r：重复元素的data.table计数

Question

我有点困惑。 我有一个类似的数据表：

library(data.table)
DT<-data.table(contract=c("c","c","c","d","d","d","d","f","f","f","f"),events=c("a","b","c","a","b","b","c","a","b","c","d"))


contract events
  c       a
  c       b
  c       c
  d       a
  d       b
  d       b
  d       c
  f       a
  f       b
  f       c
  f       d

我想通过跟踪重复来计算合同中的事件数量，最终得到类似的东西：

contract events seqevents
      c       a       1
      c       b       2
      c       c       3
      d       a       1
      d       b       2
      d       b       2
      d       c       3
      f       a       1
      f       b       2
      f       c       3
      f       d       4

首先，我尝试通过与此代码签订合同来获取简单的事件计数，然后尝试找出一种方法来解释同一事件的重复：

DT<-DT[ , seqevents := 1:.N , by = c("contract","events") ]

但是输出不正确，尽管它适用于不同的数据集。 关于它为什么不起作用的任何建议，我如何得到如上所述的计数？非常感谢