如何在R中的每个组中选择“x”最新值？

我试图在R中的数据框中选择/筛选每个组中的最新值。例如，我想从以下数据框中的每个名称组中选择3个最新值，即最接近今天的日期：

Player  Date    Result
 Sam    03/15/2015  1
 Sam    03/22/2015  0
 Sam    04/04/2015  2
 Sam    04/12/2015  1
 Sam    04/18/2015  1
 Sam    04/26/2015  0
 Sam    08/08/2015  3
Steve   02/17/2015  0
Steve   02/21/2015  0
Steve   03/04/2015  4
Steve   03/11/2015  2
Steve   03/15/2015  1
Steve   03/22/2015  0
Steve   04/12/2015  0
Steve   04/18/2015  2
Steve   04/26/2015  1
Steve   04/29/2015  2
Steve   08/16/2015  4
Jasper  03/15/2015  3
Jasper  03/22/2015  3.5
Jasper  04/04/2015  4
Jasper  04/12/2015  4
Jasper  04/18/2015  5
Jasper  04/26/2015  0

我已经编写了as.date代码，因此R现在可以理解日期格式，但我现在可以使用什么代码只选择每组中的3个最新值

我们可以使用dplyr。我们使用as.Date将“Date”类转换为Date类。按“Player”分组后，我们将“Date”列向下排列，并使用slice获取最近的3个值。如果不想更改“Date”类，可以删除mutate步骤，并在arrange中进行转换，即arrangedescas.datedatedate，“%m/%d/%Y”

或者在我们按“Player”分组后，我们可以通过指定“n”和“wt”变量来使用top\n进行排序

 df1 %>% 
   mutate(Date=as.Date(Date, '%m/%d/%Y')) %>%
   group_by(Player)  %>%
   top_n(n = 3, Date)
#  Player       Date Result
#1    Sam 2015-04-18      1
#2    Sam 2015-04-26      0
#3    Sam 2015-08-08      3
#4  Steve 2015-04-26      1
#5  Steve 2015-04-29      2
#6  Steve 2015-08-16      4
#7 Jasper 2015-04-12      4
#8 Jasper 2015-04-18      5
#9 Jasper 2015-04-26      0

使用data.table，我们将“data.frame”转换为“data.table”setDTdf1。按照“玩家”分组，我们在转换为日期类后，对“日期”进行排序，并使用头部可以得到每组的前3行

library(data.table)
setDT(df1)[order(-as.IDate(Date, '%m/%d/%Y')),head(.SD, 3) , by = Player]
#   Player       Date Result
#1:  Steve 08/16/2015      4
#2:  Steve 04/29/2015      2
#3:  Steve 04/26/2015      1
#4:    Sam 08/08/2015      3
#5:    Sam 04/26/2015      0
#6:    Sam 04/18/2015      1
#7: Jasper 04/26/2015      0
#8: Jasper 04/18/2015      5
#9: Jasper 04/12/2015      4

数据

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你自己已经试过什么了？为什么不起作用？这个解决方案的可能副本看起来很棒，非常感谢！我将能够在几天内正确地测试它，并将报告给您确认。@WillT-E谢谢。很高兴帮助你。我和高层一起去的，效果很好。好的是，如果其中一个组长度小于“n”，它不会给出错误消息。“它只包括对该组可用的任何观察结果。”WillT-E感谢您的反馈。slice也应该以这种方式工作。例如，df1%>%mutateDate=as.DateDate，'%m/%d/%Y%%>%group\U byPlayer%>%arrangedescDate%>%1:7

library(data.table)
setDT(df1)[order(-as.IDate(Date, '%m/%d/%Y')),head(.SD, 3) , by = Player]
#   Player       Date Result
#1:  Steve 08/16/2015      4
#2:  Steve 04/29/2015      2
#3:  Steve 04/26/2015      1
#4:    Sam 08/08/2015      3
#5:    Sam 04/26/2015      0
#6:    Sam 04/18/2015      1
#7: Jasper 04/26/2015      0
#8: Jasper 04/18/2015      5
#9: Jasper 04/12/2015      4

df1 <- structure(list(Player = c("Sam", "Sam", "Sam", "Sam", "Sam", 
"Sam", "Sam", "Steve", "Steve", "Steve", "Steve", "Steve", "Steve", 
"Steve", "Steve", "Steve", "Steve", "Steve", "Jasper", "Jasper", 
"Jasper", "Jasper", "Jasper", "Jasper"), Date = c("03/15/2015", 
"03/22/2015", "04/04/2015", "04/12/2015", "04/18/2015", "04/26/2015", 
"08/08/2015", "02/17/2015", "02/21/2015", "03/04/2015", "03/11/2015", 
"03/15/2015", "03/22/2015", "04/12/2015", "04/18/2015", "04/26/2015", 
"04/29/2015", "08/16/2015", "03/15/2015", "03/22/2015", "04/04/2015", 
"04/12/2015", "04/18/2015", "04/26/2015"), Result = c(1, 0, 2, 
1, 1, 0, 3, 0, 0, 4, 2, 1, 0, 0, 2, 1, 2, 4, 3, 3.5, 4, 4, 5, 
0)), .Names = c("Player", "Date", "Result"),
class = "data.frame", row.names = c(NA,  -24L))