是这样吗?
> df <- data_frame(month = c(1, 2, 1, 1, 2), year = c(rep(2000, 3), rep(2011, 2)), sales = runif(5) * 1000)
> df
# A tibble: 5 × 3
month year sales
<dbl> <dbl> <dbl>
1 1 2000 818.5468
2 2 2000 794.9369
3 1 2000 477.8024
4 1 2011 692.4940
5 2 2011 891.0750
> ddply(df, .(year), summarise, sales = sum(sales)) %>% tbl_df
# A tibble: 2 × 2
year sales
<dbl> <dbl>
1 2000 2091.286
2 2011 1583.569