其中包含用户和顺序事件以及中间的非事件。

DT = data.table(user = c("1001","1001","1001","1001","1001","1001",
                          "1002","1002","1002","1002"), 
               event = c(NA,"e1",NA,NA,NA,"e2",
                           "e1",NA,NA,"e2"))

我希望能够在用户组发生事件之前计算行数(非事件)。预期结果:

   user  event  rows.before.event
 1: 1001    NA                 NA
 2: 1001    e1                  1
 3: 1001    NA                 NA
 4: 1001    NA                 NA
 5: 1001    NA                 NA
 6: 1001    e2                  3
 7: 1002    e1                  0
 8: 1002    NA                 NA
 9: 1002    NA                 NA
10: 1002    e2                  2

试过 rleid()但没有成功。欢迎任何建议。

Answer 1

DT[, count := .N-1, by = .(user, rev(cumsum(rev(!is.na(event)))))][
   is.na(event), count := NA]
#    user event count
# 1: 1001    NA    NA
# 2: 1001    e1     1
# 3: 1001    NA    NA
# 4: 1001    NA    NA
# 5: 1001    NA    NA
# 6: 1001    e2     3
# 7: 1002    e1     0
# 8: 1002    NA    NA
# 9: 1002    NA    NA
#10: 1002    e2     2