r - 在多行的单列中查找最大日期

Question

我有以下数据框:

id       <- c(1,1,2,3,3)
date     <- c("23-01-08","01-11-07","30-11-07","17-12-07","12-12-08")
df       <- data.frame(id,date)
df$date2 <- as.Date(as.character(df$date), format = "%d-%m-%y")


id     date      date2
1   23-01-08 2008-01-23
1   01-11-07 2007-11-01
2   30-11-07 2007-11-30
3   17-12-07 2007-12-17
3   12-12-08 2008-12-12

现在我需要创建第四列并为每个 id 插入最大交易日期在那里面。
决赛 table 应如下:

id     date      date2        max
1   23-01-08 2008-01-23 2008-01-23
1   01-11-07 2007-11-01   0
2   30-11-07 2007-11-30 2007-11-30 
3   17-12-07 2007-12-17   0
3   12-12-08 2008-12-12 2008-12-12

如果你能帮我解决这个问题，我将不胜感激。

Answer 1

id<-c(1,1,2,3,3)
date<-c("23-01-08","01-11-07","30-11-07","17-12-07","12-12-08")
df<-data.frame(id,date)
df$date2<-as.Date(as.character(df$date), format = "%d-%m-%y")
# aggregate can be used for this type of thing
d = aggregate(df$date2,by=list(df$id),max)
# And merge the result of aggregate 
# with the original data frame
df2 = merge(df,d,by.x=1,by.y=1)
df2

  id     date      date2          x
1  1 23-01-08 2008-01-23 2008-01-23
2  1 01-11-07 2007-11-01 2008-01-23
3  2 30-11-07 2007-11-30 2007-11-30
4  3 17-12-07 2007-12-17 2008-12-12
5  3 12-12-08 2008-12-12 2008-12-12

编辑:由于您希望当日期与最大日期不匹配时最后一列为“空”，因此您可以尝试下一行。

df2[df2[,3]!=df2[,4],4]=NA

df2
  id     date      date2          x
1  1 23-01-08 2008-01-23 2008-01-23
2  1 01-11-07 2007-11-01       <NA>
3  2 30-11-07 2007-11-30 2007-11-30
4  3 17-12-07 2007-12-17       <NA>
5  3 12-12-08 2008-12-12 2008-12-12

当然，清理列名等总是很好的，但我把它留给你。