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 are Structs?
An Elixir key/value data structure. They are also extensions of the Map module and thus share similar functionality.
What are some differences between Structs and Maps?
- Structs do not have access to Enum functions like Maps do
- The keys for a Struct must be included in its definition.
- If a key is provided to a Struct that the Struct is unaware of an error will be raised.
- Only atoms can be used as keys in a Struct.
How is a Struct defined?
From inside a module using the defstruct macro. Ex:
defmodule User do
defstruct email: "lisa@gmail.com", age: 29, name: "Lisa"
endHow would you instantiate a new Struct using the previous cards User struct definition?
The examples below are two separate structs created from the User struct.
lisa = %User{} => %User{age: 29, email: "lisa@gmail.com", name: "Lisa"}
jim = %User{name: "Jim", email: "jim@gmail.com"}
=> %User{age: 29, email: "jim@gmail.com", name: "Jim"}How do you update a Struct?
The same way you would update a Map. Using the | operator:
jim = %User{name: "Jim", email: "jim@gmail.com", age: 24}
jim = %{jim | age: 30}
=> %User{age: 30, email: "jim@gmail.com", name: "Jim"}How do you access a value in a Struct?
The same way you would access a value in a Map:
jim = %User{name: "Jim", email: "jim@gmail.com", age: 24}
jim.email
=> "jim@gmail.com"How do you delete a key/value from a Struct?
You don’t. Struct definitions are constant. If you need a more dynamic key/value store, a Map should be used.
Can you set a Struct key without a default value?
Yes, but it must come at the end of the beginning of the definition. Ex:
defmodule User do
defstruct [:email, age: 29, name: "Lisa"]
end