View Source Enum (Elixir v1.15.7)
Functions for working with collections (known as enumerables).
In Elixir, an enumerable is any data type that implements the
Enumerable protocol. Lists ([1, 2, 3]), Maps (%{foo: 1, bar: 2})
and Ranges (1..3) are common data types used as enumerables:
iex> Enum.map([1, 2, 3], fn x -> x * 2 end)
[2, 4, 6]
iex> Enum.sum([1, 2, 3])
6
iex> Enum.map(1..3, fn x -> x * 2 end)
[2, 4, 6]
iex> Enum.sum(1..3)
6
iex> map = %{"a" => 1, "b" => 2}
iex> Enum.map(map, fn {k, v} -> {k, v * 2} end)
[{"a", 2}, {"b", 4}]However, many other enumerables exist in the language, such as MapSets
and the data type returned by File.stream!/3 which allows a file to be
traversed as if it was an enumerable.
The functions in this module work in linear time. This means that, the
time it takes to perform an operation grows at the same rate as the length
of the enumerable. This is expected on operations such as Enum.map/2.
After all, if we want to traverse every element on a list, the longer the
list, the more elements we need to traverse, and the longer it will take.
This linear behaviour should also be expected on operations like count/1,
member?/2, at/2 and similar. While Elixir does allow data types to
provide performant variants for such operations, you should not expect it
to always be available, since the Enum module is meant to work with a
large variety of data types and not all data types can provide optimized
behaviour.
Finally, note the functions in the Enum module are eager: they will
traverse the enumerable as soon as they are invoked. This is particularly
dangerous when working with infinite enumerables. In such cases, you should
use the Stream module, which allows you to lazily express computations,
without traversing collections, and work with possibly infinite collections.
See the Stream module for examples and documentation.
Summary
Functions
Returns true if all elements in enumerable are truthy.
Returns true if fun.(element) is truthy for all elements in enumerable.
Returns true if at least one element in enumerable is truthy.
Returns true if fun.(element) is truthy for at least one element in enumerable.
Finds the element at the given index (zero-based).
Splits enumerable on every element for which fun returns a new
value.
Shortcut to chunk_every(enumerable, count, count).
Returns list of lists containing count elements each, where
each new chunk starts step elements into the enumerable.
Chunks the enumerable with fine grained control when every chunk is emitted.
Given an enumerable of enumerables, concatenates the enumerables into
a single list.
Concatenates the enumerable on the right with the enumerable on the
left.
Returns the size of the enumerable.
Returns the count of elements in the enumerable for which fun returns
a truthy value.
Counts the enumerable stopping at limit.
Counts the elements in the enumerable for which fun returns a truthy value, stopping at limit.
Enumerates the enumerable, returning a list where all consecutive
duplicated elements are collapsed to a single element.
Enumerates the enumerable, returning a list where all consecutive
duplicated elements are collapsed to a single element.
Drops the amount of elements from the enumerable.
Returns a list of every nth element in the enumerable dropped,
starting with the first element.
Drops elements at the beginning of the enumerable while fun returns a
truthy value.
Invokes the given fun for each element in the enumerable.
Determines if the enumerable is empty.
Finds the element at the given index (zero-based).
Finds the element at the given index (zero-based).
Filters the enumerable, i.e. returns only those elements
for which fun returns a truthy value.
Returns the first element for which fun returns a truthy value.
If no such element is found, returns default.
Similar to find/3, but returns the index (zero-based)
of the element instead of the element itself.
Similar to find/3, but returns the value of the function
invocation instead of the element itself.
Maps the given fun over enumerable and flattens the result.
Maps and reduces an enumerable, flattening the given results (only one level deep).
Returns a map with keys as unique elements of enumerable and values
as the count of every element.
Returns a map with keys as unique elements given by key_fun and values
as the count of every element.
Splits the enumerable into groups based on key_fun.
Intersperses separator between each element of the enumeration.
Inserts the given enumerable into a collectable.
Inserts the given enumerable into a collectable according to the
transformation function.
Joins the given enumerable into a string using joiner as a
separator.
Returns a list where each element is the result of invoking
fun on each corresponding element of enumerable.
Returns a list of results of invoking fun on every nth
element of enumerable, starting with the first element.
Maps and intersperses the given enumerable in one pass.
Maps and joins the given enumerable in one pass.
Invokes the given function to each element in the enumerable to reduce
it to a single element, while keeping an accumulator.
Returns the maximal element in the enumerable according
to Erlang's term ordering.
Returns the maximal element in the enumerable as calculated
by the given fun.
Checks if element exists within the enumerable.
Returns the minimal element in the enumerable according
to Erlang's term ordering.
Returns the minimal element in the enumerable as calculated
by the given fun.
Returns a tuple with the minimal and the maximal elements in the enumerable according to Erlang's term ordering.
Returns a tuple with the minimal and the maximal elements in the enumerable as calculated by the given function.
Returns the product of all elements.
Returns a random element of an enumerable.
Invokes fun for each element in the enumerable with the
accumulator.
Invokes fun for each element in the enumerable with the accumulator.
Reduces enumerable until fun returns {:halt, term}.
Returns a list of elements in enumerable excluding those for which the function fun returns
a truthy value.
Returns a list of elements in enumerable in reverse order.
Reverses the elements in enumerable, appends the tail, and returns
it as a list.
Reverses the enumerable in the range from initial start_index
through count elements.
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.
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.
Returns a list with the elements of enumerable shuffled.
Returns a subset list of the given enumerable by index_range.
Returns a subset list of the given enumerable, from start_index (zero-based)
with amount number of elements if available.
Slides a single or multiple elements given by range_or_single_index from enumerable
to insertion_index.
Sorts the enumerable according to Erlang's term ordering.
Sorts the enumerable by the given function.
Sorts the mapped results of the enumerable according to the provided sorter
function.
Splits the enumerable into two enumerables, leaving count
elements in the first one.
Splits enumerable in two at the position of the element for which
fun returns a falsy value (false or nil) for the first time.
Splits the enumerable in two lists according to the given function fun.
Returns the sum of all elements.
Takes an amount of elements from the beginning or the end of the enumerable.
Returns a list of every nth element in the enumerable,
starting with the first element.
Takes count random elements from enumerable.
Takes the elements from the beginning of the enumerable while fun returns
a truthy value.
Converts enumerable to a list.
Enumerates the enumerable, removing all duplicated elements.
Enumerates the enumerable, by removing the elements for which
function fun returned duplicate elements.
Opposite of zip/2. Extracts two-element tuples from the
given enumerable and groups them together.
Returns the enumerable with each element wrapped in a tuple
alongside its index.
Zips corresponding elements from a finite collection of enumerables into a list of tuples.
Zips corresponding elements from two enumerables into a list of tuples.
Reduces over all of the given enumerables, halting as soon as any enumerable is empty.
Reduces over two enumerables halting as soon as either enumerable is empty.
Zips corresponding elements from a finite collection of enumerables
into list, transforming them with the zip_fun function as it goes.
Zips corresponding elements from two enumerables into a list, transforming them with
the zip_fun function as it goes.
Types
@type acc() :: any()
@type default() :: any()
@type element() :: any()
@type index() :: integer()
Zero-based index. It can also be a negative integer.
@type t() :: Enumerable.t()
Functions
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)
trueAs 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.
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)
falseFinds 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)
:noneSplits 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).
chunk_every(enumerable, count, step, leftover \\ [])
View Source (since 1.5.0)@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 theenumerable.
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]]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]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)
10Enumerates 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]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]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]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"
#=> :okDetermines if the enumerable is empty.
Returns true if enumerable is empty, otherwise false.
Examples
iex> Enum.empty?([])
true
iex> Enum.empty?([1, 2, 3])
falseFinds 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)
:errorFinds 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)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)
1Similar 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!"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}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}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}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"}]
}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}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"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]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"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}max(enumerable, sorter \\ &>=/2, empty_fallback \\ fn -> raise Enum.EmptyError end)
View Source@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])
3The 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)
0max_by(enumerable, fun, sorter \\ &>=/2, empty_fallback \\ fn -> raise Enum.EmptyError end)
View Source@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)
nilChecks 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)
falseWhen called outside guards, the in and not in
operators work by using this function.
min(enumerable, sorter \\ &<=/2, empty_fallback \\ fn -> raise Enum.EmptyError end)
View Source@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])
1The 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)
0min_by(enumerable, fun, sorter \\ &<=/2, empty_fallback \\ fn -> raise Enum.EmptyError end)
View Source@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)
nilmin_max(enumerable, empty_fallback \\ fn -> raise Enum.EmptyError end)
View Source@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}min_max_by(enumerable, fun, sorter_or_empty_fallback \\ &</2, empty_fallback \\ fn -> raise Enum.EmptyError end)
View Source@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)
nilReturns 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])
24.0Returns 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)
309Invokes 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)
24Invokes 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)
9Reduce 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()
endIn 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.
Reduces enumerable until fun returns {:halt, term}.
The return value for fun is expected to be
{:cont, acc}to continue the reduction withaccas 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]Returns a list of elements in enumerable in reverse order.
Examples
iex> Enum.reverse([1, 2, 3])
[3, 2, 1]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]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]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]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]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)
[]slide(enumerable, range_or_single_index, insertion_index)
View Source (since 1.13.0)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
lastis out of bounds, the range is truncated to the last element.If the normalized index range's
firstis 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]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.
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]}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], []}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)
{[], []}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)
25Takes 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]Converts enumerable to a list.
Examples
iex> Enum.to_list(1..3)
[1, 2, 3]Enumerates the enumerable, removing all duplicated elements.
Examples
iex> Enum.uniq([1, 2, 3, 3, 2, 1])
[1, 2, 3]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}]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}]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}]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}]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]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]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]