List
Linked lists hold zero, one, or more elements in the chosen order.
Lists in Elixir are specified between square brackets:
[1, "two", 3, :four]
Two lists can be concatenated and subtracted using the
Kernel.++/2 and Kernel.--/2 operators:
[1, 2, 3] ++ [4, 5, 6][1, true, 2, false, 3, true] -- [true, false]
An element can be prepended to a list using |:
new = 0
list = [1, 2, 3]
[new | list]Lists in Elixir are effectively linked lists, which means they are internally represented in pairs containing the head and the tail of a list:
[head | tail] = [1, 2, 3]
headtail
Similarly, we could write the list [1, 2, 3] using only
such pairs (called cons cells):
[1 | [2 | [3 | []]]]Some lists, called improper lists, do not have an empty list as the second element in the last cons cell:
[1 | [2 | [3 | 4]]]
Although improper lists are generally avoided, they are used in some
special circumstances like iodata and chardata entities (see the IO module).
Due to their cons cell based representation, prepending an element to a list is always fast (constant time), while appending becomes slower as the list grows in size (linear time):
list = [1, 2, 3]
# fast
[0 | list]# slow
list ++ [4]
Most of the functions in this module work in linear time. This means that,
that the time it takes to perform an operation grows at the same rate as the
length of the list. For example length/1 and last/1 will run in linear
time because they need to iterate through every element of the list, but
first/1 will run in constant time because it only needs the first element.
Lists also implement the Enumerable protocol, so many functions to work with
lists are found in the Enum module. Additionally, the following functions and
operators for lists are found in Kernel:
-
++/2 -
--/2 -
hd/1 -
tl/1 -
in/2 -
length/1
Charlists
If a list is made of non-negative integers, where each integer represents a Unicode code point, the list can also be called a charlist. These integers must:
-
be within the range
0..0x10FFFF(0..1_114_111); -
and be out of the range
0xD800..0xDFFF(55_296..57_343), which is reserved in Unicode for UTF-16 surrogate pairs.
Elixir uses single quotes to define charlists:
'héllo'In particular, charlists will be printed back by default in single quotes if they contain only printable ASCII characters:
'abc'Even though the representation changed, the raw data does remain a list of numbers, which can be handled as such:
inspect('abc', charlists: :as_list)Enum.map('abc', fn num -> 1000 + num end)
You can use the IEx.Helpers.i/1 helper to get a condensed rundown on
charlists in IEx when you encounter them, which shows you the type, description
and also the raw representation in one single summary.
The rationale behind this behaviour is to better support
Erlang libraries which may return text as charlists
instead of Elixir strings. In Erlang, charlists are the default
way of handling strings, while in Elixir it’s binaries. One
example of such functions is Application.loaded_applications/0:
Application.loaded_applications()
# => [
# => {:stdlib, 'ERTS CXC 138 10', '2.6'},
# => {:compiler, 'ERTS CXC 138 10', '6.0.1'},
# => {:elixir, 'elixir', '1.0.0'},
# => {:kernel, 'ERTS CXC 138 10', '4.1'},
# => {:logger, 'logger', '1.0.0'}
# => ]
A list can be checked if it is made of only printable ASCII
characters with ascii_printable?/2.
Improper lists are never deemed as charlists.
Function ascii_printable?/2
Checks if list is a charlist made only of printable ASCII characters.
Takes an optional limit as a second argument. ascii_printable?/2 only
checks the printability of the list up to the limit.
A printable charlist in Elixir contains only the printable characters in the standard seven-bit ASCII character encoding, which are characters ranging from 32 to 126 in decimal notation, plus the following control characters:
-
?\a- Bell -
?\b- Backspace -
?\t- Horizontal tab -
?\n- Line feed -
?\v- Vertical tab -
?\f- Form feed -
?\r- Carriage return -
?\e- Escape
For more information read the Character groupssection in the Wikipedia article of the ASCII standard.
Examples
List.ascii_printable?('abc')List.ascii_printable?('abc' ++ [0])List.ascii_printable?('abc' ++ [0], 2)Improper lists are not printable, even if made only of ASCII characters:
List.ascii_printable?('abc' ++ ?d)Function delete/2
Deletes the given element from the list. Returns a new list without
the element.
If the element occurs more than once in the list, just
the first occurrence is removed.
Examples
List.delete([:a, :b, :c], :a)List.delete([:a, :b, :c], :d)List.delete([:a, :b, :b, :c], :b)List.delete([], :b)Function delete_at/2
Produces a new list by removing the value at the specified index.
Negative indices indicate an offset from the end of the list.
If index is out of bounds, the original list is returned.
Examples
List.delete_at([1, 2, 3], 0)List.delete_at([1, 2, 3], 10)List.delete_at([1, 2, 3], -1)Function duplicate/2
Duplicates the given element n times in a list.
n is an integer greater than or equal to 0.
If n is 0, an empty list is returned.
Examples
List.duplicate("hello", 0)List.duplicate("hi", 1)List.duplicate("bye", 2)List.duplicate([1, 2], 3)Function first/2
Returns the first element in list or default if list is empty.
first/2 has been introduced in Elixir v1.12.0, while first/1 has been available since v1.0.0.
Examples
List.first([])List.first([], 1)List.first([1])List.first([1, 2, 3])Function flatten/1
Flattens the given list of nested lists.
Empty list elements are discarded.
Examples
List.flatten([1, [[2], 3]])List.flatten([[], [[], []]])Function flatten/2
Flattens the given list of nested lists.
The list tail will be added at the end of
the flattened list.
Empty list elements from list are discarded,
but not the ones from tail.
Examples
List.flatten([1, [[2], 3]], [4, 5])List.flatten([1, [], 2], [3, [], 4])Function foldl/3
Folds (reduces) the given list from the left with a function. Requires an accumulator, which can be any value.
Examples
List.foldl([5, 5], 10, fn x, acc -> x + acc end)List.foldl([1, 2, 3, 4], 0, fn x, acc -> x - acc end)List.foldl([1, 2, 3], {0, 0}, fn x, {a1, a2} -> {a1 + x, a2 - x} end)Function foldr/3
Folds (reduces) the given list from the right with a function. Requires an accumulator, which can be any value.
Examples
List.foldr([1, 2, 3, 4], 0, fn x, acc -> x - acc end)List.foldr([1, 2, 3, 4], %{sum: 0, product: 1}, fn x, %{sum: a1, product: a2} ->
%{sum: a1 + x, product: a2 * x}
end)Function improper?/1
Returns true if list is an improper list. Otherwise returns false.
Examples
List.improper?([1, 2 | 3])List.improper?([1, 2, 3])Function insert_at/3
Returns a list with value inserted at the specified index.
Note that index is capped at the list length. Negative indices
indicate an offset from the end of the list.
Examples
List.insert_at([1, 2, 3, 4], 2, 0)List.insert_at([1, 2, 3], 10, 0)List.insert_at([1, 2, 3], -1, 0)List.insert_at([1, 2, 3], -10, 0)Function keydelete/3
Receives a list of tuples and deletes the first tuple
where the element at position matches the
given key. Returns the new list.
Examples
List.keydelete([a: 1, b: 2], :a, 0)List.keydelete([a: 1, b: 2], 2, 1)List.keydelete([a: 1, b: 2], :c, 0)Function keyfind/4
Receives a list of tuples and returns the first tuple
where the element at position in the tuple matches the
given key.
If no matching tuple is found, default is returned.
Examples
List.keyfind([a: 1, b: 2], :a, 0)List.keyfind([a: 1, b: 2], 2, 1)List.keyfind([a: 1, b: 2], :c, 0)Function keyfind!/3
Receives a list of tuples and returns the first tuple
where the element at position in the tuple matches the
given key.
If no matching tuple is found, an error is raised.
Examples
List.keyfind!([a: 1, b: 2], :a, 0)List.keyfind!([a: 1, b: 2], 2, 1)List.keyfind!([a: 1, b: 2], :c, 0)Function keymember?/3
Receives a list of tuples and returns true if there is
a tuple where the element at position in the tuple matches
the given key.
Examples
List.keymember?([a: 1, b: 2], :a, 0)List.keymember?([a: 1, b: 2], 2, 1)List.keymember?([a: 1, b: 2], :c, 0)Function keyreplace/4
Receives a list of tuples and if the identified element by key at position
exists, it is replaced with new_tuple.
Examples
List.keyreplace([a: 1, b: 2], :a, 0, {:a, 3})List.keyreplace([a: 1, b: 2], :a, 1, {:a, 3})Function keysort/2
Receives a list of tuples and sorts the elements
at position of the tuples. The sort is stable.
Examples
List.keysort([a: 5, b: 1, c: 3], 1)List.keysort([a: 5, c: 1, b: 3], 0)Function keystore/4
Receives a list of tuples and replaces the element
identified by key at position with new_tuple.
If the element does not exist, it is added to the end of the list.
Examples
List.keystore([a: 1, b: 2], :a, 0, {:a, 3})List.keystore([a: 1, b: 2], :c, 0, {:c, 3})Function keytake/3
Receives a list of tuples and returns the first tuple
where the element at position in the tuple matches the
given key, as well as the list without found tuple.
If such a tuple is not found, nil will be returned.
Examples
List.keytake([a: 1, b: 2], :a, 0)List.keytake([a: 1, b: 2], 2, 1)List.keytake([a: 1, b: 2], :c, 0)Function last/2
Returns the last element in list or default if list is empty.
last/2 has been introduced in Elixir v1.12.0, while last/1 has been available since v1.0.0.
Examples
List.last([])List.last([], 1)List.last([1])List.last([1, 2, 3])Function myers_difference/2
Returns a keyword list that represents an edit script.
The algorithm is outlined in the “An O(ND) Difference Algorithm and Its Variations” paper by E. Myers.
An edit script is a keyword list. Each key describes the “editing action” to
take in order to bring list1 closer to being equal to list2; a key can be
:eq, :ins, or :del. Each value is a sublist of either list1 or list2
that should be inserted (if the corresponding key :ins), deleted (if the
corresponding key is :del), or left alone (if the corresponding key is
:eq) in list1 in order to be closer to list2.
See myers_difference/3 if you want to handle nesting in the diff scripts.
Examples
List.myers_difference([1, 4, 2, 3], [1, 2, 3, 4])Function myers_difference/3
Returns a keyword list that represents an edit script with nested diffs.
This is an extension of myers_difference/2 where a diff_script function
can be given in case it is desired to compute nested differences. The function
may return a list with the inner edit script or nil in case there is no
such script. The returned inner edit script will be under the :diff key.
Examples
List.myers_difference(["a", "db", "c"], ["a", "bc"], &String.myers_difference/2)Function pop_at/3
Returns and removes the value at the specified index in the list.
Negative indices indicate an offset from the end of the list.
If index is out of bounds, the original list is returned.
Examples
List.pop_at([1, 2, 3], 0)List.pop_at([1, 2, 3], 5)List.pop_at([1, 2, 3], 5, 10)List.pop_at([1, 2, 3], -1)Function replace_at/3
Returns a list with a replaced value at the specified index.
Negative indices indicate an offset from the end of the list.
If index is out of bounds, the original list is returned.
Examples
List.replace_at([1, 2, 3], 0, 0)List.replace_at([1, 2, 3], 10, 0)List.replace_at([1, 2, 3], -1, 0)List.replace_at([1, 2, 3], -10, 0)Function starts_with?/2
Returns true if list starts with the given prefix list; otherwise returns false.
If prefix is an empty list, it returns true.
Examples
List.starts_with?([1, 2, 3], [1, 2])List.starts_with?([1, 2], [1, 2, 3])List.starts_with?([:alpha], [])List.starts_with?([], [:alpha])Function to_atom/1
Converts a charlist to an atom.
Elixir supports conversions from charlists which contains any Unicode code point.
Inlined by the compiler.
Examples
List.to_atom('Elixir')List.to_atom('🌢 Elixir')Function to_charlist/1
Converts a list of integers representing Unicode code points, lists or strings into a charlist.
Note that this function expects a list of integers representing
Unicode code points. If you have a list of bytes, you must instead use
the :binary module.
Examples
List.to_charlist([0x00E6, 0x00DF])List.to_charlist([0x0061, "bc"])List.to_charlist([0x0064, "ee", ['p']])Function to_existing_atom/1
Converts a charlist to an existing atom. Raises an ArgumentError
if the atom does not exist.
Elixir supports conversions from charlists which contains any Unicode code point.
Inlined by the compiler.
Examples
_ = :my_atom
List.to_existing_atom('my_atom')_ = :"🌢 Elixir"
List.to_existing_atom('🌢 Elixir')Function to_float/1
Returns the float whose text representation is charlist.
Inlined by the compiler.
Examples
List.to_float('2.2017764e+0')Function to_integer/1
Returns an integer whose text representation is charlist.
Inlined by the compiler.
Examples
List.to_integer('123')Function to_integer/2
Returns an integer whose text representation is charlist in base base.
Inlined by the compiler.
The base needs to be between 2 and 36.
Examples
List.to_integer('3FF', 16)Function to_string/1
Converts a list of integers representing code points, lists or strings into a string.
To be converted to a string, a list must either be empty or only contain the following elements:
- strings
- integers representing Unicode code points
- a list containing one of these three elements
Note that this function expects a list of integers representingUnicode code points. If you have a list of bytes, you must instead use
the :binary module.
Examples
List.to_string([0x00E6, 0x00DF])List.to_string([0x0061, "bc"])List.to_string([0x0064, "ee", ['p']])List.to_string([])Function to_tuple/1
Converts a list to a tuple.
Inlined by the compiler.
Examples
List.to_tuple([:share, [:elixir, 163]])Function update_at/3
Returns a list with an updated value at the specified index.
Negative indices indicate an offset from the end of the list.
If index is out of bounds, the original list is returned.
Examples
List.update_at([1, 2, 3], 0, &(&1 + 10))List.update_at([1, 2, 3], 10, &(&1 + 10))List.update_at([1, 2, 3], -1, &(&1 + 10))List.update_at([1, 2, 3], -10, &(&1 + 10))Function wrap/1
Wraps term in a list if this is not list.
If term is already a list, it returns the list.
If term is nil, it returns an empty list.
Examples
List.wrap("hello")List.wrap([1, 2, 3])List.wrap(nil)Function zip/1
Zips corresponding elements from each list in list_of_lists.
The zipping finishes as soon as any list terminates.
Examples
List.zip([[1, 2], [3, 4], [5, 6]])List.zip([[1, 2], [3], [5, 6]])
