您显然对将角色[month]用作Date变量不感兴趣.考虑到我没错，您可以执行以下操作:

You apparently are not interested in taking your Character [month] as a Date variable. Considering that I'm not wrong you could simply do something like this:

library(dplyr)

tab %>%
  group_by(month, variable) %>%
  summarise(a_sum=sum(amount),
            a_mean=(mean(amount)))

并得到这个:

Source: local data frame [3 x 4]
Groups: month

  month variable a_sum a_mean
1 1-Jan        x  4000   2000
2 2-Feb        y  3000   3000
3 2-Feb        z  5000   5000