How to create dummy that equals 1 (and 0 otherwise) if an id appears only once? (in R)

structure(list(id = c(1L, 1L, 2L, 3L, 3L, 4L), hire_year = c(2017L, 
2017L, 2017L, 2017L, 2016L, 2016L)), class = "data.frame", row.names = c(NA, 
-6L))
  id hire_year
1  1      2017
2  1      2017
3  2      2017
4  3      2017
5  3      2016
6  4      2016

**Expected output**
  id hire_year dummy
1  1      2017     0
2  1      2017     0
3  2      2017     1
4  3      2017     0
5  3      2016     0
6  4      2016     1

How to create dummy that equals 1 (and 0 otherwise) if an id appears only once?

>Solution :

With tidyverse, we can group by the id, then use the number of observations within an ifelse statement.

library(tidyverse)

df %>%
  group_by(id) %>%
  mutate(dummy = ifelse(n() == 1, 1, 0))

Or we could add the number of observations, then change the value based on the condition.

df %>% 
  add_count(id, name = "dummy") %>% 
  mutate(n = ifelse(n == 1, 1, 0))

Output

  id hire_year dummy
1  1      2017     0
2  1      2017     0
3  2      2017     1
4  3      2017     0
5  3      2016     0
6  4      2016     1