Golang tutorial - 2 Constant

#go #beginners

Constant in Golang

In many programming languages, there is a special variable called constant. Constant is a variable that the value cannot be changed. To declare a constant variable in Go, use const keyword.
The declaration of constant can be seen in this code below.

const pi = 3.14               //1. using const keyword followed by constant name
const myConst string = "test" //2. using const keyword with specified type
const (
    a = 12
    b = 23
    c = 34
) //3. declare multiple constants easily with const(..) just like declaring variables

Here it is the complete code example using const.

package main

import "fmt"

const pi = 3.14               
const myConst string = "test" 
const (
    a = 12
    b = 23
    c = 34
) 
func main() {
    area := pi * 7 * 7
    fmt.Println("Area of circle : ", area)
    fmt.Println("Other const", myConst, a, b, c)
}

The output from that code is :

Area of circle :  153.86
Other const test 12 23 34

If we try to change a value from a const, this code throw an error

package main

import "fmt"

func main() {
    const a = 12
    a = 45 //change a value from const
    fmt.Println("value of a", a)
}

The error message from that code :

.\main.go:7:4: cannot assign to a

iota

There is a special const in Go called iota. basically, iota is a const that has a default value equals 0 and will increased automatically by 1 if assigned into another constant.

Here it is the code example using iota.

package main

import "fmt"

const (
    a = iota // iota = 0
    b = iota // iota = 1
    c = iota // iota = 2
)

const d = iota // iota value reset to 0 because assign into another variable with const keyword

func main() {
    fmt.Println(a)
    fmt.Println(b)
    fmt.Println(c)
    fmt.Println(d)
}

The output from that code :

The another example of using iota can be seen in this code.

package main

import (
    "fmt"
)

const (
    _      = iota      //ignore the default value with _ notation
    bronze = 10 * iota //iota = 1
    silver = 10 * iota // iota = 2
    gold   = 10 * iota // iota = 3

)

func main() {
    score1 := 200 * bronze
    score2 := 200 * silver
    score3 := 200 * gold
    fmt.Println("The scores : ", score1, score2, score3)
}