Constants and iota in Go

Constants are values that do not change during program execution. Go provides the const keyword to define constants and iota to create enumerated values efficiently.

Constants make programs safer, clearer, and easier to maintain.

Why Use Constants?

Constants are used when a value should remain fixed throughout the program.

• Improve code readability
• Prevent accidental modification
• Make programs easier to update
• Represent fixed business rules

Examples include tax rates, limits, configuration flags, and status codes.

Declaring Constants

Constants are declared using the const keyword.

const pi = 3.14159
const appName = "Dataplexa"
const maxUsers = 1000

Once declared, a constant value cannot be changed.

Typed vs Untyped Constants

Go supports both typed and untyped constants.

Untyped Constants

The type is inferred when the constant is used.

const discount = 10
finalPrice := 100 - discount

Typed Constants

The type is explicitly defined.

const taxRate float64 = 7.5

Typed constants enforce stricter type rules.

Declaring Multiple Constants

Go allows grouping constants using parentheses.

const (
    minAge = 18
    maxAge = 60
    country = "USA"
)

This improves organization and readability.

What Is iota?

iota is a special identifier in Go used to create sequential constants automatically.

It starts from 0 and increments by 1 for each constant in a block.

Basic iota Example

const (
    Sunday = iota
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday
)

Values assigned:

• Sunday → 0
• Monday → 1
• Tuesday → 2
• ...

Using iota for Status Codes

A common real-world use case is defining application states.

const (
    StatusPending = iota
    StatusApproved
    StatusRejected
)

This ensures consistent and readable status handling across the application.

Skipping Values with iota

You can skip values using the blank identifier.

const (
    _ = iota
    Low
    Medium
    High
)

Here, Low starts from 1 instead of 0.

iota with Expressions

You can combine iota with arithmetic operations.

const (
    KB = 1 << (10 * iota)
    MB
    GB
    TB
)

This is commonly used for memory sizes.

Real-World Example

In a subscription system:

const (
    FreePlan = iota
    BasicPlan
    PremiumPlan
)

Using constants prevents errors caused by magic numbers.

Rules and Limitations

• Constants cannot use :=
• Constants cannot be assigned runtime values
• Constants are evaluated at compile time

Example of invalid constant:

// invalid
const timeNow = time.Now()

Practice Exercise

Task

Create constants for:

• Three user roles using iota
• A fixed tax percentage
• A maximum login limit

Print their values using fmt.Println().

What You Will Learn Next

In the next lesson, you will learn about operators in Go, which allow you to perform calculations, comparisons, and logical operations.