How do you get duplicates in a list in elixir? - functional-programming

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

Answer 3

If you want to get a unique list of all duplicates
def get_uniq_duplicates(all) do
all |> Enum.reduce({[], []}, fn val, {once, duplicates} ->
if once |> Enum.member?(val) do
if duplicates |> Enum.member?(val) do
{once, duplicates}
else
{once, duplicates ++ [val]}
end
else
{once ++ [val], duplicates}
end
end) |> elem(1)
end
If you want a list of duplicates where only one copy of each value has been removed, eg. ["a", "b", "c", "c", "c"] -> ["c", "c"]
Then you can use the simpler:
def get_duplicates(all) do
all |> Enum.reduce({[], []}, fn val, {once, duplicates} ->
if once |> Enum.member?(val) do
{once, duplicates ++ [val]}
else
{once ++ [val], duplicates}
end
end) |> elem(1)
end

Answer 4

Using Enum.group_by/3
["ash", "bob", "cat", "bob", "ash"]
|> Enum.group_by(&(&1))
|> Enum.filter(&((&1 |> Enum.count()) > 1))
|> Enum.map(&(&1 |> Enum.uniq()))
|> List.flatten()
This method supports searching structs for specific criteria.