Go Constants
Understanding immutable values in Go programming
🔒 What are Go Constants?
Constants are fixed values that cannot be changed during program execution. They store unchanging data like mathematical values, configuration settings, or fixed text throughout your application.
// Constants cannot be changed
const pi float64 = 3.14159
const appName string = "MyApp"
const maxUsers int = 1000
fmt.Println(pi, appName, maxUsers)
Output:
3.14159 MyApp 1000
Key Constants Concepts
Immutable
Values cannot be changed after declaration
const name = "Alice"Compile Time
Values must be known at compile time
const size = 100Typed/Untyped
Can have explicit or inferred types
const pi = 3.14Block Declaration
Group constants together
const (
x = 1
y = 2
)🔹 Basic Constant Declaration
Simple ways to declare constants:
// Basic constant declarations
const companyName string = "TechCorp"
const foundedYear int = 2010
const pi float64 = 3.14159
const isActive bool = true
// Constants with type inference
const version = "1.0.0"
const maxConnections = 100
const rate = 0.05
fmt.Printf("Company: %s (Founded: %d)\n", companyName, foundedYear)
fmt.Printf("Version: %s, Max Connections: %d\n", version, maxConnections)
fmt.Printf("Pi: %.3f, Rate: %.2f, Active: %t\n", pi, rate, isActive)Output:
Company: TechCorp (Founded: 2010)
Version: 1.0.0, Max Connections: 100
Pi: 3.142, Rate: 0.05, Active: true
🔹 Multiple Constants Declaration
Declare multiple constants at once:
// Multiple constants in one line
const width, height int = 800, 600
const firstName, lastName = "John", "Doe"
// Block declaration for better organization
const (
StatusActive = "active"
StatusInactive = "inactive"
StatusPending = "pending"
MinAge = 18
MaxAge = 65
DefaultTimeout = 30
MaxRetries = 3
)
fmt.Printf("Screen: %dx%d\n", width, height)
fmt.Printf("User: %s %s\n", firstName, lastName)
fmt.Printf("Status: %s, Age Range: %d-%d\n", StatusActive, MinAge, MaxAge)
fmt.Printf("Timeout: %ds, Retries: %d\n", DefaultTimeout, MaxRetries)Output:
Screen: 800x600
User: John Doe
Status: active, Age Range: 18-65
Timeout: 30s, Retries: 3
🔹 Constants vs Variables
Understanding the key differences:
// Constants - cannot be changed
const appVersion = "2.1.0"
const maxFileSize = 1024
// Variables - can be changed
var currentUser = "alice"
var fileCount = 5
fmt.Printf("App Version: %s (constant)\n", appVersion)
fmt.Printf("Max File Size: %d MB (constant)\n", maxFileSize)
fmt.Printf("Current User: %s (variable)\n", currentUser)
fmt.Printf("File Count: %d (variable)\n", fileCount)
// This works - changing variables
currentUser = "bob"
fileCount = 10
// This would cause an error - cannot change constants
// appVersion = "2.2.0" // Error!
// maxFileSize = 2048 // Error!
fmt.Printf("Updated User: %s, Files: %d\n", currentUser, fileCount)Output:
App Version: 2.1.0 (constant)
Max File Size: 1024 MB (constant)
Current User: alice (variable)
File Count: 5 (variable)
Updated User: bob, Files: 10
🔹 Practical Constants Example
Real-world usage of constants in applications:
// Application configuration constants
const (
AppName = "E-Commerce Store"
AppVersion = "1.2.3"
DatabaseURL = "localhost:5432"
// HTTP Status codes
StatusOK = 200
StatusNotFound = 404
StatusError = 500
// Business rules
FreeShippingLimit = 50.0
TaxRate = 0.08
MaxItemsInCart = 20
)
// Using constants in calculations
var orderTotal float64 = 75.50
var itemCount int = 3
fmt.Printf("=== %s v%s ===\n", AppName, AppVersion)
fmt.Printf("Order Total: $%.2f\n", orderTotal)
if orderTotal >= FreeShippingLimit {
fmt.Println("✓ Free shipping applies!")
} else {
fmt.Printf("Add $%.2f more for free shipping\n", FreeShippingLimit-orderTotal)
}
tax := orderTotal * TaxRate
finalTotal := orderTotal + tax
fmt.Printf("Tax (%.1f%%): $%.2f\n", TaxRate*100, tax)
fmt.Printf("Final Total: $%.2f\n", finalTotal)
fmt.Printf("Items in cart: %d/%d\n", itemCount, MaxItemsInCart)Output:
=== E-Commerce Store v1.2.3 ===
Order Total: $75.50
✓ Free shipping applies!
Tax (8.0%): $6.04
Final Total: $81.54
Items in cart: 3/20