Get first and last values per group – dplyr group_by with last() and first()

Question

The code below should group the data by year and then create two new columns with the first and last value of each year.

library(dplyr)

set.seed(123)

d <- data.frame(
    group = rep(1:3, each = 3),
    year = rep(seq(2000,2002,1),3),
    value = sample(1:9, r = T))

d %>% 
    group_by(group) %>%
    mutate(
        first = dplyr::first(value),
        last = dplyr::last(value)
    )

However, it does not work as it should. The expected result would be

  group  year value first  last
  <int> <dbl> <int> <int> <int>
1     1  2000     3     3     4
2     1  2001     8     3     4
3     1  2002     4     3     4
4     2  2000     8     8     1
5     2  2001     9     8     1
6     2  2002     1     8     1
7     3  2000     5     5     5
8     3  2001     9     5     5
9     3  2002     5     5     5

Yet, I get this (it takes the first and the last value over the entire data frame, not just the groups):

  group  year value first  last
  <int> <dbl> <int> <int> <int>
1     1  2000     3     3     5
2     1  2001     8     3     5
3     1  2002     4     3     5
4     2  2000     8     3     5
5     2  2001     9     3     5
6     2  2002     1     3     5
7     3  2000     5     3     5
8     3  2001     9     3     5
9     3  2002     5     3     5

Answer 1

dplyr::mutate() did the trick

d %>% 
    group_by(group) %>%
    dplyr::mutate(
        first = dplyr::first(value),
        last = dplyr::last(value)
    )

Answer 2

You can also try by using summarise function within dpylr to get the first and last values of unique groups

 d %>% 
    group_by(group) %>% 
        summarise(first_value = first(na.omit(values)),
            last_value = last(na.omit(values))) %>% 
               left_join(d, ., by = 'group')