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

For any solution, you might as well correct your date variable first, as shown by @akrun:

df$date <- as.Date(df$date, '%m/%d/%Y')

Base R

df[
  tapply(1:nrow(df),df$ID,function(ii) ii[which.max(df$date[ii])])
,]

This uses a selection of row numbers to subset the data. You can see the selection by running the middle line (between the []s) on its own.

Data.table

Similar to @rawr's:

require(data.table)
DT <- data.table(df)

unique(DT[order(date)], by="ID", fromLast=TRUE)
# or
unique(DT[order(-date)], by="ID")