Keyword Lists
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{:jason, "~> 1.4"},
{:kino, "~> 0.9", override: true},
{:youtube, github: "brooklinjazz/youtube"},
{:hidden_cell, github: "brooklinjazz/hidden_cell"}
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Upon completing this lesson, a student should be able to answer the following questions.
- What are keyword lists, and why might we want to store data in a key-value data structure?
- How do we access a value in a keyword list?
- How do we use list operators with keyword lists?
- What is the internal representation of a keyword list in Elixir?
Keyword Lists
Keyword lists are lists with keys. Each key associates with a value.
flowchart
key1: --> Value1
key2: --> Value2
key3: --> Value3Keyword lists belong to a family of data types known as associative data structures. you’ll also often hear it referred to as a key-value data structure.
Why Use A Keyword List?
Keyword lists guarantee order, and are more performant to create than maps. They are performant for small amounts of data, but less performant for large amounts of data than maps.
Therefore, we want to use a keyword lists for smaller amounts of data, or when our data should be ordered.
For example, we might have a “top five playlist” of our favourite songs stored in a keyword list. The name could represent the song, and the value could represent the song for the artist.
[artist1: "song 1", artist2: "song 2", artist3: "song 3", artist4: "song 4", artist5: "song 5"]However, for a large playlist we might choose to use a Map since order doesn’t matter, and we need the playlist to be performant for a large amount of data. We’ll cover maps in the next lesson.
# Map Example
%{
artist1: "song 1"
artist2: "song 1"
artist3: "song 1"
artist4: "song 1"
artist5: "song 1"
artist6: "song 1"
...
}Most commonly, keyword lists are used to provide a list of key-value options.
For example, when we want to label some data, IO.inspect accepts a keyword lists of options. The :label option labels the data.
IO.inspect("data", [label: "label for my data"])When a keyword list is inside of a function, Elixir removes the square brackets so the above becomes the following when we format it.
IO.inspect("data", label: "label for my data")
For a full list of options for IO.inspect, see: Inspect.Opts
Breakdown Of Keyword Lists
In Elixir, the key is an atom, and the value can be any data structure, even another keyword list.
[key: "value"]
[atom: "anything!"]
["You can use quotes for the atom key!": ""]
[snake_case_is_convention: ""]
[ButThatIsNotEnforced: ""]Keys in a keyword list do not have to be unique.
[my_key: "value", my_key: "value"]
Elixir allows you to work with keyword lists in this nice [key: "value"] syntax. However,
keyword lists are actually just a list of tuples where the first element is an atom.
[{:key, "value"}]
In the Elixir cell below, you can see how Elixir converts lists of tuples matching the {:atom, value} syntax
into a keyword list.
[{:key, "value"}, {:key, "value"}]If you break the convention in the keyword list, Elixir reverts to displaying the keyword list as a list of tuples.
[{}, key: "value"]The keyword list syntax must come at the end of a list, or we’ll cause a SyntaxError.
[key: "value", ""]List order is guaranteed, so the same is true for keyword lists.
Your Turn
In the Elixir cell below, create a keyword list of your favourite super hero. Include their :name and :secret_identity.
Example solution
[name: "Spiderman", secret_identity: "Peter Parker"]Enter your solution below.
Accessing A Key
We can access values in a keyword list by their associated key using square bracket [key] syntax where key is the atom key.
keyword_list = [key: "value"]
keyword_list[:key]Your Turn
Access the :hello key in the following keyword list.
keyword_list = [hello: "world"]Keyword List Operators
We can also use -- and ++ list operators with keyword lists.
[one: 1, two: 2] -- [one: 1]If you add a list to a keyword list, you’ll notice that the keyword list element will evaluate as a tuple again. Remember that keyword lists are simply lists of tuples with an atom and a value.
[one: 1] ++ [1]Your Turn
In the Elixir cell below, use ++ to add [one: 1] to [two: 2] to make [one: 1, two: 2].
Remove [two: 2] from [one: 1, two: 2] to make [one: 1].
Pattern Matching
Much like with lists, we can use pattern matching to bind values in a keyword list to variables.
[hello: my_variable] = [hello: "world"]
my_variableMuch like a list, we must match on the entire keyword list, otherwise we’ll cause a MatchError.
[one: one] = [one: 1, two: 2]So long as we have a match for every element in the keyword list, we can use pattern matching.
[one: one, two: two] = [one: 1, two: 2]
We can use the [head | tail] syntax to pattern match on the tail of the keyword list to avoid this issue.
[head | _tail] = [one: 1, two: 2]
head
If we want to pattern match on the keys in the list, we need to use the tuple syntax on the left hand side of the = match operator.
[{key, value}] = [key: "value"]keyvalueYour Turn
Bind 1 in the following keyword list to a variable one.
Example solution
[{_, one} | _tail] = [one: 1, two: 2, three: 3, four: 4]Enter your solution below.
[one: 1, two: 2, three: 3, four: 4]Further Reading
Consider the following resource(s) to deepen your understanding of the topic.
Commit Your Progress
DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.
Run git status to ensure there are no undesirable changes.
Then run the following in your command line from the curriculum folder to commit your progress.
$ git add .
$ git commit -m "finish Keyword Lists reading"
$ git pushWe’re proud to offer our open-source curriculum free of charge for anyone to learn from at their own pace.
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