How to add count of unique values by group to R data.frame

Question

I wish to count the number of unique values by grouping of a second variable, and then add the count to the existing data.frame as a new column. For example, if the existing

Answer 1

Here's a solution with the dplyr package - it has n_distinct() as a wrapper for length(unique()).

df %>%
  group_by(color) %>%
  mutate(unique_types = n_distinct(type))