r - r-为每个组选择最后n次出现

Question

情况

我有一个数据框df:

df <- structure(list(person = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 
    3L, 3L), .Label = c("pA", "pB", "pC"), class = "factor"), date = structure(c(16071, 
    16102, 16130, 16161, 16071, 16102, 16130, 16071, 16102), class = "Date")), .Names = c("person", 
    "date"), row.names = c(NA, -9L), class = "data.frame")

> df
  person       date
1     pA 2014-01-01
2     pA 2014-02-01
3     pA 2014-03-01
4     pA 2014-04-01
5     pB 2014-01-01
6     pB 2014-02-01
7     pB 2014-03-01
8     pC 2014-01-01
9     pC 2014-02-01

问题

如何为每个人选择按日期排序的最后2个(或'n')条目，这样我就得到了一个数据框 df1:

> df1
  person       date
1     pA 2014-03-01
2     pA 2014-04-01
3     pB 2014-02-01
4     pB 2014-03-01
5     pC 2014-01-01
6     pC 2014-02-01

？

我试过组合

library(dplyr)
df1 <- df %>%
    group_by(person) %>% 
    select(tail(df, 2))

没有喜悦。

Answer 1

您可以尝试slice

library(dplyr)
df %>% 
    group_by(person) %>% 
    arrange(date, person) %>%
    slice((n()-1):n())
#  person       date
#1     pA 2014-03-01
#2     pA 2014-04-01
#3     pB 2014-02-01
#4     pB 2014-03-01
#5     pC 2014-01-01
#6     pC 2014-02-01

或代替最后一步

   do(tail(., 2))