Get last row of each group in R [duplicate]

Answer 1

tail() returns the last 6 items of a subsettable object. When using aggregate(), the parameters to the FUN argument are passed immediately after the function using a comma; here 1 refers to n = 1, which tells tail() to only return the last item.

aggregate(a[, c('NUM', 'VAL')], list(a$ID), tail, 1)

# Group.1 NUM VAL
# 1       A   2   0
# 2       B   3   0
# 3       C   9   0

Answer 2

You might try:

a %>% 
  group_by(ID) %>% 
  arrange(NUM) %>%  
  slice(n())

Answer 3

One dplyr option could be:

a %>%
 arrange(ID, NUM) %>% 
 group_by(ID) %>% 
 summarise_all(last)

  ID      NUM   VAL
  <fct> <dbl> <dbl>
1 A        2.    0.
2 B        4.    1.
3 C        9.    0.

Or since dplyr 1.0.0:

a %>%
 arrange(ID, NUM) %>% 
 group_by(ID) %>% 
 summarise(across(everything(), last))

Answer 4

You can use top_n. (grouping already sorts by ID, and sorting by NUM isn't necessary since there's only 1 value)

library(dplyr)

a %>% 
  group_by(ID) %>% 
  top_n(1, NUM)

# # A tibble: 3 x 3
# # Groups:   ID [3]
#   ID      NUM   VAL
#   <fct> <dbl> <dbl>
# 1 A         2     0
# 2 B         4     1
# 3 C         9     0