Swift Constants

Constant Characteristics

let pi = 3.14159
// pi = 3.14 // Error!

let maxUsers = 1000
// Compiler optimizes this

let apiKey = "abc123"
// Safe in concurrent code

let gravity = 9.81
// Clearly won't change

🔹 Declaring Constants

Constants are declared using the let keyword:

// Method 1: Let Swift infer the type
let companyName = "Apple Inc."     // String
let foundedYear = 1976             // Int
let stockPrice = 150.25            // Double

// Method 2: Explicitly specify the type
let country: String = "United States"
let employees: Int = 150000
let marketCap: Double = 2500000000000.0

// Method 3: Declare first, assign later (only once!)
let ceoName: String
ceoName = "Tim Cook"
// ceoName = "Someone Else"  // This would cause an error!

print("Company: \(companyName)")
print("Founded: \(foundedYear)")
print("Stock Price: $\(stockPrice)")
print("Country: \(country)")
print("Employees: \(employees)")
print("CEO: \(ceoName)")

🔹 Constants vs Variables

Understanding when to use constants vs variables:

// Constants (use let) - values that never change
let pi = 3.14159
let appVersion = "2.1.0"
let maxLoginAttempts = 3
let welcomeMessage = "Welcome to our app!"

// Variables (use var) - values that can change
var currentUser = "guest"
var loginAttempts = 0
var isLoggedIn = false
var sessionTime = 0

print("=== App Constants ===")
print("Pi: \(pi)")
print("Version: \(appVersion)")
print("Max attempts: \(maxLoginAttempts)")
print("Message: \(welcomeMessage)")

print("\n=== Current State ===")
print("User: \(currentUser)")
print("Attempts: \(loginAttempts)")
print("Logged in: \(isLoggedIn)")
print("Session: \(sessionTime) minutes")

// Simulate user login
currentUser = "john_doe"
loginAttempts = 1
isLoggedIn = true
sessionTime = 15

print("\n=== After Login ===")
print("User: \(currentUser)")
print("Attempts: \(loginAttempts)")
print("Logged in: \(isLoggedIn)")
print("Session: \(sessionTime) minutes")

🔹 Common Use Cases for Constants

Here are typical scenarios where constants are perfect:

// Configuration values
let serverURL = "https://api.myapp.com"
let apiVersion = "v1"
let timeoutSeconds = 30

// Mathematical constants
let earthRadius = 6371.0  // kilometers
let speedOfLight = 299792458  // meters per second
let goldenRatio = 1.618033988749

// App settings
let defaultLanguage = "en"
let supportEmail = "[email protected]"
let privacyPolicyURL = "https://myapp.com/privacy"

// Color values (in a real app, these might be hex codes)
let primaryColor = "Blue"
let secondaryColor = "Gray"
let errorColor = "Red"

print("=== Server Configuration ===")
print("URL: \(serverURL)/\(apiVersion)")
print("Timeout: \(timeoutSeconds) seconds")

print("\n=== Physical Constants ===")
print("Earth radius: \(earthRadius) km")
print("Speed of light: \(speedOfLight) m/s")
print("Golden ratio: \(goldenRatio)")

print("\n=== App Settings ===")
print("Language: \(defaultLanguage)")
print("Support: \(supportEmail)")
print("Privacy: \(privacyPolicyURL)")

print("\n=== Theme Colors ===")
print("Primary: \(primaryColor)")
print("Secondary: \(secondaryColor)")
print("Error: \(errorColor)")

🔹 Best Practices

Follow these guidelines when using constants:

// Good practice: Use let by default, var only when needed
let userName = "Alice"        // Won't change during session
let maxFileSize = 10485760   // 10MB limit, never changes
var uploadProgress = 0.0     // Will change during upload
var filesUploaded = 0        // Counter that increments

// Computed constants
let welcomeText = "Welcome, \(userName)!"
let fileSizeInMB = Double(maxFileSize) / 1048576.0

print("User: \(userName)")
print("Message: \(welcomeText)")
print("Max file size: \(fileSizeInMB) MB")
print("Progress: \(uploadProgress)%")
print("Files uploaded: \(filesUploaded)")

// Simulate file upload
uploadProgress = 45.5
filesUploaded = 2

print("\nAfter uploading:")
print("Progress: \(uploadProgress)%")
print("Files uploaded: \(filesUploaded)")