Module 7: Structs

Preparation

As covered in a previous module, a map is a type of data structure where keys point to values, and the value associated with a particular key can easily be looked up. This works really well for purposes where the set of keys change over time.

But this is not always the case. Sometimes, what we want is to define a map and know that a specific set of keys are present. This is particularly useful when defining types. For instance, we might define a person as:

defmodule Person do
  defstruct [:name, :age, :nationality, :alive?]

  def first_name(person) do
    person.name |> String.split(" ") |> Enum.at(0)
  end
end

Lets create an instance of this type:

person = %Person{name: "Keira Knightley", age: 39, nationality: :english, alive?: true}

We can call associated functions:

Person.first_name(person)

Is this person alive?

person.alive?

Sometimes a field of a struct needs to be updated. For instance, once a year the age of a living person should be incremented:

person = Map.put(person, :age, person.age + 1)

We can treat the struct as a map and add keys:

 person = Map.put(person, :cpr, "1234567890")

Note the presence of the key __struct__. This is how Elixir differentiates between maps and structs. This particular key tells Elixir that this map adheres to template defined in the Person struct.

Exercise

In the definition of Person, we skipped over this detail:

def first_name(person) do
  person.name |> String.split(" ") |> Enum.at(0)
end

Use the cell below to experimentally figure out how this function works. You may want to hover the mouse over some of the functions.

Create a new defintion of Person in the cell below. Start out with a copy of the above definition, and add a function called birthday that takes a person and returns the same person except that the returned person has an age that is one higher than that of the input person.

Now, test this function:

Lets introduce some people:

people = [
  person,
  %Person{name: "Helen Mirren", age: 79, nationality: :english, alive?: false},
  %Person{name: "William Shakespeare", age: 52, nationality: :english, alive?: true},
  %Person{name: "Edward Norton", age: 55, nationality: :american, alive?: true},
  %Person{name: "Amy Acker", age: 48, nationality: :american, alive?: true},
  %Person{name: "Hedy Lamarr", age: 85, nationality: :austrian, alive?: false},
  %Person{name: "José Valim", age: nil, nationality: :brazilian, alive?: true},
  %Person{name: "Grace Hopper", age: 85, nationality: :american, alive?: false},
]

Calculate the average age of these people.

Hints:

• The nil value is used to indicate the absence of a (valid) value.
• The built-in length function will return the length of a list.

Next step …

When done, go to the next exercise here.