9.1. Structs

Do you remember tuples? We can store different types of data together in a tuple. Structs are very similar to that. We can have any kind of variables in it. For example, a simple struct to store a person's details would be:

struct Person{
    var name:String
    var age:Int
    var city:String
}

Here the struct Person holds three properties (we call the variables inside structs properties): two strings name and city, and one integer age. Please note that in Swift the name of a struct or class should start with an uppercase character, like Person.

Now since struct creates a custom data type for us, we can use Person as a data type everywhere. So, let's create a new variable (or instance, more appropriately) of type Person.

var newPerson: Person = Person(name: "Bob Smith", age: 30, city: "Boston")

//print the variable person of type Person...
print(newPerson)
//print newPerson's name...
print(newPerson.name)

It will print:

Person(name: "Bob Smith", age: 30, city: "Boston")
Bob Smith

So we can create instances of our custom data type Person and once created, we can access the properties of the instances. Also, we can modify the properties of the instances, e.g. we can modify the age of newPerson like:

//modifying the inner variables of newPerson
newPerson.age = 36
//printing newPerson after changing the age...
print(newPerson)

It prints:

Person(name: "Bob Smith", age: 36, city: "Boston")

The value of newPerson's age got changed to 36.

Computed properties

Swift allows us to have special properties called computed properties in a struct. Basically, it means that we do not supply the value for that property. When we create a variable of a struct, the struct automatically calculates the value and sets it. For example, from the age property of a Person we can easily determine if the person is a minor or an adult, right?

Let's modify the Person struct to add a new computed property called isMinor:

struct Person{
    var name:String
    var age:Int
    var city:String
    
//    computed property .....
    var isMinor:Bool{
        if age < 18{
            return true
        }else{
            return false
        }
    }
}

Here, we are computing the value of the property isMinor. If the value of age is less than 18, we return true, that basically sets the value of isMinor to true. Else, it sets the value of the property to false. Now, let's add the following code:

print(newPerson)

if(newPerson.isMinor){
    print("\(newPerson.name) is a minor!")
}else{
    print("\(newPerson.name) is an adult!")
}

It prints:

Person(name: "Bob Smith", age: 36, city: "Boston")
Bob Smith is an adult!

So depending on different values of the property age, it automatically computes the value of the property isMinor.