Filter dataframe by maximum values in each group [duplicate]

Answer 1

aggregate should also work:

aggregate(date ~ id, df, max)

Answer 2

Here's a simple and fast approach using data.table package

library(data.table)
setDT(df)[, .SD[which.max(date)], id]
#    id date
# 1:  1 2012
# 2:  3 2014
# 3:  2 2014

Or (could be a bit faster because of keyed by

setkey(setDT(df), id)[, .SD[which.max(date)], id]

Or using OPs idea via the data.table package

unique(setorder(setDT(df), id, -date), by = "id")

Or

setorder(setDT(df), id, -date)[!duplicated(id)]

Or base R solution

with(df, tapply(date, id, function(x) x[which.max(x)]))
##    1    2    3 
## 2012 2014 2014 

Another way

library(dplyr)
df %>%
  group_by(id) %>%
  filter(date == max(date)) # Will keep all existing columns but allow multiple rows in case of ties
# Source: local data table [3 x 2]
# Groups: id
# 
#   id date
# 1  1 2012
# 2  2 2014
# 3  3 2014

Or

df %>%
  group_by(id) %>%
  slice(which.max(date)) # Will keep all columns but won't return multiple rows in case of ties

Or

df %>%
  group_by(id) %>%
  summarise(max(date)) # Will remove all other columns and wont return multiple rows in case of ties