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

You can try

library(dplyr)
df %>% 
  group_by(ID) %>%
  slice(which.max(as.Date(date, '%m/%d/%Y')))

data

df <- data.frame(ID= rep(1:3, each=3), date=c('02/20/1989',
'03/14/2001', '02/25/1990',  '04/20/2002', '02/04/2005', '02/01/2008',
'08/22/2011','08/20/2009', '08/25/2010' ), stringsAsFactors=FALSE)