ExArray

ExArray.Array is a module that provides Array-ish functionality to Elixir.

The ExArray.Array structure is useful when fast random access, by index, is needed. It can be dynamically grown and shrunk by removing or adding elements from any position in the structure.

When passing indexes to any function in the module, negative integers are supported. Negative integers are treated as an offset from the end of the structure. For example, if the structure contains 5 elements, the index -1 will return the last element, -2 will return the second to last element, and so on. If the negative index runs over the beginning of the structure, the appropriate failure condition will occur, either raises or an error tuple, depending on the method being called. In the example with 5 elements, the index -6 will result in an error, as there is no element before the first element. When a positive index is passed, the index is treated as an offset from the beginning and the appropriate failure condition will occur if the index runs over the end of the structure.

Full documentation can be found at https://hexdocs.pm/exarray.

Construction

ExArray.Array can be constructed in a number of ways. The most common is to use the new/0 function to create an empty structure. Alternatively, you can use the new/1 function to create a structure containing the elements in the given list.

iex> ExArray.Array.new()
%ExArray.Array{length: 0, contents: %{}}

iex> ExArray.Array.new([1,2,3])
%ExArray.Array{length: 3, contents: %{0 => 1, 1 => 2, 2 => 3}}

Accessing elements

Elements can be accessed by index using either the get/2 or get!/2 functions. The difference between these two functions is that get!/2 will raise an error if the index is out of bounds, while get/2 will return an error tuple.

iex> ExArray.Array.get(ExArray.Array.new([1, 2, 3]), 1)
{:ok, 2}

iex> ExArray.Array.get(ExArray.Array.new([1, 2, 3]), 3)
{:error, :out_of_bounds}

iex> ExArray.Array.get!(ExArray.Array.new([1, 2, 3]), 1)
2

iex> ExArray.Array.get!(ExArray.Array.new([1, 2, 3]), 3)
** (ArgumentError) Index 3 is out of bounds for length 3

Adding Elements

Adding elements to the end of the structure is fast as no other elements need to be re-indexed. This is done using the add/2 function. The add/2 function returns a new structure with the element added to the end.

This operation should always succeed

iex> ExArray.Array.add(ExArray.Array.new([1, 2, 3]), 4)
%ExArray.Array{length: 4, contents: %{0 => 1, 1 => 2, 2 => 3, 3 => 4}}

Adding elements to the beginning or middle of the structure is slower as all elements after the index need to be re-indexed. This is done using add_at/3 or add_at!/3. These functions return a new structure with the element added at the given index. If an element existed at the given index, it and all other elements after it will be shifted one to the right.

iex> ExArray.Array.add_at(ExArray.Array.new([1, 2, 3]), 1, 4) 
{:ok, %ExArray.Array{length: 4, contents: %{0 => 1, 1 => 4, 2 => 2, 3 => 3}}}

iex> ExArray.Array.add_at(ExArray.Array.new([1, 2, 3]), 4, 4)
{:error, :out_of_bounds}

iex> ExArray.Array.add_at!(ExArray.Array.new([1, 2, 3]), 1, 4)
%ExArray.Array{length: 4, contents: %{0 => 1, 1 => 4, 2 => 2, 3 => 3}}

iex> ExArray.Array.add_at!(ExArray.Array.new([1, 2, 3]), 4, 4)
** (ArgumentError) Index 4 is out of bounds for length 3

Removing Elements

Removing elements from the end of the structure is fast as no other elements need to be re-indexed. This is done using the remove/1 function. The remove/1 function returns a new structure with the last element removed.

If the ExArray.Array is empty, the remove/1 function will return the original structure.

This operation should always succeed

iex> ExArray.Array.remove(ExArray.Array.new([1, 2, 3]))
%ExArray.Array{length: 2, contents: %{0 => 1, 1 => 2}}

iex> ExArray.Array.remove(ExArray.Array.new())
%ExArray.Array{length: 0, contents: %{}}

Removing elements from the beginning or middle of the structure is slower as all elements after the index need to be re-indexed. This is done using remove_at/2 or remove_at!/2. These functions return a new structure with the element at the given index removed. All elements after it will be shifted one to the left.

iex> ExArray.Array.remove_at(ExArray.Array.new([1, 2, 3]), 1)
{:ok, %ExArray.Array{length: 2, contents: %{0 => 1, 1 => 3}}}

iex> ExArray.Array.remove_at(ExArray.Array.new([1, 2, 3]), 4)
{:error, :out_of_bounds}

iex> ExArray.Array.remove_at!(ExArray.Array.new([1, 2, 3]), 1)
%ExArray.Array{length: 2, contents: %{0 => 1, 1 => 3}}

iex> ExArray.Array.remove_at!(ExArray.Array.new([1, 2, 3]), 4)
** (ArgumentError) Index 4 is out of bounds for length 3

Replacing Elements

An element can be replaced at the given index without requiring re-indexing. Use set/3 and its raising counterpart for these operations.

iex> ExArray.Array.new([1, 2, 3]) |> ExArray.Array.set(1, 4)
{:ok, %ExArray.Array{length: 3, contents: %{0 => 1, 1 => 4, 2 => 3}}}

iex> ExArray.Array.new([1, 2, 3]) |> ExArray.Array.set(-1, 4)
{:ok, %ExArray.Array{length: 3, contents: %{0 => 1, 1 => 2, 2 => 4}}}

iex> ExArray.Array.new([1, 2, 3]) |> ExArray.Array.set(4, 4)
{:error, :out_of_bounds} 

Collectable Operations

ExArray.Array implements the Collectable protocol. This means that it can be used with Enum.into and for special forms

iex> Enum.into([1, 2, 3], ExArray.Array.new())
%ExArray.Array{length: 3, contents: %{0 => 1, 1 => 2, 2 => 3}}

iex> for x <- [1, 2, 3], into: ExArray.Array.new(), do: x
%ExArray.Array{length: 3, contents: %{0 => 1, 1 => 2, 2 => 3}}

Enum Operations

ExArray.Array implements the Enumerable protocol. This means that it can be used with the functions in the Enum module.

iex> ExArray.Array.new([1, 2, 3]) |> Enum.map(&(&1 * 2))
[2, 4, 6]

iex> ExArray.Array.new([1, 2, 3]) |> Enum.reduce(&(&1 + &2))
6

iex> ExArray.Array.new([1, 2, 3]) |> Enum.filter(&(&1 > 1))
[2, 3]

iex> ExArray.Array.new([1, 2, 3]) |> Enum.at(1)
2

Installation

If available in Hex, the package can be installed by adding ex_array to your list of dependencies in mix.exs:

def deps do
  [
    {:exarray, "~> 0.2.0"}
  ]
end

Acknowledgements

Special thanks to the following individuals for providing feedback on public API design:

• @llalon
• @rgmz