如何创建一个新列,Letter通过Year+ + 的唯一梳子组累计标识列中的新值外观Month？
数据样本.
require(data.table)
dt <- data.table(Letter = c(LETTERS[c(5, 1:2, 1:2, 1:4, 3:6)]),
Year = 2018,
Month = c(rep(5,5), rep(6,4), rep(7,4)))
打印.
Letter Year Month
1: E 2018 5
2: A 2018 5
3: B 2018 5
4: A 2018 5
5: B 2018 5
6: A 2018 6
7: B 2018 6
8: C 2018 6
9: D 2018 6
10: C 2018 7
11: D 2018 7
12: E 2018 7 …