How do you get duplicates in a list in elixir?

Question

If you want to get duplicates instead of uniq values in a list, how would you do this in a quick, dense script that uses pattern matching?

For example, with an input of ["ash", "bob", "cat", "bob", "ash"] how could I get ["ash", "bob"] ?

Answer 1

Since you specified you wanted a quick, dense script, I think you should consider this solution:

l = ["ash", "bob", "cat", "bob", "ash", "ash"]

# to get all duplicates
l -- Enum.uniq(l) # => ["bob", "ash", "ash"]

# to get a unique list of duplicates
Enum.uniq(l -- Enum.uniq(l)) # => ["bob", "ash"]

Answer 2

Here is how I would do it:

["ash", "bob", "cat", "bob", "ash"]
|> (&((&1 -- (&1 |> Enum.uniq())) |> Enum.uniq())).()

This is the same as doing:

my_list = ["ash", "bob", "cat", "bob", "ash"]
(my_list -- (my_list |> Enum.uniq())) |> Enum.uniq()

What is happening:

1. get a list of all the unique values (the complement to what we want): my_list |> Enum.uniq()
2. Use list subtraction to get the complement of these unique values.
3. Use another call to Enum.uniq to get these "duplicates" in unique list.