Day 8
Structs Part 1
Creating your own structs
In Swift, a struct is used to store variables of different data types.
structures, which are a ānamed typeā. Like a String, Int or Array, you can define your own structures to create named types to later use in your code
Variables inside structs are called properties.
Structs can be given their own variables and constants, and their own functions, then created and used however you want.
struct Sport {
var name: String
}
That defines the type, so now we can create and use an instance of it:
var tennis = Sport(name: "Tennis")
print(tennis.name)
Output:
Tennis
tennis.name = "Lawn tennis"
print(tennis.name)
Output:
Lawn tennis
Properties can have default values just like regular variables, and you can usually rely on Swiftās type inference.
Computed properties
struct Sport {
var name: String
}
That has a name property that stores a String. These are called stored properties, because Swift has a different kind of property called a computed property ā a property that runs code to figure out its value.
struct Sport {
var name: String
var isOlympicSport: Bool
var olympicStatus: String {
if isOlympicSport {
return "\(name) is an Olympic sport"
} else {
return "\(name) is not an Olympic sport"
}
}
}
let chessBoxing = Sport(name: "Chessboxing", isOlympicSport: false)
print(chessBoxing.olympicStatus)
Output:
Chessboxing is not an Olympic sport
Property observers
struct Progressa {
var task: String
var amount: Int {
didSet {
print("\(task) is now \(amount)% complete")
}
}
}
var progress = Progressa(task: "Loading data", amount: 0)
progress.amount = 30
progress.amount = 80
progress.amount = 100
Output:
Loading data is now 30% complete
Loading data is now 80% complete
Loading data is now 100% complete
What we want to happen is for Swift to print a message every time amount changes, and we can use a didSet property observer for that. This will run some code every time amount changes:
You can also use willSet to take action before a property changes, but that is rarely used.
Methods
Structs can have functions inside them, and those functions can use the properties of the struct as they need to. Functions inside structs are called methods.
struct City {
var population: Int
func collectTaxes() -> Int {
return population * 1000
}
}
let london = City(population: 9_000_000)
london.collectTaxes()
Mutating methods
If a struct has a variable property but the instance of the struct was created as a constant, that property canāt be changed.
When you want to change a property inside a method, you need to mark it using the mutating keyword, like this:
struct Person {
var name: String
mutating func makeAnonymous() {
name = "Anonymous"
}
}
var person = Person(name: "Ed")
person.makeAnonymous()
print(person.name)
Output:
Anonymous
Properties and methods of strings
let string = "Welcome to 100Days Of Swift"
print(string.count) //27
They have a hasPrefix() method that returns true if the string starts with specific letters:
print(string.hasPrefix("Sau")) //false
print(string.hasPrefix("Welcome")) //true
print(string.uppercased()) //WELCOME TO 100DAYS OF SWIFT
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