Structures

Individual variables or constants only go so far.

What if we want to describe, say, a person?

What attributes does a person have?

These might include:

• name
• hair
• colour
• age
• height
• mass

Definition of a structure

This is the definition of a structure:

struct Person {
    
    // MARK: Stored properties
    // These are properties that have an assigned value
    let age: Int
    let hairColour: String
    let name: String
    let height: Double
    let mass: Double
}

Action

Add the code shown above to your Structures playground now.

A structure is a data type, just like String, Int, and Double are data types.

So by defining our own structure, we are literally creating a new data type in Swift!

Creating an instance

How do we use a structure, though?

To create an instance of a type, we must provide values for each property of the type.

Here, two instances of the Person data type are created, as constants:

let joyce = Person(age: 16,
                   hairColour: "black",
                   name: "Joyce",
                   height: 160,
                   mass: 30)
 
let gordon = Person(age: 46,
                    hairColour: "red",
                    name: "Gordon",
                    height: 180,
                    mass: 78)

Action

Try creating an instance of the Person type named me to represent yourself. Make it a constant, using the let keyword, like the instances of the Person structure shown above.

Once an instance of a structure is created, we can “ask about” the values held in each property, and use these values in programs we author.

Action

Add the following code to the bottom of your Structures playground and try it out.

me.age

What shows up in the results sidebar, at right?

Variables and constants

What happens if you try to change one of the properties of the structure?

Action

Add this code at the bottom of the playground:

me.age = 21

Remember, the structure was defined with the age property as a constant. This means the the value of age cannot ever change:

struct Person {
    
    // MARK: Stored properties
    // These are properties that have an assigned value
    let age: Int           // Defined using "let", so it is constant
    let hairColour: String
    let name: String       
    let height: Double
    let mass: Double
}

Action

Change the definition of the name constant so that it is variable – use var instead of let.

Now, try running your playground again – what happens?

We still cannot change the value of the age property because the instance of the Person structure itself was defined as a constant.

Action

Change the code that creates an instance of the Person named me in your playground, so that the instance is a variable, rather than a constant.

To summarize, a structure works like any other data type.

When an instance of a structure is declared as a constant, using the let keyword, that instance cannot be changed.

When an instance of a structure is declared as a variable, using the var keyword, and there are properties of that structure also declared as variables, we can change the values of individual properties.

Discussion

Discuss with a friend in class.

What other properties of the Person structure should be defined as variables?

To answer this question, consider whether the values those properties hold would change over time.

Make any necessary changes to the Person structure in your Xcode playground.

Exercise

Here is a close-up of a retro-style hockey card for an imaginary player.

Here is an image of several similar cards.

Make a new Xcode playground named HockeyCards.

Write structure(s) in your playground to describe the data shown in the hockey cards.

Then create instances of your structure for at least three hockey cards.