Swift Variables: Syntax, Usage, and Examples
Variables in Swift store and manage data in a program. They hold values that can change during execution, making them essential for dynamic applications. Swift provides different ways to declare variables, each with specific use cases.
How to Declare Variables in Swift
Use the var keyword to declare a variable in Swift. The variable’s type is inferred based on the assigned value, but you can also specify it explicitly.
swift
var name = "Alice" // String inferred
var age: Int = 30 // Explicit type annotation
var isLoggedIn = false // Boolean inferred
Once a variable is declared, update its value freely.
swift
name = "Bob" // Allowed, because name is declared as a variable
age = 35 // Allowed
Use let instead of var when the value should not change. Constants prevent accidental modifications.
swift
let birthYear = 1993
// birthYear = 2000 // Error: Cannot assign to a constant
When to Use Variables in Swift
Variables allow you to manage state, store data, and create dynamic behaviors. Use variables in Swift for:
1. Storing User Input
When building interactive applications, variables store user responses.
swift
var username = ""
username = "Charlie"
print("Welcome, \(username)!")
2. Tracking Application State
Use variables to track whether a user is logged in, a feature is enabled, or a session is active.
swift
var isDarkModeEnabled = false
isDarkModeEnabled = true
3. Performing Calculations
Mathematical operations require variables to store and update values dynamically.
swift
var score = 0
score += 10 // Increases score by 10
print("Current score: \(score)")
Examples of Variables in Swift
Variables are widely used in Swift applications. Here are some examples:
Declaring Variables
Declare and modify variables in Swift.
swift
var city = "Paris"
city = "New York"
print(city) // Output: New York
Using Variables in Functions
Pass and modify variables inside functions.
swift
func greetUser(name: String) {
print("Hello, \(name)!")
}
var user = "Alice"
greetUser(name: user)
Variables in Loops
Variables store counters or accumulators inside loops.
swift
var total = 0
for i in 1...5 {
total += i
}
print(total) // Output: 15
Variables with Optionals
Optional variables can hold a value or nil.
swift
var optionalName: String? = "Alice"
print(optionalName ?? "Guest") // Output: Alice
optionalName = nil
print(optionalName ?? "Guest") // Output: Guest
Learn More About Variables in Swift
Swift provides additional functionality for working with variables, including computed properties, global variables, and environment variables.
Variable vs. Constant
Use var for values that change and let for values that remain constant.
swift
var score = 10
score += 5 // Allowed
let maxScore = 100
// maxScore = 200 // Error: Cannot assign to a constant
Static Variables in Swift
Use static var to define properties shared across all instances of a class or struct.
swift
struct Settings {
static var theme = "Light"
}
print(Settings.theme) // Output: Light
Computed Variables in Swift
Computed properties calculate values dynamically instead of storing them.
swift
struct Rectangle {
var width: Double
var height: Double
var area: Double {
return width * height
}
}
let rect = Rectangle(width: 5, height: 10)
print(rect.area) // Output: 50
Global Variables
Define global variables outside of functions or types for use throughout the program.
swift
var appLanguage = "English"
func printLanguage() {
print("Current language: \(appLanguage)")
}
printLanguage() // Output: Current language: English
Override Variables in Subclasses
Use override var to modify inherited properties.
swift
class Animal {
var sound: String {
return "Unknown"
}
}
class Dog: Animal {
override var sound: String {
return "Bark"
}
}
let myDog = Dog()
print(myDog.sound) // Output: Bark
Swift Environment Variables
Retrieve environment variables using ProcessInfo.
swift
if let path = ProcessInfo.processInfo.environment["PATH"] {
print("System Path: \(path)")
}
Checking the Type of a Variable
Use type(of:) to determine a variable’s type at runtime.
swift
let num = 42
print(type(of: num)) // Output: Int
Printing Variables in Swift
Use print() to display variable values.
swift
var message = "Hello, Swift!"
print(message) // Output: Hello, Swift!
Public and Private Variables
Swift allows different access levels for variables.
swift
class User {
public var name = "Alice" // Accessible from anywhere
private var password = "1234" // Accessible only inside the class
}
let user = User()
print(user.name) // Allowed
// print(user.password) // Error: password is private
Best Practices for Using Variables in Swift
- Use
letwhen possible – If a value should not change, declare it as a constant. - Choose meaningful names – Avoid generic names like
xortemp. - Group related variables – Organize variables in structures or classes.
- Avoid magic numbers – Define constants instead of using hardcoded values.
- Minimize the use of global variables – Keep variables within the smallest possible scope to reduce unintended side effects.
Looking to dive deeper into Swift variables and other essential Swift concepts? Check out our Swift programming course.