Constants and iota

Declaring Constants

The const keyword declares a value that is fixed at compile time and can never be reassigned. Constants are useful for values like mathematical constants, configuration limits, or labels that should never accidentally change during the program's execution.

Example: Declaring Constants

package main

import "fmt"

const MaxRetries = 3

func main() {
	fmt.Println("Max retries allowed:", MaxRetries)
}

Grouped Constants

Like var, multiple related constants can be declared together in a parenthesized const block, which keeps a group of fixed values organized and easy to scan.

Example: Grouped Constants

package main

import "fmt"

const (
	StatusOK       = 200
	StatusNotFound = 404
	StatusError    = 500
)

func main() {
	fmt.Println(StatusOK, StatusNotFound, StatusError)
}

Introducing iota

Inside a const block, the identifier iota starts at 0 on the first line and increases by 1 for each subsequent line, even if you don't write it explicitly on every line. This gives you a compact way to generate a sequence of related constant values without typing each number by hand.

Note: iota resets to 0 at the start of each new const( ) block.

Example: Introducing iota

package main

import "fmt"

const (
	Sunday = iota
	Monday
	Tuesday
	Wednesday
)

func main() {
	fmt.Println(Sunday, Monday, Tuesday, Wednesday)
}

Using iota for Enumerated Values

iota is often combined with simple expressions to create meaningful enumerations, such as byte-size units or status categories, where the exact numeric value matters less than each constant being distinct and ordered.

Example: Using iota for Enumerated Values

package main

import "fmt"

const (
	_  = iota // skip 0
	KB = 1 << (10 * iota)
	MB
	GB
)

func main() {
	fmt.Println("KB:", KB, "MB:", MB, "GB:", GB)
}