Table of Contents

Overview

rune in Go is an alias for int32 meaning it is an integer value. This integer value is meant to represent a Unicode Code Point. To understand rune you have to know what Unicode is. Below is short description but you can refer to the famous blog post about it –

The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)

What is Unicode

Unicode is a superset of ASCII characters which assigns a unique number to every character that exists. This unique number is called Unicode Code Point.For eg

Digit 0 is represented as Unicode Point U+0030 (Decimal Value – 48)

Small Case b is represented as Unicode Point U+0062 (Decimal Value – 98)

A pound symbol £ is represented as Unicode Point U+00A3 (Decimal Value – 163)

Visit https://en.wikipedia.org/wiki/List_of_Unicode_characters to know about Unicode Point of other characters. But Unicode doesn’t talk about how these code points will be saved in memory. This is where utf-8 comes into picture

UTF-8

utf-8 saves every Unicode Point either using 1, 2, 3 or 4 bytes. ASCII points are stored using 1 byte. That is why rune is an alias for int32 because a Unicode Point can be of max 4 bytes in Go as source code in GO is encoded using utf-8, hence every string is also encoded in utf-8

Every rune is intended to refer to one Unicode Point. For eg if you print a string after typecasting it to a rune array then it will print the Unicode Point for each of character. For for below string “0b£” output will be – [U+0030 U+0062 U+00A3]

fmt.Printf("%U\n", []rune("0b£"))

When to Use

You should use a rune when you intend to save Unicode Code Point in the rune value. A rune array should be used when all values in the array are meant to be a Unicode Code Point.

Rune is also used to represent a character.

Declare Rune

A rune is declared using a character between single quotes like below declaring a variable named ‘rPound’

rPound := '£'

After declaring Rune you can perform below things as well

Print Type – Output will be int32

fmt.Printf("Type: %s\n", reflect.TypeOf(rPound))

Print Unicode Code Point – Output will be U+00A3

fmt.Printf("Unicode CodePoint: %U\n", rPound)

Print Character – Output will be £

fmt.Printf("Character: %c\n", r)

Code:

Below is the code illustrating each point we discussed

package main

import (
    "fmt"
    "reflect"
    "unsafe"
)

func main() {
    r := 'a'
    
    //Print Size
    fmt.Printf("Size: %d\n", unsafe.Sizeof(r))
    
    //Print Type
    fmt.Printf("Type: %s\n", reflect.TypeOf(r))
    
    //Print Code Point
    fmt.Printf("Unicode CodePoint: %U\n", r)
    
    //Print Character
    fmt.Printf("Character: %c\n", r)
    s := "0b£"
    
    //This will print the Unicode Points
    fmt.Printf("%U\n", []rune(s))
    
    //This will the decimal value of Unicode Code Point
    fmt.Println([]rune(s))
}

Output:

Size: 4
Type: int32
Unicode CodePoint: U+0061
Character: a
[U+0030 U+0062 U+00A3]
[48 98 163]

Rune array to string and vice versa

Rune array to string

package main

import "fmt"

func main() {
    runeArray := []rune{'a', 'b', '£'}
    s := string(runeArray)
    fmt.Println(s)
}

Output:

ab£

String to Rune Array

package main

import "fmt"

func main() {
    s := "ab£"
    r := []rune(s)
    fmt.Printf("%U\n", r)
}

Output:

[U+0061 U+0062 U+00A3]

rune