Select row with most recent date by group

Question

I have a data frame in R where the rows represent events, and one column is the date of the event. The thing the event is happening to is described by an ID column. So for e

Answer 1

It's probably a character flaw but I sometimes resist picking up new packages. The "base R" functions can often do the job. In this case I think the alue of the dplyr package shows through since I stumbled in creating a good solution since the ave function returned a character value for a logical test, which I still don't understand. So I think dplyr is a real gem. And if I could I'd like to insist that any upvotes be preceded by an upvote to akrun's answer. (It's hard to believe this hasn't already been asked and answered on SO.)

Anyway:

> df[ as.logical(
        ave(df$date, df$ID, FUN=function(d) as.Date(d , '%m/%d/%Y') == 
                                             max(as.Date(d, '%m/%d/%Y'))))
      , ]
  ID       date
2  1 03/14/2001
6  2 02/01/2008
7  3 08/22/2011

I thought this should work (fail) :

> df[ ave(df$date, df$ID, FUN=function(d) as.Date(d , '%m/%d/%Y') ==max(as.Date(d, '%m/%d/%Y'))) , ]
     ID date
NA   NA 
NA.1 NA 
NA.2 NA 
NA.3 NA 
NA.4 NA 
NA.5 NA 
NA.6 NA 
NA.7 NA 
NA.8 NA 

Here's another base R solution that worked the first time with no surprises:

> do.call( rbind, by(df, df$ID, function(d) d[ which.max(as.Date(d$date, '%m/%d/%Y')), ] ) )
  ID       date
1  1 03/14/2001
2  2 02/01/2008
3  3 08/22/2011

Here's one inspired by @rawr's notion of taking the last one from an ordered subset:

> do.call( rbind, by(df, df$ID, function(d) tail( d[ order(as.Date(d$date, '%m/%d/%Y')), ] ,1)) )
  ID       date
1  1 03/14/2001
2  2 02/01/2008
3  3 08/22/2011