dplyr function on grouped data to get the value of a variable when a different variable is equal to its min value

I have the following dataset:

id <- c(1,1,1,2,2,2)
year <- c(2012, 2013, 2014, 2012, 2013, 2014)
assignment <- c(0,1,1,0,0,0)
df <- cbind.data.frame(id, year, assignment)

I want to mutate a new variable which takes the value of whatever the assignment variable was equal to in the minimum year value per group. For example, for ID 1, the value of assignment in the smallest year (2012) is 0, so the new variable will be zero in all instances of ID 1.

>Solution :

You could group_by id, arrange by year and get the first value of assignment:

library(dplyr, warn=FALSE)

df %>% 
  group_by(id) %>% 
  arrange(year) %>% 
  mutate(new_assignment = first(assignment)) %>% 
  ungroup()
#> # A tibble: 6 × 4
#>      id  year assignment new_assignment
#>   <dbl> <dbl>      <dbl>          <dbl>
#> 1     1  2012          0              0
#> 2     2  2012          0              0
#> 3     1  2013          1              0
#> 4     2  2013          0              0
#> 5     1  2014          1              0
#> 6     2  2014          0              0