List

Functions that work on (linked) lists.

Lists in Elixir are specified between square brackets:

iex> [1, "two", 3, :four]
[1, "two", 3, :four]

Two lists can be concatenated and subtracted using the Kernel.++/2 and Kernel.--/2 operators:

iex> [1, 2, 3] ++ [4, 5, 6]
[1, 2, 3, 4, 5, 6]
iex> [1, true, 2, false, 3, true] -- [true, false]
[1, 2, 3, true]

Lists in Elixir are effectively linked lists, which means they are internally represented in pairs containing the head and the tail of a list:

iex> [head | tail] = [1, 2, 3]
iex> head
1
iex> tail
[2, 3]

Similarly, we could write the list [1, 2, 3] using only such pairs (called cons cells):

iex> [1 | [2 | [3 | []]]]
[1, 2, 3]

Some lists, called improper lists, do not have an empty list as the second element in the last cons cell:

iex> [1 | [2 | [3 | 4]]]
[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):

iex> list = [1, 2, 3]
iex> [0 | list]   # fast
[0, 1, 2, 3]
iex> list ++ [4]  # slow
[1, 2, 3, 4]

The Kernel module contains many functions to manipulate lists and that are allowed in guards. For example, Kernel.hd/1 to retrieve the head, Kernel.tl/1 to fetch the tail and Kernel.length/1 for calculating the length. Keep in mind that, similar to appending to a list, calculating the length needs to traverse the whole list.

Charlists

If a list is made of non-negative integers, it can also be called a charlist. Elixir uses single quotes to define charlists:

iex> 'héllo'
[104, 233, 108, 108, 111]

In particular, charlists may be printed back in single quotes if they contain only ASCII-printable codepoints:

iex> 'abc'
'abc'

The rationale behind this behaviour is to better support Erlang libraries which may return text as charlists instead of Elixir strings. 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'}]

List and Enum modules

This module aims to provide operations that are specific to lists, like conversion between data types, updates, deletions and key lookups (for lists of tuples). For traversing lists in general, developers should use the functions in the Enum module that work across a variety of data types.

In both Enum and List modules, any kind of index access on a list is linear. Negative indexes are also supported but they imply the list will be iterated twice, one to calculate the proper index and another to perform the operation.

Summary

Functions

delete(list, item)

Deletes the given item from the list. Returns a new list without the item

delete_at(list, index)

Produces a new list by removing the value at the specified index

duplicate(elem, n)

Duplicates the given element n times in a list

first(list)

Returns the first element in list or nil if list is empty

flatten(list)

Flattens the given list of nested lists

flatten(list, tail)

Flattens the given list of nested lists. The list tail will be added at the end of the flattened list

foldl(list, acc, function)

Folds (reduces) the given list from the left with a function. Requires an accumulator

foldr(list, acc, function)

Folds (reduces) the given list from the right with a function. Requires an accumulator

insert_at(list, index, value)

Returns a list with value inserted at the specified index

keydelete(list, key, position)

Receives a list of tuples and deletes the first tuple where the item at position matches the given key. Returns the new list

keyfind(list, key, position, default \\ nil)

Receives a list of tuples and returns the first tuple where the item at position in the tuple matches the given key

keymember?(list, key, position)

Receives a list of tuples and returns true if there is a tuple where the item at position in the tuple matches the given key

keyreplace(list, key, position, new_tuple)

Receives a list of tuples and replaces the item identified by key at position if it exists

keysort(list, position)

Receives a list of tuples and sorts the items at position of the tuples. The sort is stable

keystore(list, key, position, new_tuple)

Receives a list of tuples and replaces the item identified by key at position

keytake(list, key, position)

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

last(list)

Returns the last element in list or nil if list is empty

myers_difference(list1, list2)

Returns a keyword list that represents an edit script

pop_at(list, index, default \\ nil)

Returns and removes the value at the specified index in the list

replace_at(list, index, value)

Returns a list with a replaced value at the specified index

starts_with?(list, prefix)

Returns true if list starts with the given prefix list; otherwise returns false

to_atom(charlist)

Converts a charlist to an atom

to_existing_atom(charlist)

Converts a charlist to an existing atom. Raises an ArgumentError if the atom does not exist

to_float(charlist)

Returns the float whose text representation is charlist

to_integer(charlist)

Returns an integer whose text representation is charlist

to_integer(charlist, base)

Returns an integer whose text representation is charlist in base base

to_string(list)

Converts a list of integers representing codepoints, lists or strings into a string

to_tuple(list)

Converts a list to a tuple

update_at(list, index, fun)

Returns a list with an updated value at the specified index

wrap(list)

Wraps the argument in a list

zip(list_of_lists)

Zips corresponding elements from each list in list_of_lists

Functions

delete(list, item)

delete(list(), any()) :: list()

Deletes the given item from the list. Returns a new list without the item.

If the item occurs more than once in the list, just the first occurrence is removed.

Examples

iex> List.delete([:a, :b, :c], :a)
[:b, :c]

iex> List.delete([:a, :b, :b, :c], :b)
[:a, :b, :c]

delete_at(list, index)

delete_at(list(), integer()) :: list()

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

iex> List.delete_at([1, 2, 3], 0)
[2, 3]

iex> List.delete_at([1, 2, 3], 10)
[1, 2, 3]

iex> List.delete_at([1, 2, 3], -1)
[1, 2]

duplicate(elem, n)

duplicate(elem, non_neg_integer()) :: [elem] when elem: var

Duplicates the given element n times in a list.

Examples

iex> List.duplicate("hello", 3)
["hello", "hello", "hello"]

iex> List.duplicate([1, 2], 2)
[[1, 2], [1, 2]]

first(list)

first([elem]) :: nil | elem when elem: var

Returns the first element in list or nil if list is empty.

Examples

iex> List.first([])
nil

iex> List.first([1])
1

iex> List.first([1, 2, 3])
1

flatten(list)

flatten(deep_list) :: list() when deep_list: [any() | deep_list]

Flattens the given list of nested lists.

Examples

iex> List.flatten([1, [[2], 3]])
[1, 2, 3]

flatten(list, tail)

flatten(deep_list, [elem]) :: [elem] when deep_list: [elem | deep_list], elem: var

Flattens the given list of nested lists. The list tail will be added at the end of the flattened list.

Examples

iex> List.flatten([1, [[2], 3]], [4, 5])
[1, 2, 3, 4, 5]

foldl(list, acc, function)

foldl([elem], acc, (elem, acc -> acc)) :: acc when elem: var, acc: var

Folds (reduces) the given list from the left with a function. Requires an accumulator.

Examples

iex> List.foldl([5, 5], 10, fn(x, acc) -> x + acc end)
20

iex> List.foldl([1, 2, 3, 4], 0, fn(x, acc) -> x - acc end)
2

foldr(list, acc, function)

foldr([elem], acc, (elem, acc -> acc)) :: acc when elem: var, acc: var

Folds (reduces) the given list from the right with a function. Requires an accumulator.

Examples

iex> List.foldr([1, 2, 3, 4], 0, fn(x, acc) -> x - acc end)
-2

insert_at(list, index, value)

insert_at(list(), integer(), any()) :: list()

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

iex> List.insert_at([1, 2, 3, 4], 2, 0)
[1, 2, 0, 3, 4]

iex> List.insert_at([1, 2, 3], 10, 0)
[1, 2, 3, 0]

iex> List.insert_at([1, 2, 3], -1, 0)
[1, 2, 3, 0]

iex> List.insert_at([1, 2, 3], -10, 0)
[0, 1, 2, 3]

keydelete(list, key, position)

keydelete([tuple()], any(), non_neg_integer()) :: [tuple()]

Receives a list of tuples and deletes the first tuple where the item at position matches the given key. Returns the new list.

Examples

iex> List.keydelete([a: 1, b: 2], :a, 0)
[b: 2]

iex> List.keydelete([a: 1, b: 2], 2, 1)
[a: 1]

iex> List.keydelete([a: 1, b: 2], :c, 0)
[a: 1, b: 2]

keyfind(list, key, position, default \\ nil)

keyfind([tuple()], any(), non_neg_integer(), any()) :: any()

Receives a list of tuples and returns the first tuple where the item at position in the tuple matches the given key.

Examples

iex> List.keyfind([a: 1, b: 2], :a, 0)
{:a, 1}

iex> List.keyfind([a: 1, b: 2], 2, 1)
{:b, 2}

iex> List.keyfind([a: 1, b: 2], :c, 0)
nil

keymember?(list, key, position)

keymember?([tuple()], any(), non_neg_integer()) :: boolean()

Receives a list of tuples and returns true if there is a tuple where the item at position in the tuple matches the given key.

Examples

iex> List.keymember?([a: 1, b: 2], :a, 0)
true

iex> List.keymember?([a: 1, b: 2], 2, 1)
true

iex> List.keymember?([a: 1, b: 2], :c, 0)
false

keyreplace(list, key, position, new_tuple)

keyreplace([tuple()], any(), non_neg_integer(), tuple()) :: [tuple()]

Receives a list of tuples and replaces the item identified by key at position if it exists.

Examples

iex> List.keyreplace([a: 1, b: 2], :a, 0, {:a, 3})
[a: 3, b: 2]

keysort(list, position)

keysort([tuple()], non_neg_integer()) :: [tuple()]

Receives a list of tuples and sorts the items at position of the tuples. The sort is stable.

Examples

iex> List.keysort([a: 5, b: 1, c: 3], 1)
[b: 1, c: 3, a: 5]

iex> List.keysort([a: 5, c: 1, b: 3], 0)
[a: 5, b: 3, c: 1]

keystore(list, key, position, new_tuple)

keystore([tuple()], any(), non_neg_integer(), tuple()) :: [tuple(), ...]

Receives a list of tuples and replaces the item identified by key at position.

If the item does not exist, it is added to the end of the list.

Examples

iex> List.keystore([a: 1, b: 2], :a, 0, {:a, 3})
[a: 3, b: 2]

iex> List.keystore([a: 1, b: 2], :c, 0, {:c, 3})
[a: 1, b: 2, c: 3]

keytake(list, key, position)

keytake([tuple()], any(), non_neg_integer()) ::
  {tuple(), [tuple()]} |
  nil

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

iex> List.keytake([a: 1, b: 2], :a, 0)
{{:a, 1}, [b: 2]}

iex> List.keytake([a: 1, b: 2], 2, 1)
{{:b, 2}, [a: 1]}

iex> List.keytake([a: 1, b: 2], :c, 0)
nil

last(list)

last([elem]) :: nil | elem when elem: var

Returns the last element in list or nil if list is empty.

Examples

iex> List.last([])
nil

iex> List.last([1])
1

iex> List.last([1, 2, 3])
3

myers_difference(list1, list2)

myers_difference(list(), list()) ::
  [{:eq | :ins | :del, list()}] |
  nil

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.

Examples

iex> List.myers_difference([1, 4, 2, 3], [1, 2, 3, 4])
[eq: [1], del: [4], eq: [2, 3], ins: [4]]

pop_at(list, index, default \\ nil)

pop_at(list(), integer(), any()) :: {any(), list()}

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

iex> List.pop_at([1, 2, 3], 0)
{1, [2, 3]}
iex> List.pop_at([1, 2, 3], 5)
{nil, [1, 2, 3]}
iex> List.pop_at([1, 2, 3], 5, 10)
{10, [1, 2, 3]}
iex> List.pop_at([1, 2, 3], -1)
{3, [1, 2]}

replace_at(list, index, value)

replace_at(list(), integer(), any()) :: list()

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

iex> List.replace_at([1, 2, 3], 0, 0)
[0, 2, 3]

iex> List.replace_at([1, 2, 3], 10, 0)
[1, 2, 3]

iex> List.replace_at([1, 2, 3], -1, 0)
[1, 2, 0]

iex> List.replace_at([1, 2, 3], -10, 0)
[1, 2, 3]

starts_with?(list, prefix)

starts_with?(list(), list()) :: boolean()

starts_with?(list(), []) :: true

starts_with?([], [...]) :: false

Returns true if list starts with the given prefix list; otherwise returns false.

If prefix is an empty list, it returns true.

Examples

iex> List.starts_with?([1, 2, 3], [1, 2])
true

iex> List.starts_with?([1, 2], [1, 2, 3])
false

iex> List.starts_with?([:alpha], [])
true

iex> List.starts_with?([], [:alpha])
false

to_atom(charlist)

to_atom(charlist()) :: atom()

Converts a charlist to an atom.

Currently Elixir does not support conversions from charlists which contains Unicode codepoints greater than 0xFF.

Inlined by the compiler.

Examples

iex> List.to_atom('elixir')
:elixir

to_existing_atom(charlist)

to_existing_atom(charlist()) :: atom()

Converts a charlist to an existing atom. Raises an ArgumentError if the atom does not exist.

Currently Elixir does not support conversions from charlists which contains Unicode codepoints greater than 0xFF.

Inlined by the compiler.

Examples

iex> _ = :my_atom
iex> List.to_existing_atom('my_atom')
:my_atom

iex> List.to_existing_atom('this_atom_will_never_exist')
** (ArgumentError) argument error

to_float(charlist)

to_float(charlist()) :: float()

Returns the float whose text representation is charlist.

Inlined by the compiler.

Examples

iex> List.to_float('2.2017764e+0')
2.2017764

to_integer(charlist)

to_integer(charlist()) :: integer()

Returns an integer whose text representation is charlist.

Inlined by the compiler.

Examples

iex> List.to_integer('123')
123

to_integer(charlist, base)

to_integer(charlist(), 2..36) :: integer()

Returns an integer whose text representation is charlist in base base.

Inlined by the compiler.

Examples

iex> List.to_integer('3FF', 16)
1023

to_string(list)

to_string(:unicode.charlist()) :: String.t()

Converts a list of integers representing codepoints, lists or strings into a string.

Notice that this function expects a list of integers representing UTF-8 codepoints. If you have a list of bytes, you must instead use the :binary module.

Examples

iex> List.to_string([0x00E6, 0x00DF])
"æß"

iex> List.to_string([0x0061, "bc"])
"abc"

to_tuple(list)

to_tuple(list()) :: tuple()

Converts a list to a tuple.

Inlined by the compiler.

Examples

iex> List.to_tuple([:share, [:elixir, 163]])
{:share, [:elixir, 163]}

update_at(list, index, fun)

update_at([elem], integer(), (elem -> any())) :: list() when elem: var

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

iex> List.update_at([1, 2, 3], 0, &(&1 + 10))
[11, 2, 3]

iex> List.update_at([1, 2, 3], 10, &(&1 + 10))
[1, 2, 3]

iex> List.update_at([1, 2, 3], -1, &(&1 + 10))
[1, 2, 13]

iex> List.update_at([1, 2, 3], -10, &(&1 + 10))
[1, 2, 3]

wrap(list)

wrap(list() | any()) :: list()

Wraps the argument in a list.

If the argument is already a list, returns the list. If the argument is nil, returns an empty list.

Examples

iex> List.wrap("hello")
["hello"]

iex> List.wrap([1, 2, 3])
[1, 2, 3]

iex> List.wrap(nil)
[]

zip(list_of_lists)

zip([list()]) :: [tuple()]

Zips corresponding elements from each list in list_of_lists.

The zipping finishes as soon as any list terminates.

Examples

iex> List.zip([[1, 2], [3, 4], [5, 6]])
[{1, 3, 5}, {2, 4, 6}]

iex> List.zip([[1, 2], [3], [5, 6]])
[{1, 3, 5}]