Understanding Elixir Maps for new developers

This and the other “Deck” posts are a repurposing of flashcard study decks to Q&A blog posts. Google was not showing love to this content as a set of flashcards and I didn’t want to delete them entirely, I hope you find it useful.

What is an Elixir Map?

They are the most often used key/value store data type in Elixir.

What is a Map used for?

Maps are used to associate data elements with meaningful keys and the data associated with that key can be retrieved or updated using its key.

What are different ways to add a new value to a Map?

Using the Map module:

Map.put(%{name: "Humphry Dobs", email: "thedobinator@emailsalot.com"}, :age, 24)

Or Using the Kernel module:

put_in %{name: "Humphry Dobs", email: "thedobinator@emailsalot.com"}[:age], 24

Does Elixir contain an operator for updating a keys value?

Yes, the operator is |, but key has to exist, otherwise the code will error. The first example below updates the value for key one:, the second example tries to update the value for key three:, which doesn’t exist so a KeyError is thrown.

=>map = %{one: 1, two: 2} 
=> %{map | one: "one"} 
%{one: "one", two: 2}

=> map = %{one: 1, two: 2}
=> %{map | three: 3} 
(KeyError)

How can you read a value from a Map without pattern matching?

Through access syntax, [].

=> %{:key => "value"}[:key] 
"value"

If the key is an atom you can also use the static lookup syntax.

=> %{:key => "value"}.key 
"value"

Or through the Map module.

=> Map.get(%{a: 1}, :a) 
1

How can you read a value from a Map using pattern matching?

=> %{:a => a} = %{:a => 12, :b => 14} 
=> a 
12

The variable a above is set using pattern matching. It will return the value of the key :a. In this case that is 12.

What happens when you try to pattern match on a key that doesn't exist?

An error is thrown. The below will return a MatchError.

%{:c => c} = %{:a => 12, :b => 14} 
=> (MatchError)

Can you use a variable for defining a key?

Yes. In the example below the variable t is a variable used as a key for the value “dog”.

=> t = :type
=> %{t => "dog", :name => "Abby"}[:type] 
"dog"