@spec all?(t()) :: boolean()

Returns true if all elements in enumerable are truthy.

When an element has a falsy value (false or nil) iteration stops immediately and false is returned. In all other cases true is returned.

Examples

iex> Enum.all?([1, 2, 3])
true

iex> Enum.all?([1, nil, 3])
false

iex> Enum.all?([])
true

@spec all?(t(), (element() -> as_boolean(term()))) :: boolean()

Returns true if fun.(element) is truthy for all elements in enumerable.

Iterates over enumerable and invokes fun on each element. If fun ever returns a falsy value (false or nil), iteration stops immediately and false is returned. Otherwise, true is returned.

Examples

iex> Enum.all?([2, 4, 6], fn x -> rem(x, 2) == 0 end)
true

iex> Enum.all?([2, 3, 4], fn x -> rem(x, 2) == 0 end)
false

iex> Enum.all?([], fn _ -> nil end)
true

As the last example shows, Enum.all?/2 returns true if enumerable is empty, regardless of fun. In an empty enumerable there is no element for which fun returns a falsy value, so the result must be true. This is a well-defined logical argument for empty collections.

@spec any?(t()) :: boolean()

Returns true if at least one element in enumerable is truthy.

When an element has a truthy value (neither false nor nil) iteration stops immediately and true is returned. In all other cases false is returned.

Examples

iex> Enum.any?([false, false, false])
false

iex> Enum.any?([false, true, false])
true

iex> Enum.any?([])
false

@spec any?(t(), (element() -> as_boolean(term()))) :: boolean()

Returns true if fun.(element) is truthy for at least one element in enumerable.

Iterates over the enumerable and invokes fun on each element. When an invocation of fun returns a truthy value (neither false nor nil) iteration stops immediately and true is returned. In all other cases false is returned.

Examples

iex> Enum.any?([2, 4, 6], fn x -> rem(x, 2) == 1 end)
false

iex> Enum.any?([2, 3, 4], fn x -> rem(x, 2) == 1 end)
true

iex> Enum.any?([], fn x -> x > 0 end)
false

@spec at(t(), index(), default()) :: element() | default()

Finds the element at the given index (zero-based).

Returns default if index is out of bounds.

A negative index can be passed, which means the enumerable is enumerated once and the index is counted from the end (for example, -1 finds the last element).

Examples

iex> Enum.at([2, 4, 6], 0)
2

iex> Enum.at([2, 4, 6], 2)
6

iex> Enum.at([2, 4, 6], 4)
nil

iex> Enum.at([2, 4, 6], 4, :none)
:none

@spec chunk_by(t(), (element() -> any())) :: [list()]

Splits enumerable on every element for which fun returns a new value.

Returns a list of lists.

Examples

iex> Enum.chunk_by([1, 2, 2, 3, 4, 4, 6, 7, 7], &(rem(&1, 2) == 1))
[[1], [2, 2], [3], [4, 4, 6], [7, 7]]

@spec chunk_every(t(), pos_integer()) :: [list()]

Shortcut to chunk_every(enumerable, count, count).

@spec chunk_every(t(), pos_integer(), pos_integer(), t() | :discard) :: [list()]

Returns list of lists containing count elements each, where each new chunk starts step elements into the enumerable.

step is optional and, if not passed, defaults to count, i.e. chunks do not overlap. Chunking will stop as soon as the collection ends or when we emit an incomplete chunk.

If the last chunk does not have count elements to fill the chunk, elements are taken from leftover to fill in the chunk. If leftover does not have enough elements to fill the chunk, then a partial chunk is returned with less than count elements.

If :discard is given in leftover, the last chunk is discarded unless it has exactly count elements.

Examples

iex> Enum.chunk_every([1, 2, 3, 4, 5, 6], 2)
[[1, 2], [3, 4], [5, 6]]

iex> Enum.chunk_every([1, 2, 3, 4, 5, 6], 3, 2, :discard)
[[1, 2, 3], [3, 4, 5]]

iex> Enum.chunk_every([1, 2, 3, 4, 5, 6], 3, 2, [7])
[[1, 2, 3], [3, 4, 5], [5, 6, 7]]

iex> Enum.chunk_every([1, 2, 3, 4], 3, 3, [])
[[1, 2, 3], [4]]

iex> Enum.chunk_every([1, 2, 3, 4], 10)
[[1, 2, 3, 4]]

iex> Enum.chunk_every([1, 2, 3, 4, 5], 2, 3, [])
[[1, 2], [4, 5]]

iex> Enum.chunk_every([1, 2, 3, 4], 3, 3, Stream.cycle([0]))
[[1, 2, 3], [4, 0, 0]]

@spec chunk_while(
  t(),
  acc(),
  (element(), acc() -> {:cont, chunk, acc()} | {:cont, acc()} | {:halt, acc()}),
  (acc() -> {:cont, chunk, acc()} | {:cont, acc()})
) :: Enumerable.t()
when chunk: any()

Chunks the enumerable with fine grained control when every chunk is emitted.

chunk_fun receives the current element and the accumulator and must return:

• {:cont, chunk, acc} to emit a chunk and continue with the accumulator
• {:cont, acc} to not emit any chunk and continue with the accumulator
• {:halt, acc} to halt chunking over the enumerable.

after_fun is invoked with the final accumulator when iteration is finished (or halted) to handle any trailing elements that were returned as part of an accumulator, but were not emitted as a chunk by chunk_fun. It must return:

• {:cont, chunk, acc} to emit a chunk. The chunk will be appended to the list of already emitted chunks.
• {:cont, acc} to not emit a chunk

The acc in after_fun is required in order to mirror the tuple format from chunk_fun but it will be discarded since the traversal is complete.

Returns a list of emitted chunks.

Examples

iex> chunk_fun = fn element, acc ->
...>   if rem(element, 2) == 0 do
...>     {:cont, Enum.reverse([element | acc]), []}
...>   else
...>     {:cont, [element | acc]}
...>   end
...> end
iex> after_fun = fn
...>   [] -> {:cont, []}
...>   acc -> {:cont, Enum.reverse(acc), []}
...> end
iex> Enum.chunk_while(1..10, [], chunk_fun, after_fun)
[[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]
iex> Enum.chunk_while([1, 2, 3, 5, 7], [], chunk_fun, after_fun)
[[1, 2], [3, 5, 7]]

@spec concat(t()) :: t()

Given an enumerable of enumerables, concatenates the enumerables into a single list.

Examples

iex> Enum.concat([1..3, 4..6, 7..9])
[1, 2, 3, 4, 5, 6, 7, 8, 9]

iex> Enum.concat([[1, [2], 3], [4], [5, 6]])
[1, [2], 3, 4, 5, 6]

@spec concat(t(), t()) :: t()

Concatenates the enumerable on the right with the enumerable on the left.

This function produces the same result as the ++/2 operator for lists.

Examples

iex> Enum.concat(1..3, 4..6)
[1, 2, 3, 4, 5, 6]

iex> Enum.concat([1, 2, 3], [4, 5, 6])
[1, 2, 3, 4, 5, 6]

@spec count(t()) :: non_neg_integer()

Returns the size of the enumerable.

Examples

iex> Enum.count([1, 2, 3])
3

@spec count(t(), (element() -> as_boolean(term()))) :: non_neg_integer()

Returns the count of elements in the enumerable for which fun returns a truthy value.

Examples

iex> Enum.count([1, 2, 3, 4, 5], fn x -> rem(x, 2) == 0 end)
2

@spec count_until(t(), pos_integer()) :: non_neg_integer()

Counts the enumerable stopping at limit.

This is useful for checking certain properties of the count of an enumerable without having to actually count the entire enumerable. For example, if you wanted to check that the count was exactly, at least, or more than a value.

If the enumerable implements Enumerable.count/1, the enumerable is not traversed and we return the lower of the two numbers. To force enumeration, use count_until/3 with fn _ -> true end as the second argument.

Examples

iex> Enum.count_until(1..20, 5)
5
iex> Enum.count_until(1..20, 50)
20
iex> Enum.count_until(1..10, 10) == 10 # At least 10
true
iex> Enum.count_until(1..11, 10 + 1) > 10 # More than 10
true
iex> Enum.count_until(1..5, 10) < 10 # Less than 10
true
iex> Enum.count_until(1..10, 10 + 1) == 10 # Exactly ten
true

@spec count_until(t(), (element() -> as_boolean(term())), pos_integer()) ::
  non_neg_integer()

Counts the elements in the enumerable for which fun returns a truthy value, stopping at limit.

See count/2 and count_until/2 for more information.

Examples

iex> Enum.count_until(1..20, fn x -> rem(x, 2) == 0 end, 7)
7
iex> Enum.count_until(1..20, fn x -> rem(x, 2) == 0 end, 11)
10

@spec dedup(t()) :: list()

Enumerates the enumerable, returning a list where all consecutive duplicated elements are collapsed to a single element.

Elements are compared using ===/2.

If you want to remove all duplicated elements, regardless of order, see uniq/1.

Examples

iex> Enum.dedup([1, 2, 3, 3, 2, 1])
[1, 2, 3, 2, 1]

iex> Enum.dedup([1, 1, 2, 2.0, :three, :three])
[1, 2, 2.0, :three]

@spec dedup_by(t(), (element() -> term())) :: list()

Enumerates the enumerable, returning a list where all consecutive duplicated elements are collapsed to a single element.

The function fun maps every element to a term which is used to determine if two elements are duplicates.

Examples

iex> Enum.dedup_by([{1, :a}, {2, :b}, {2, :c}, {1, :a}], fn {x, _} -> x end)
[{1, :a}, {2, :b}, {1, :a}]

iex> Enum.dedup_by([5, 1, 2, 3, 2, 1], fn x -> x > 2 end)
[5, 1, 3, 2]

@spec drop(t(), integer()) :: list()

Drops the amount of elements from the enumerable.

If a negative amount is given, the amount of last values will be dropped. The enumerable will be enumerated once to retrieve the proper index and the remaining calculation is performed from the end.

Examples

iex> Enum.drop([1, 2, 3], 2)
[3]

iex> Enum.drop([1, 2, 3], 10)
[]

iex> Enum.drop([1, 2, 3], 0)
[1, 2, 3]

iex> Enum.drop([1, 2, 3], -1)
[1, 2]

@spec drop_every(t(), non_neg_integer()) :: list()

Returns a list of every nth element in the enumerable dropped, starting with the first element.

The first element is always dropped, unless nth is 0.

The second argument specifying every nth element must be a non-negative integer.

Examples

iex> Enum.drop_every(1..10, 2)
[2, 4, 6, 8, 10]

iex> Enum.drop_every(1..10, 0)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

iex> Enum.drop_every([1, 2, 3], 1)
[]

@spec drop_while(t(), (element() -> as_boolean(term()))) :: list()

Drops elements at the beginning of the enumerable while fun returns a truthy value.

Examples

iex> Enum.drop_while([1, 2, 3, 2, 1], fn x -> x < 3 end)
[3, 2, 1]

@spec each(t(), (element() -> any())) :: :ok

Invokes the given fun for each element in the enumerable.

Returns :ok.

Examples

Enum.each(["some", "example"], fn x -> IO.puts(x) end)
"some"
"example"
#=> :ok

@spec empty?(t()) :: boolean()

Determines if the enumerable is empty.

Returns true if enumerable is empty, otherwise false.

Examples

iex> Enum.empty?([])
true

iex> Enum.empty?([1, 2, 3])
false

@spec fetch(t(), index()) :: {:ok, element()} | :error

Finds the element at the given index (zero-based).

Returns {:ok, element} if found, otherwise :error.

A negative index can be passed, which means the enumerable is enumerated once and the index is counted from the end (for example, -1 fetches the last element).

Examples

iex> Enum.fetch([2, 4, 6], 0)
{:ok, 2}

iex> Enum.fetch([2, 4, 6], -3)
{:ok, 2}

iex> Enum.fetch([2, 4, 6], 2)
{:ok, 6}

iex> Enum.fetch([2, 4, 6], 4)
:error

@spec fetch!(t(), index()) :: element()

Finds the element at the given index (zero-based).

Raises OutOfBoundsError if the given index is outside the range of the enumerable.

Examples

iex> Enum.fetch!([2, 4, 6], 0)
2

iex> Enum.fetch!([2, 4, 6], 2)
6

iex> Enum.fetch!([2, 4, 6], 4)
** (Enum.OutOfBoundsError) out of bounds error

@spec filter(t(), (element() -> as_boolean(term()))) :: list()

Filters the enumerable, i.e. returns only those elements for which fun returns a truthy value.

See also reject/2 which discards all elements where the function returns a truthy value.

Examples

iex> Enum.filter([1, 2, 3], fn x -> rem(x, 2) == 0 end)
[2]

Keep in mind that filter is not capable of filtering and transforming an element at the same time. If you would like to do so, consider using flat_map/2. For example, if you want to convert all strings that represent an integer and discard the invalid one in one pass:

strings = ["1234", "abc", "12ab"]

Enum.flat_map(strings, fn string ->
  case Integer.parse(string) do
    # transform to integer
    {int, _rest} -> [int]
    # skip the value
    :error -> []
  end
end)

@spec find(t(), default(), (element() -> any())) :: element() | default()

Returns the first element for which fun returns a truthy value. If no such element is found, returns default.

Examples

iex> Enum.find([2, 3, 4], fn x -> rem(x, 2) == 1 end)
3

iex> Enum.find([2, 4, 6], fn x -> rem(x, 2) == 1 end)
nil
iex> Enum.find([2, 4, 6], 0, fn x -> rem(x, 2) == 1 end)
0

@spec find_index(t(), (element() -> any())) :: non_neg_integer() | nil

Similar to find/3, but returns the index (zero-based) of the element instead of the element itself.

Examples

iex> Enum.find_index([2, 4, 6], fn x -> rem(x, 2) == 1 end)
nil

iex> Enum.find_index([2, 3, 4], fn x -> rem(x, 2) == 1 end)
1

@spec find_value(t(), any(), (element() -> any())) :: any() | nil

Similar to find/3, but returns the value of the function invocation instead of the element itself.

The return value is considered to be found when the result is truthy (neither nil nor false).

Examples

iex> Enum.find_value([2, 3, 4], fn x ->
...>   if x > 2, do: x * x
...> end)
9

iex> Enum.find_value([2, 4, 6], fn x -> rem(x, 2) == 1 end)
nil

iex> Enum.find_value([2, 3, 4], fn x -> rem(x, 2) == 1 end)
true

iex> Enum.find_value([1, 2, 3], "no bools!", &is_boolean/1)
"no bools!"

@spec flat_map(t(), (element() -> t())) :: list()

Maps the given fun over enumerable and flattens the result.

This function returns a new enumerable built by appending the result of invoking fun on each element of enumerable together; conceptually, this is similar to a combination of map/2 and concat/1.

Examples

iex> Enum.flat_map([:a, :b, :c], fn x -> [x, x] end)
[:a, :a, :b, :b, :c, :c]

iex> Enum.flat_map([{1, 3}, {4, 6}], fn {x, y} -> x..y end)
[1, 2, 3, 4, 5, 6]

iex> Enum.flat_map([:a, :b, :c], fn x -> [[x]] end)
[[:a], [:b], [:c]]

@spec flat_map_reduce(t(), acc(), fun) :: {[any()], acc()}
when fun: (element(), acc() -> {t(), acc()} | {:halt, acc()})

Maps and reduces an enumerable, flattening the given results (only one level deep).

It expects an accumulator and a function that receives each enumerable element, and must return a tuple containing a new enumerable (often a list) with the new accumulator or a tuple with :halt as first element and the accumulator as second.

Examples

iex> enumerable = 1..100
iex> n = 3
iex> Enum.flat_map_reduce(enumerable, 0, fn x, acc ->
...>   if acc < n, do: {[x], acc + 1}, else: {:halt, acc}
...> end)
{[1, 2, 3], 3}

iex> Enum.flat_map_reduce(1..5, 0, fn x, acc -> {[[x]], acc + x} end)
{[[1], [2], [3], [4], [5]], 15}

@spec frequencies(t()) :: map()

Returns a map with keys as unique elements of enumerable and values as the count of every element.

Examples

iex> Enum.frequencies(~w{ant buffalo ant ant buffalo dingo})
%{"ant" => 3, "buffalo" => 2, "dingo" => 1}

@spec frequencies_by(t(), (element() -> any())) :: map()

Returns a map with keys as unique elements given by key_fun and values as the count of every element.

Examples

iex> Enum.frequencies_by(~w{aa aA bb cc}, &String.downcase/1)
%{"aa" => 2, "bb" => 1, "cc" => 1}

iex> Enum.frequencies_by(~w{aaa aA bbb cc c}, &String.length/1)
%{3 => 2, 2 => 2, 1 => 1}

@spec group_by(t(), (element() -> any()), (element() -> any())) :: map()

Splits the enumerable into groups based on key_fun.

The result is a map where each key is given by key_fun and each value is a list of elements given by value_fun. The order of elements within each list is preserved from the enumerable. However, like all maps, the resulting map is unordered.

Examples

iex> Enum.group_by(~w{ant buffalo cat dingo}, &String.length/1)
%{3 => ["ant", "cat"], 5 => ["dingo"], 7 => ["buffalo"]}

iex> Enum.group_by(~w{ant buffalo cat dingo}, &String.length/1, &String.first/1)
%{3 => ["a", "c"], 5 => ["d"], 7 => ["b"]}

The key can be any Elixir value. For example, you may use a tuple to group by multiple keys:

iex> collection = [
...>   %{id: 1, lang: "Elixir", seq: 1},
...>   %{id: 1, lang: "Java", seq: 1},
...>   %{id: 1, lang: "Ruby", seq: 2},
...>   %{id: 2, lang: "Python", seq: 1},
...>   %{id: 2, lang: "C#", seq: 2},
...>   %{id: 2, lang: "Haskell", seq: 2},
...> ]
iex> Enum.group_by(collection, &{&1.id, &1.seq})
%{
  {1, 1} => [%{id: 1, lang: "Elixir", seq: 1}, %{id: 1, lang: "Java", seq: 1}],
  {1, 2} => [%{id: 1, lang: "Ruby", seq: 2}],
  {2, 1} => [%{id: 2, lang: "Python", seq: 1}],
  {2, 2} => [%{id: 2, lang: "C#", seq: 2}, %{id: 2, lang: "Haskell", seq: 2}]
}
iex> Enum.group_by(collection, &{&1.id, &1.seq}, &{&1.id, &1.lang})
%{
  {1, 1} => [{1, "Elixir"}, {1, "Java"}],
  {1, 2} => [{1, "Ruby"}],
  {2, 1} => [{2, "Python"}],
  {2, 2} => [{2, "C#"}, {2, "Haskell"}]
}

@spec intersperse(t(), element()) :: list()

Intersperses separator between each element of the enumeration.

Examples

iex> Enum.intersperse([1, 2, 3], 0)
[1, 0, 2, 0, 3]

iex> Enum.intersperse([1], 0)
[1]

iex> Enum.intersperse([], 0)
[]

@spec into(Enumerable.t(), Collectable.t()) :: Collectable.t()

Inserts the given enumerable into a collectable.

Note that passing a non-empty list as the collectable is deprecated. If you're collecting into a non-empty keyword list, consider using Keyword.merge(collectable, Enum.to_list(enumerable)). If you're collecting into a non-empty list, consider something like Enum.to_list(enumerable) ++ collectable.

Examples

iex> Enum.into([1, 2], [])
[1, 2]

iex> Enum.into([a: 1, b: 2], %{})
%{a: 1, b: 2}

iex> Enum.into(%{a: 1}, %{b: 2})
%{a: 1, b: 2}

iex> Enum.into([a: 1, a: 2], %{})
%{a: 2}

@spec into(Enumerable.t(), Collectable.t(), (term() -> term())) :: Collectable.t()

Inserts the given enumerable into a collectable according to the transformation function.

Examples

iex> Enum.into([1, 2, 3], [], fn x -> x * 3 end)
[3, 6, 9]

iex> Enum.into(%{a: 1, b: 2}, %{c: 3}, fn {k, v} -> {k, v * 2} end)
%{a: 2, b: 4, c: 3}

@spec join(t(), String.t()) :: String.t()

Joins the given enumerable into a string using joiner as a separator.

If joiner is not passed at all, it defaults to an empty string.

All elements in the enumerable must be convertible to a string, otherwise an error is raised.

Examples

iex> Enum.join([1, 2, 3])
"123"

iex> Enum.join([1, 2, 3], " = ")
"1 = 2 = 3"

iex> Enum.join([["a", "b"], ["c", "d", "e", ["f", "g"]], "h", "i"], " ")
"ab cdefg h i"

@spec map(t(), (element() -> any())) :: list()

Returns a list where each element is the result of invoking fun on each corresponding element of enumerable.

For maps, the function expects a key-value tuple.

Examples

iex> Enum.map([1, 2, 3], fn x -> x * 2 end)
[2, 4, 6]

iex> Enum.map([a: 1, b: 2], fn {k, v} -> {k, -v} end)
[a: -1, b: -2]

@spec map_every(t(), non_neg_integer(), (element() -> any())) :: list()

Returns a list of results of invoking fun on every nth element of enumerable, starting with the first element.

The first element is always passed to the given function, unless nth is 0.

The second argument specifying every nth element must be a non-negative integer.

If nth is 0, then enumerable is directly converted to a list, without fun being ever applied.

Examples

iex> Enum.map_every(1..10, 2, fn x -> x + 1000 end)
[1001, 2, 1003, 4, 1005, 6, 1007, 8, 1009, 10]

iex> Enum.map_every(1..10, 3, fn x -> x + 1000 end)
[1001, 2, 3, 1004, 5, 6, 1007, 8, 9, 1010]

iex> Enum.map_every(1..5, 0, fn x -> x + 1000 end)
[1, 2, 3, 4, 5]

iex> Enum.map_every([1, 2, 3], 1, fn x -> x + 1000 end)
[1001, 1002, 1003]

@spec map_intersperse(t(), element(), (element() -> any())) :: list()

Maps and intersperses the given enumerable in one pass.

Examples

iex> Enum.map_intersperse([1, 2, 3], :a, &(&1 * 2))
[2, :a, 4, :a, 6]

@spec map_join(t(), String.t(), (element() -> String.Chars.t())) :: String.t()

Maps and joins the given enumerable in one pass.

If joiner is not passed at all, it defaults to an empty string.

All elements returned from invoking the mapper must be convertible to a string, otherwise an error is raised.

Examples

iex> Enum.map_join([1, 2, 3], &(&1 * 2))
"246"

iex> Enum.map_join([1, 2, 3], " = ", &(&1 * 2))
"2 = 4 = 6"

@spec map_reduce(t(), acc(), (element(), acc() -> {element(), acc()})) ::
  {list(), acc()}

Invokes the given function to each element in the enumerable to reduce it to a single element, while keeping an accumulator.

Returns a tuple where the first element is the mapped enumerable and the second one is the final accumulator.

The function, fun, receives two arguments: the first one is the element, and the second one is the accumulator. fun must return a tuple with two elements in the form of {result, accumulator}.

For maps, the first tuple element must be a {key, value} tuple.

Examples

iex> Enum.map_reduce([1, 2, 3], 0, fn x, acc -> {x * 2, x + acc} end)
{[2, 4, 6], 6}

@spec max(t(), (element(), element() -> boolean()) | module(), (-> empty_result)) ::
  element() | empty_result
when empty_result: any()

Returns the maximal element in the enumerable according to Erlang's term ordering.

By default, the comparison is done with the >= sorter function. If multiple elements are considered maximal, the first one that was found is returned. If you want the last element considered maximal to be returned, the sorter function should not return true for equal elements.

If the enumerable is empty, the provided empty_fallback is called. The default empty_fallback raises Enum.EmptyError.

Examples

iex> Enum.max([1, 2, 3])
3

The fact this function uses Erlang's term ordering means that the comparison is structural and not semantic. For example:

iex> Enum.max([~D[2017-03-31], ~D[2017-04-01]])
~D[2017-03-31]

In the example above, max/2 returned March 31st instead of April 1st because the structural comparison compares the day before the year. For this reason, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> Enum.max([~D[2017-03-31], ~D[2017-04-01]], Date)
~D[2017-04-01]

Finally, if you don't want to raise on empty enumerables, you can pass the empty fallback:

iex> Enum.max([], &>=/2, fn -> 0 end)
0

@spec max_by(
  t(),
  (element() -> any()),
  (element(), element() -> boolean()) | module(),
  (-> empty_result)
) :: element() | empty_result
when empty_result: any()

Returns the maximal element in the enumerable as calculated by the given fun.

By default, the comparison is done with the >= sorter function. If multiple elements are considered maximal, the first one that was found is returned. If you want the last element considered maximal to be returned, the sorter function should not return true for equal elements.

Calls the provided empty_fallback function and returns its value if enumerable is empty. The default empty_fallback raises Enum.EmptyError.

Examples

iex> Enum.max_by(["a", "aa", "aaa"], fn x -> String.length(x) end)
"aaa"

iex> Enum.max_by(["a", "aa", "aaa", "b", "bbb"], &String.length/1)
"aaa"

The fact this function uses Erlang's term ordering means that the comparison is structural and not semantic. Therefore, if you want to compare structs, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> users = [
...>   %{name: "Ellis", birthday: ~D[1943-05-11]},
...>   %{name: "Lovelace", birthday: ~D[1815-12-10]},
...>   %{name: "Turing", birthday: ~D[1912-06-23]}
...> ]
iex> Enum.max_by(users, &(&1.birthday), Date)
%{name: "Ellis", birthday: ~D[1943-05-11]}

Finally, if you don't want to raise on empty enumerables, you can pass the empty fallback:

iex> Enum.max_by([], &String.length/1, fn -> nil end)
nil

@spec member?(t(), element()) :: boolean()

Checks if element exists within the enumerable.

Membership is tested with the match (===/2) operator.

Examples

iex> Enum.member?(1..10, 5)
true
iex> Enum.member?(1..10, 5.0)
false

iex> Enum.member?([1.0, 2.0, 3.0], 2)
false
iex> Enum.member?([1.0, 2.0, 3.0], 2.000)
true

iex> Enum.member?([:a, :b, :c], :d)
false

When called outside guards, the in and not in operators work by using this function.

@spec min(t(), (element(), element() -> boolean()) | module(), (-> empty_result)) ::
  element() | empty_result
when empty_result: any()

Returns the minimal element in the enumerable according to Erlang's term ordering.

By default, the comparison is done with the <= sorter function. If multiple elements are considered minimal, the first one that was found is returned. If you want the last element considered minimal to be returned, the sorter function should not return true for equal elements.

If the enumerable is empty, the provided empty_fallback is called. The default empty_fallback raises Enum.EmptyError.

Examples

iex> Enum.min([1, 2, 3])
1

The fact this function uses Erlang's term ordering means that the comparison is structural and not semantic. For example:

iex> Enum.min([~D[2017-03-31], ~D[2017-04-01]])
~D[2017-04-01]

In the example above, min/2 returned April 1st instead of March 31st because the structural comparison compares the day before the year. For this reason, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> Enum.min([~D[2017-03-31], ~D[2017-04-01]], Date)
~D[2017-03-31]

Finally, if you don't want to raise on empty enumerables, you can pass the empty fallback:

iex> Enum.min([], fn -> 0 end)
0

@spec min_by(
  t(),
  (element() -> any()),
  (element(), element() -> boolean()) | module(),
  (-> empty_result)
) :: element() | empty_result
when empty_result: any()

Returns the minimal element in the enumerable as calculated by the given fun.

By default, the comparison is done with the <= sorter function. If multiple elements are considered minimal, the first one that was found is returned. If you want the last element considered minimal to be returned, the sorter function should not return true for equal elements.

Calls the provided empty_fallback function and returns its value if enumerable is empty. The default empty_fallback raises Enum.EmptyError.

Examples

iex> Enum.min_by(["a", "aa", "aaa"], fn x -> String.length(x) end)
"a"

iex> Enum.min_by(["a", "aa", "aaa", "b", "bbb"], &String.length/1)
"a"

The fact this function uses Erlang's term ordering means that the comparison is structural and not semantic. Therefore, if you want to compare structs, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> users = [
...>   %{name: "Ellis", birthday: ~D[1943-05-11]},
...>   %{name: "Lovelace", birthday: ~D[1815-12-10]},
...>   %{name: "Turing", birthday: ~D[1912-06-23]}
...> ]
iex> Enum.min_by(users, &(&1.birthday), Date)
%{name: "Lovelace", birthday: ~D[1815-12-10]}

Finally, if you don't want to raise on empty enumerables, you can pass the empty fallback:

iex> Enum.min_by([], &String.length/1, fn -> nil end)
nil

@spec min_max(t(), (-> empty_result)) :: {element(), element()} | empty_result
when empty_result: any()

Returns a tuple with the minimal and the maximal elements in the enumerable according to Erlang's term ordering.

If multiple elements are considered maximal or minimal, the first one that was found is returned.

Calls the provided empty_fallback function and returns its value if enumerable is empty. The default empty_fallback raises Enum.EmptyError.

Examples

iex> Enum.min_max([2, 3, 1])
{1, 3}

iex> Enum.min_max([], fn -> {nil, nil} end)
{nil, nil}

@spec min_max_by(
  t(),
  (element() -> any()),
  (element(), element() -> boolean()) | module(),
  (-> empty_result)
) :: {element(), element()} | empty_result
when empty_result: any()

Returns a tuple with the minimal and the maximal elements in the enumerable as calculated by the given function.

If multiple elements are considered maximal or minimal, the first one that was found is returned.

Examples

iex> Enum.min_max_by(["aaa", "bb", "c"], fn x -> String.length(x) end)
{"c", "aaa"}

iex> Enum.min_max_by(["aaa", "a", "bb", "c", "ccc"], &String.length/1)
{"a", "aaa"}

iex> Enum.min_max_by([], &String.length/1, fn -> {nil, nil} end)
{nil, nil}

The fact this function uses Erlang's term ordering means that the comparison is structural and not semantic. Therefore, if you want to compare structs, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> users = [
...>   %{name: "Ellis", birthday: ~D[1943-05-11]},
...>   %{name: "Lovelace", birthday: ~D[1815-12-10]},
...>   %{name: "Turing", birthday: ~D[1912-06-23]}
...> ]
iex> Enum.min_max_by(users, &(&1.birthday), Date)
{
  %{name: "Lovelace", birthday: ~D[1815-12-10]},
  %{name: "Ellis", birthday: ~D[1943-05-11]}
}

Finally, if you don't want to raise on empty enumerables, you can pass the empty fallback:

iex> Enum.min_max_by([], &String.length/1, fn -> nil end)
nil

@spec product(t()) :: number()

Returns the product of all elements.

Raises ArithmeticError if enumerable contains a non-numeric value.

Examples

iex> Enum.product([])
1
iex> Enum.product([2, 3, 4])
24
iex> Enum.product([2.0, 3.0, 4.0])
24.0

@spec random(t()) :: element()

Returns a random element of an enumerable.

Raises Enum.EmptyError if enumerable is empty.

This function uses Erlang's :rand module to calculate the random value. Check its documentation for setting a different random algorithm or a different seed.

The implementation is based on the reservoir sampling algorithm. It assumes that the sample being returned can fit into memory; the input enumerable doesn't have to, as it is traversed just once.

If a range is passed into the function, this function will pick a random value between the range limits, without traversing the whole range (thus executing in constant time and constant memory).

Examples

The examples below use the :exsss pseudorandom algorithm since it's the default from Erlang/OTP 22:

# Although not necessary, let's seed the random algorithm
iex> :rand.seed(:exsss, {100, 101, 102})
iex> Enum.random([1, 2, 3])
2
iex> Enum.random([1, 2, 3])
1
iex> Enum.random(1..1_000)
309

@spec reduce(t(), (element(), acc() -> acc())) :: acc()

Invokes fun for each element in the enumerable with the accumulator.

Raises Enum.EmptyError if enumerable is empty.

The first element of the enumerable is used as the initial value of the accumulator. Then, the function is invoked with the next element and the accumulator. The result returned by the function is used as the accumulator for the next iteration, recursively. When the enumerable is done, the last accumulator is returned.

Since the first element of the enumerable is used as the initial value of the accumulator, fun will only be executed n - 1 times where n is the length of the enumerable. This function won't call the specified function for enumerables that are one-element long.

If you wish to use another value for the accumulator, use Enum.reduce/3.

Examples

iex> Enum.reduce([1, 2, 3, 4], fn x, acc -> x * acc end)
24

@spec reduce(t(), acc(), (element(), acc() -> acc())) :: acc()

Invokes fun for each element in the enumerable with the accumulator.

The initial value of the accumulator is acc. The function is invoked for each element in the enumerable with the accumulator. The result returned by the function is used as the accumulator for the next iteration. The function returns the last accumulator.

Examples

iex> Enum.reduce([1, 2, 3], 0, fn x, acc -> x + acc end)
6

iex> Enum.reduce(%{a: 2, b: 3, c: 4}, 0, fn {_key, val}, acc -> acc + val end)
9

Reduce as a building block

Reduce (sometimes called fold) is a basic building block in functional programming. Almost all of the functions in the Enum module can be implemented on top of reduce. Those functions often rely on other operations, such as Enum.reverse/1, which are optimized by the runtime.

For example, we could implement map/2 in terms of reduce/3 as follows:

def my_map(enumerable, fun) do
  enumerable
  |> Enum.reduce([], fn x, acc -> [fun.(x) | acc] end)
  |> Enum.reverse()
end

In the example above, Enum.reduce/3 accumulates the result of each call to fun into a list in reverse order, which is correctly ordered at the end by calling Enum.reverse/1.

Implementing functions like map/2, filter/2 and others are a good exercise for understanding the power behind Enum.reduce/3. When an operation cannot be expressed by any of the functions in the Enum module, developers will most likely resort to reduce/3.

@spec reduce_while(t(), any(), (element(), any() -> {:cont, any()} | {:halt, any()})) ::
  any()

Reduces enumerable until fun returns {:halt, term}.

The return value for fun is expected to be

• {:cont, acc} to continue the reduction with acc as the new accumulator or
• {:halt, acc} to halt the reduction

If fun returns {:halt, acc} the reduction is halted and the function returns acc. Otherwise, if the enumerable is exhausted, the function returns the accumulator of the last {:cont, acc}.

Examples

iex> Enum.reduce_while(1..100, 0, fn x, acc ->
...>   if x < 5 do
...>     {:cont, acc + x}
...>   else
...>     {:halt, acc}
...>   end
...> end)
10
iex> Enum.reduce_while(1..100, 0, fn x, acc ->
...>   if x > 0 do
...>     {:cont, acc + x}
...>   else
...>     {:halt, acc}
...>   end
...> end)
5050

@spec reject(t(), (element() -> as_boolean(term()))) :: list()

Returns a list of elements in enumerable excluding those for which the function fun returns a truthy value.

See also filter/2.

Examples

iex> Enum.reject([1, 2, 3], fn x -> rem(x, 2) == 0 end)
[1, 3]

@spec reverse(t()) :: list()

Returns a list of elements in enumerable in reverse order.

Examples

iex> Enum.reverse([1, 2, 3])
[3, 2, 1]

@spec reverse(t(), t()) :: list()

Reverses the elements in enumerable, appends the tail, and returns it as a list.

This is an optimization for enumerable |> Enum.reverse() |> Enum.concat(tail).

Examples

iex> Enum.reverse([1, 2, 3], [4, 5, 6])
[3, 2, 1, 4, 5, 6]

@spec reverse_slice(t(), non_neg_integer(), non_neg_integer()) :: list()

Reverses the enumerable in the range from initial start_index through count elements.

If count is greater than the size of the rest of the enumerable, then this function will reverse the rest of the enumerable.

Examples

iex> Enum.reverse_slice([1, 2, 3, 4, 5, 6], 2, 4)
[1, 2, 6, 5, 4, 3]

@spec scan(t(), (element(), any() -> any())) :: list()

Applies the given function to each element in the enumerable, storing the result in a list and passing it as the accumulator for the next computation. Uses the first element in the enumerable as the starting value.

Examples

iex> Enum.scan(1..5, &(&1 + &2))
[1, 3, 6, 10, 15]

@spec scan(t(), any(), (element(), any() -> any())) :: list()

Applies the given function to each element in the enumerable, storing the result in a list and passing it as the accumulator for the next computation. Uses the given acc as the starting value.

Examples

iex> Enum.scan(1..5, 0, &(&1 + &2))
[1, 3, 6, 10, 15]

@spec shuffle(t()) :: list()

Returns a list with the elements of enumerable shuffled.

This function uses Erlang's :rand module to calculate the random value. Check its documentation for setting a different random algorithm or a different seed.

Examples

The examples below use the :exsss pseudorandom algorithm since it's the default from Erlang/OTP 22:

# Although not necessary, let's seed the random algorithm
iex> :rand.seed(:exsss, {1, 2, 3})
iex> Enum.shuffle([1, 2, 3])
[3, 2, 1]
iex> Enum.shuffle([1, 2, 3])
[2, 1, 3]

@spec slice(t(), Range.t()) :: list()

Returns a subset list of the given enumerable by index_range.

index_range must be a Range. Given an enumerable, it drops elements before index_range.first (zero-base), then it takes elements until element index_range.last (inclusively).

Indexes are normalized, meaning that negative indexes will be counted from the end (for example, -1 means the last element of the enumerable).

If index_range.last is out of bounds, then it is assigned as the index of the last element.

If the normalized index_range.first is out of bounds of the given enumerable, or this one is greater than the normalized index_range.last, then [] is returned.

If a step n (other than 1) is used in index_range, then it takes every nth element from index_range.first to index_range.last (according to the same rules described above).

Examples

iex> Enum.slice([1, 2, 3, 4, 5], 1..3)
[2, 3, 4]

iex> Enum.slice([1, 2, 3, 4, 5], 3..10)
[4, 5]

# Last three elements (negative indexes)
iex> Enum.slice([1, 2, 3, 4, 5], -3..-1)
[3, 4, 5]

For ranges where start > stop, you need to explicit mark them as increasing:

iex> Enum.slice([1, 2, 3, 4, 5], 1..-2//1)
[2, 3, 4]

The step can be any positive number. For example, to get every 2 elements of the collection:

iex> Enum.slice([1, 2, 3, 4, 5], 0..-1//2)
[1, 3, 5]

To get every third element of the first ten elements:

iex> integers = Enum.to_list(1..20)
iex> Enum.slice(integers, 0..9//3)
[1, 4, 7, 10]

If the first position is after the end of the enumerable or after the last position of the range, it returns an empty list:

iex> Enum.slice([1, 2, 3, 4, 5], 6..10)
[]

# first is greater than last
iex> Enum.slice([1, 2, 3, 4, 5], 6..5)
[]

@spec slice(t(), index(), non_neg_integer()) :: list()

Returns a subset list of the given enumerable, from start_index (zero-based) with amount number of elements if available.

Given an enumerable, it drops elements right before element start_index; then, it takes amount of elements, returning as many elements as possible if there are not enough elements.

A negative start_index can be passed, which means the enumerable is enumerated once and the index is counted from the end (for example, -1 starts slicing from the last element).

It returns [] if amount is 0 or if start_index is out of bounds.

Examples

iex> Enum.slice(1..100, 5, 10)
[6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

# amount to take is greater than the number of elements
iex> Enum.slice(1..10, 5, 100)
[6, 7, 8, 9, 10]

iex> Enum.slice(1..10, 5, 0)
[]

# using a negative start index
iex> Enum.slice(1..10, -6, 3)
[5, 6, 7]
iex> Enum.slice(1..10, -11, 5)
[1, 2, 3, 4, 5]

# out of bound start index
iex> Enum.slice(1..10, 10, 5)
[]

@spec slide(t(), Range.t() | index(), index()) :: list()

Slides a single or multiple elements given by range_or_single_index from enumerable to insertion_index.

The semantics of the range to be moved match the semantics of Enum.slice/2. Specifically, that means:

• Indices are normalized, meaning that negative indexes will be counted from the end (for example, -1 means the last element of the enumerable). This will result in two traversals of your enumerable on types like lists that don't provide a constant-time count.

• If the normalized index range's last is out of bounds, the range is truncated to the last element.

• If the normalized index range's first is out of bounds, the selected range for sliding will be empty, so you'll get back your input list.

• Decreasing ranges (such as 5..0//1) also select an empty range to be moved, so you'll get back your input list.

• Ranges with any step but 1 will raise an error.

Examples

# Slide a single element
iex> Enum.slide([:a, :b, :c, :d, :e, :f, :g], 5, 1)
[:a, :f, :b, :c, :d, :e, :g]

# Slide a range of elements backward
iex> Enum.slide([:a, :b, :c, :d, :e, :f, :g], 3..5, 1)
[:a, :d, :e, :f, :b, :c, :g]

# Slide a range of elements forward
iex> Enum.slide([:a, :b, :c, :d, :e, :f, :g], 1..3, 5)
[:a, :e, :f, :b, :c, :d, :g]

# Slide with negative indices (counting from the end)
iex> Enum.slide([:a, :b, :c, :d, :e, :f, :g], 3..-1//1, 2)
[:a, :b, :d, :e, :f, :g, :c]
iex> Enum.slide([:a, :b, :c, :d, :e, :f, :g], -4..-2, 1)
[:a, :d, :e, :f, :b, :c, :g]

# Insert at negative indices (counting from the end)
iex> Enum.slide([:a, :b, :c, :d, :e, :f, :g], 3, -1)
[:a, :b, :c, :e, :f, :g, :d]

@spec sort(t()) :: list()

Sorts the enumerable according to Erlang's term ordering.

This function uses the merge sort algorithm. Do not use this function to sort structs, see sort/2 for more information.

Examples

iex> Enum.sort([3, 2, 1])
[1, 2, 3]

@spec sort(
  t(),
  (element(), element() -> boolean())
  | :asc
  | :desc
  | module()
  | {:asc | :desc, module()}
) :: list()

Sorts the enumerable by the given function.

This function uses the merge sort algorithm. The given function should compare two arguments, and return true if the first argument precedes or is in the same place as the second one.

Examples

iex> Enum.sort([1, 2, 3], &(&1 >= &2))
[3, 2, 1]

The sorting algorithm will be stable as long as the given function returns true for values considered equal:

iex> Enum.sort(["some", "kind", "of", "monster"], &(byte_size(&1) <= byte_size(&2)))
["of", "some", "kind", "monster"]

If the function does not return true for equal values, the sorting is not stable and the order of equal terms may be shuffled. For example:

iex> Enum.sort(["some", "kind", "of", "monster"], &(byte_size(&1) < byte_size(&2)))
["of", "kind", "some", "monster"]

Ascending and descending (since v1.10.0)

sort/2 allows a developer to pass :asc or :desc as the sorter, which is a convenience for &<=/2 and &>=/2 respectively.

iex> Enum.sort([2, 3, 1], :asc)
[1, 2, 3]
iex> Enum.sort([2, 3, 1], :desc)
[3, 2, 1]

Sorting structs

Do not use </2, <=/2, >/2, >=/2 and friends when sorting structs. That's because the built-in operators above perform structural comparison and not a semantic one. Imagine we sort the following list of dates:

iex> dates = [~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]]
iex> Enum.sort(dates)
[~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]]

Note that the returned result is incorrect, because sort/1 by default uses <=/2, which will compare their structure. When comparing structures, the fields are compared in alphabetical order, which means the dates above will be compared by day, month and then year, which is the opposite of what we want.

For this reason, most structs provide a "compare" function, such as Date.compare/2, which receives two structs and returns :lt (less-than), :eq (equal to), and :gt (greater-than). If you pass a module as the sorting function, Elixir will automatically use the compare/2 function of said module:

iex> dates = [~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]]
iex> Enum.sort(dates, Date)
[~D[2019-01-01], ~D[2019-06-06], ~D[2020-03-02]]

To retrieve all dates in descending order, you can wrap the module in a tuple with :asc or :desc as first element:

iex> dates = [~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]]
iex> Enum.sort(dates, {:asc, Date})
[~D[2019-01-01], ~D[2019-06-06], ~D[2020-03-02]]
iex> dates = [~D[2019-01-01], ~D[2020-03-02], ~D[2019-06-06]]
iex> Enum.sort(dates, {:desc, Date})
[~D[2020-03-02], ~D[2019-06-06], ~D[2019-01-01]]

@spec sort_by(
  t(),
  (element() -> mapped_element),
  (element(), element() -> boolean())
  | :asc
  | :desc
  | module()
  | {:asc | :desc, module()}
) :: list()
when mapped_element: element()

Sorts the mapped results of the enumerable according to the provided sorter function.

This function maps each element of the enumerable using the provided mapper function. The enumerable is then sorted by the mapped elements using the sorter, which defaults to :asc and sorts the elements ascendingly.

sort_by/3 differs from sort/2 in that it only calculates the comparison value for each element in the enumerable once instead of once for each element in each comparison. If the same function is being called on both elements, it's more efficient to use sort_by/3.

Ascending and descending (since v1.10.0)

sort_by/3 allows a developer to pass :asc or :desc as the sorter, which is a convenience for &<=/2 and &>=/2 respectively:

iex> Enum.sort_by([2, 3, 1], &(&1), :asc)
[1, 2, 3]

iex> Enum.sort_by([2, 3, 1], &(&1), :desc)
[3, 2, 1]

Examples

Using the default sorter of :asc :

iex> Enum.sort_by(["some", "kind", "of", "monster"], &byte_size/1)
["of", "some", "kind", "monster"]

Sorting by multiple properties - first by size, then by first letter (this takes advantage of the fact that tuples are compared element-by-element):

iex> Enum.sort_by(["some", "kind", "of", "monster"], &{byte_size(&1), String.first(&1)})
["of", "kind", "some", "monster"]

Similar to sort/2, you can pass a custom sorter:

iex> Enum.sort_by(["some", "kind", "of", "monster"], &byte_size/1, :desc)
["monster", "some", "kind", "of"]

As in sort/2, avoid using the default sorting function to sort structs, as by default it performs structural comparison instead of a semantic one. In such cases, you shall pass a sorting function as third element or any module that implements a compare/2 function. For example, to sort users by their birthday in both ascending and descending order respectively:

iex> users = [
...>   %{name: "Ellis", birthday: ~D[1943-05-11]},
...>   %{name: "Lovelace", birthday: ~D[1815-12-10]},
...>   %{name: "Turing", birthday: ~D[1912-06-23]}
...> ]
iex> Enum.sort_by(users, &(&1.birthday), Date)
[
  %{name: "Lovelace", birthday: ~D[1815-12-10]},
  %{name: "Turing", birthday: ~D[1912-06-23]},
  %{name: "Ellis", birthday: ~D[1943-05-11]}
]
iex> Enum.sort_by(users, &(&1.birthday), {:desc, Date})
[
  %{name: "Ellis", birthday: ~D[1943-05-11]},
  %{name: "Turing", birthday: ~D[1912-06-23]},
  %{name: "Lovelace", birthday: ~D[1815-12-10]}
]

Performance characteristics

As detailed in the initial section, sort_by/3 calculates the comparison value for each element in the enumerable once instead of once for each element in each comparison. This implies sort_by/3 must do an initial pass on the data to compute those values.

However, if those values are cheap to compute, for example, you have already extracted the field you want to sort by into a tuple, then those extra passes become overhead. In such cases, consider using List.keysort/3 instead.

Let's see an example. Imagine you have a list of products and you have a list of IDs. You want to keep all products that are in the given IDs and return their names sorted by their price. You could write it like this:

for(
  product <- products,
  product.id in ids,
  do: product
)
|> Enum.sort_by(& &1.price)
|> Enum.map(& &1.name)

However, you could also write it like this:

for(
  product <- products,
  product.id in ids,
  do: {product.name, product.price}
)
|> List.keysort(1)
|> Enum.map(&elem(&1, 0))

Using List.keysort/3 will be a better choice for performance sensitive code as it avoids additional traversals.

@spec split(t(), integer()) :: {list(), list()}

Splits the enumerable into two enumerables, leaving count elements in the first one.

If count is a negative number, it starts counting from the back to the beginning of the enumerable.

Be aware that a negative count implies the enumerable will be enumerated twice: once to calculate the position, and a second time to do the actual splitting.

Examples

iex> Enum.split([1, 2, 3], 2)
{[1, 2], [3]}

iex> Enum.split([1, 2, 3], 10)
{[1, 2, 3], []}

iex> Enum.split([1, 2, 3], 0)
{[], [1, 2, 3]}

iex> Enum.split([1, 2, 3], -1)
{[1, 2], [3]}

iex> Enum.split([1, 2, 3], -5)
{[], [1, 2, 3]}

@spec split_while(t(), (element() -> as_boolean(term()))) :: {list(), list()}

Splits enumerable in two at the position of the element for which fun returns a falsy value (false or nil) for the first time.

It returns a two-element tuple with two lists of elements. The element that triggered the split is part of the second list.

Examples

iex> Enum.split_while([1, 2, 3, 4], fn x -> x < 3 end)
{[1, 2], [3, 4]}

iex> Enum.split_while([1, 2, 3, 4], fn x -> x < 0 end)
{[], [1, 2, 3, 4]}

iex> Enum.split_while([1, 2, 3, 4], fn x -> x > 0 end)
{[1, 2, 3, 4], []}

@spec split_with(t(), (element() -> as_boolean(term()))) :: {list(), list()}

Splits the enumerable in two lists according to the given function fun.

Splits the given enumerable in two lists by calling fun with each element in the enumerable as its only argument. Returns a tuple with the first list containing all the elements in enumerable for which applying fun returned a truthy value, and a second list with all the elements for which applying fun returned a falsy value (false or nil).

The elements in both the returned lists are in the same relative order as they were in the original enumerable (if such enumerable was ordered, like a list). See the examples below.

Examples

iex> Enum.split_with([5, 4, 3, 2, 1, 0], fn x -> rem(x, 2) == 0 end)
{[4, 2, 0], [5, 3, 1]}

iex> Enum.split_with([a: 1, b: -2, c: 1, d: -3], fn {_k, v} -> v < 0 end)
{[b: -2, d: -3], [a: 1, c: 1]}

iex> Enum.split_with([a: 1, b: -2, c: 1, d: -3], fn {_k, v} -> v > 50 end)
{[], [a: 1, b: -2, c: 1, d: -3]}

iex> Enum.split_with([], fn {_k, v} -> v > 50 end)
{[], []}

@spec sum(t()) :: number()

Returns the sum of all elements.

Raises ArithmeticError if enumerable contains a non-numeric value.

Examples

iex> Enum.sum([1, 2, 3])
6

iex> Enum.sum(1..10)
55

iex> Enum.sum(1..10//2)
25

@spec take(t(), integer()) :: list()

Takes an amount of elements from the beginning or the end of the enumerable.

If a positive amount is given, it takes the amount elements from the beginning of the enumerable.

If a negative amount is given, the amount of elements will be taken from the end. The enumerable will be enumerated once to retrieve the proper index and the remaining calculation is performed from the end.

If amount is 0, it returns [].

Examples

iex> Enum.take([1, 2, 3], 2)
[1, 2]

iex> Enum.take([1, 2, 3], 10)
[1, 2, 3]

iex> Enum.take([1, 2, 3], 0)
[]

iex> Enum.take([1, 2, 3], -1)
[3]

@spec take_every(t(), non_neg_integer()) :: list()

Returns a list of every nth element in the enumerable, starting with the first element.

The first element is always included, unless nth is 0.

The second argument specifying every nth element must be a non-negative integer.

Examples

iex> Enum.take_every(1..10, 2)
[1, 3, 5, 7, 9]

iex> Enum.take_every(1..10, 0)
[]

iex> Enum.take_every([1, 2, 3], 1)
[1, 2, 3]

@spec take_random(t(), non_neg_integer()) :: list()

Takes count random elements from enumerable.

Note that this function will traverse the whole enumerable to get the random sublist.

See random/1 for notes on implementation and random seed.

Examples

# Although not necessary, let's seed the random algorithm
iex> :rand.seed(:exsss, {1, 2, 3})
iex> Enum.take_random(1..10, 2)
[3, 1]
iex> Enum.take_random(?a..?z, 5)
'mikel'

@spec take_while(t(), (element() -> as_boolean(term()))) :: list()

Takes the elements from the beginning of the enumerable while fun returns a truthy value.

Examples

iex> Enum.take_while([1, 2, 3], fn x -> x < 3 end)
[1, 2]

@spec to_list(t()) :: [element()]

Converts enumerable to a list.

Examples

iex> Enum.to_list(1..3)
[1, 2, 3]

@spec uniq(t()) :: list()

Enumerates the enumerable, removing all duplicated elements.

Examples

iex> Enum.uniq([1, 2, 3, 3, 2, 1])
[1, 2, 3]

@spec uniq_by(t(), (element() -> term())) :: list()

Enumerates the enumerable, by removing the elements for which function fun returned duplicate elements.

The function fun maps every element to a term. Two elements are considered duplicates if the return value of fun is equal for both of them.

The first occurrence of each element is kept.

Example

iex> Enum.uniq_by([{1, :x}, {2, :y}, {1, :z}], fn {x, _} -> x end)
[{1, :x}, {2, :y}]

iex> Enum.uniq_by([a: {:tea, 2}, b: {:tea, 2}, c: {:coffee, 1}], fn {_, y} -> y end)
[a: {:tea, 2}, c: {:coffee, 1}]

@spec unzip(t()) :: {[element()], [element()]}

Opposite of zip/2. Extracts two-element tuples from the given enumerable and groups them together.

It takes an enumerable with elements being two-element tuples and returns a tuple with two lists, each of which is formed by the first and second element of each tuple, respectively.

This function fails unless enumerable is or can be converted into a list of tuples with exactly two elements in each tuple.

Examples

iex> Enum.unzip([{:a, 1}, {:b, 2}, {:c, 3}])
{[:a, :b, :c], [1, 2, 3]}

iex> Enum.unzip(%{a: 1, b: 2})
{[:a, :b], [1, 2]}

@spec with_index(t(), integer()) :: [{term(), integer()}]

@spec with_index(t(), (element(), index() -> value)) :: [value] when value: any()

Returns the enumerable with each element wrapped in a tuple alongside its index.

May receive a function or an integer offset.

If an offset is given, it will index from the given offset instead of from zero.

If a function is given, it will index by invoking the function for each element and index (zero-based) of the enumerable.

Examples

iex> Enum.with_index([:a, :b, :c])
[a: 0, b: 1, c: 2]

iex> Enum.with_index([:a, :b, :c], 3)
[a: 3, b: 4, c: 5]

iex> Enum.with_index([:a, :b, :c], fn element, index -> {index, element} end)
[{0, :a}, {1, :b}, {2, :c}]

@spec zip(enumerables) :: [tuple()] when enumerables: [t()] | t()

Zips corresponding elements from a finite collection of enumerables into a list of tuples.

The zipping finishes as soon as any enumerable in the given collection completes.

Examples

iex> Enum.zip([[1, 2, 3], [:a, :b, :c], ["foo", "bar", "baz"]])
[{1, :a, "foo"}, {2, :b, "bar"}, {3, :c, "baz"}]

iex> Enum.zip([[1, 2, 3, 4, 5], [:a, :b, :c]])
[{1, :a}, {2, :b}, {3, :c}]

@spec zip(t(), t()) :: [{any(), any()}]

Zips corresponding elements from two enumerables into a list of tuples.

The zipping finishes as soon as either enumerable completes.

Examples

iex> Enum.zip([1, 2, 3], [:a, :b, :c])
[{1, :a}, {2, :b}, {3, :c}]

iex> Enum.zip([1, 2, 3, 4, 5], [:a, :b, :c])
[{1, :a}, {2, :b}, {3, :c}]

@spec zip_reduce(t(), acc, ([term()], acc -> acc)) :: acc when acc: term()

Reduces over all of the given enumerables, halting as soon as any enumerable is empty.

The reducer will receive 2 args: a list of elements (one from each enum) and the accumulator.

In practice, the behaviour provided by this function can be achieved with:

Enum.reduce(Stream.zip(enums), acc, reducer)

But zip_reduce/3 exists for convenience purposes.

Examples

iex> enums = [[1, 1], [2, 2], [3, 3]]
...>  Enum.zip_reduce(enums, [], fn elements, acc ->
...>    [List.to_tuple(elements) | acc]
...> end)
[{1, 2, 3}, {1, 2, 3}]

iex> enums = [[1, 2], %{a: 3, b: 4}, [5, 6]]
...> Enum.zip_reduce(enums, [], fn elements, acc ->
...>   [List.to_tuple(elements) | acc]
...> end)
[{2, {:b, 4}, 6}, {1, {:a, 3}, 5}]

@spec zip_reduce(
  t(),
  t(),
  acc,
  (enum1_elem :: term(), enum2_elem :: term(), acc -> acc)
) :: acc
when acc: term()

Reduces over two enumerables halting as soon as either enumerable is empty.

In practice, the behaviour provided by this function can be achieved with:

Enum.reduce(Stream.zip(left, right), acc, reducer)

But zip_reduce/4 exists for convenience purposes.

Examples

iex> Enum.zip_reduce([1, 2], [3, 4], 0, fn x, y, acc -> x + y + acc end)
10

iex> Enum.zip_reduce([1, 2], [3, 4], [], fn x, y, acc -> [x + y | acc] end)
[6, 4]

@spec zip_with(t(), ([term()] -> term())) :: [term()]

Zips corresponding elements from a finite collection of enumerables into list, transforming them with the zip_fun function as it goes.

The first element from each of the enums in enumerables will be put into a list which is then passed to the one-arity zip_fun function. Then, the second elements from each of the enums are put into a list and passed to zip_fun, and so on until any one of the enums in enumerables runs out of elements.

Returns a list with all the results of calling zip_fun.

Examples

iex> Enum.zip_with([[1, 2], [3, 4], [5, 6]], fn [x, y, z] -> x + y + z end)
[9, 12]

iex> Enum.zip_with([[1, 2], [3, 4]], fn [x, y] -> x + y end)
[4, 6]

@spec zip_with(t(), t(), (enum1_elem :: term(), enum2_elem :: term() -> term())) :: [
  term()
]

Zips corresponding elements from two enumerables into a list, transforming them with the zip_fun function as it goes.

The corresponding elements from each collection are passed to the provided two-arity zip_fun function in turn. Returns a list that contains the result of calling zip_fun for each pair of elements.

The zipping finishes as soon as either enumerable runs out of elements.

Zipping Maps

It's important to remember that zipping inherently relies on order. If you zip two lists you get the element at the index from each list in turn. If we zip two maps together it's tempting to think that you will get the given key in the left map and the matching key in the right map, but there is no such guarantee because map keys are not ordered! Consider the following:

left =  %{:a => 1, 1 => 3}
right = %{:a => 1, :b => :c}
Enum.zip(left, right)
# [{{1, 3}, {:a, 1}}, {{:a, 1}, {:b, :c}}]

As you can see :a does not get paired with :a. If this is what you want, you should use Map.merge/3.

Examples

iex> Enum.zip_with([1, 2], [3, 4], fn x, y -> x + y end)
[4, 6]

iex> Enum.zip_with([1, 2], [3, 4, 5, 6], fn x, y -> x + y end)
[4, 6]

iex> Enum.zip_with([1, 2, 5, 6], [3, 4], fn x, y -> x + y end)
[4, 6]