Elixir lists vs Elixir tuples

Along with maps, Lists and Tuples are both ubiquitous and somewhat surprising for Elixir learners. Here's a quick guide.

Lists are linked lists

Elixir lists look just like arrays from C, Java or JavaScript, but they're not. They're not laid out contiguously in memory. They're a series of nodes where each node holds a piece of data and a pointer to the next node. The final node points to nothing.

This has performance implications. In order to read the final element in a list, you must traverse every element from the head to the tail of the list! Prepending an element to the front of the list is cheap, O(1), but accessing an element at the end is O(n)

Tuples are more like Arrays

Elixir tuples are stored contiguously in memory. Accessing any element is O(1). On the down-side, inserting or deleting an element involves copying the entire tuple, which is O(n). Remember, like everything else in the language, Tuples are immutable!

List elements are usually of the same type

This is more of a convention than a rule. Since lists implement the Enumerable protocol and it's very common to use functions like Enum.map, Enum.reduce and Enum.each that pass each element into a function, life is easier if everything in the list has the same shape.

Tuples, on the other hand are very often mixed types. They're commonly returned from functions and pattern matched against and in this case the first element is usually a status. For example:

case retrieve_stuff do
  {:ok, stuff} -> process(stuff)
  {:err, msg} -> Logger.error("Error with message: %{msg}")
end

Converting lists and tuples is easy

You can convert a list to a tuple with List.to_tuple(some_list) and you can convert a tuple to a list with Tuple.to_list(some_tuple). For example:

my_list = [1, 2, 3]
my_tuple = {:a, :b, :c}

other_tuple = List.to_tuple(my_list)   # equals {1, 2, 3}
other_list = Tuple.to_list(my_tuple)   # equals [:a, :b, :c]

Common questions

How do I get the nth element of a list?

Use Enum.at/2. For example, Enum.at([1, 2, 3, 4], 2) will return 3 since lists are zero-indexed.

How do I get the nth element of a tuple?

Use Kernel.elem/2. For example, elem({:a, :b, :c}, 0) will return :a since tuples are also zero-indexed.

How do I insert an element into a list?

Use List.insert_at/3. For example, List.insert_at([:foo, :bar, :baz], 1, :yolo) returns [:foo, :yolo, :bar, :baz].

You can delete elements in the same way with List.delete_at/2.

If you're prepending to the beginning of a list (which is very performant), then you can also use a cons syntax and insert an element like this:

my_list = [1, 2, 3]
[:hello | my_list]  # returns [:hello, 1, 2, 3]

How do I insert an element into a tuple?

Use Tuple.insert_at/3. For example, Tuple.insert_at({:ok, :stuff}, 1, :yolo) returns {:ok, :yolo, :stuff}.

You can delete elements in the same way with Tuple.delete_at/2.

How do I get the length of a list or a tuple?

Use Kernel.tuple_size/1 for tuples and use Enum.count/1 for lists.

What's a good reference if I need to know more?

It's best to go straight to the official docs!

• Elixir docs: Lists
• Elixir docs: Tuples

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