Golang: Rune

“Rune” means Unicode character. More precisely, it means a Unicode codepoint. It is a term golang invented.

When you see the word “rune”, you can think of it as any or all of the following:

• A integer. (possible values are 0 to 2^32-1)
• A golang type, with keyword rune. It is alias to the type int32
• A unicode character.

[see Golang: Basic Types]

The purpose of rune is similar to character type in some other languages.

Unicode gives each character a integer id. This integer id is called codepoint. It can be expressed in decimal notation, also in hexadecimal notation.

Note, not all codepoint are character as we think of. For example, newline, tab, non-breaking-space, right-to-left mark, combining character accents, are all codepoints.

Unicode has a standard notation for codepoint, starting with U+ followed by its codepoint in hexadecimal. E.g.

• A → U+41
• ♥ → U+2665
• 😂 → U+1F602

Rune Literal

A “rune literal” is a syntax to represent 1 Unicode character. Like this:

'c' → rune literal. Where the c is 1 single Unicode character. (but may be presented by escape sequence) e.g.

• var capA = 'A'
• var heart = '♥'
• var omg = '😂'
• var newline = '\n'
• var heart = '\u2665' same as '♥'
• var heart = '\U0001F602' same as '😂'

to see all backslash escapes, see Golang: String

package main

import "fmt"

func main() {

    var capA = 'A'
    var heart = '♥'
    var omg = '😂'

    // decimal, hex, unicode notation
    fmt.Printf("%d, %x, %U\n", capA, capA, capA)    // 65, 41, U+0041
    fmt.Printf("%d, %x, %U\n", heart, heart, heart) // 9829, 2665, U+2665
    fmt.Printf("%d, %x, %U\n", omg, omg, omg)       // 128514, 1f602, U+1F602
}

package main

import "fmt"

// rune in different input forms

func main() {
    fmt.Printf("%U\n", '\n') // U+000A

    fmt.Printf("%v\n", '♥' == '\u2665')     // true
    fmt.Printf("%v\n", '😂' == '\U0001F602') // true
}

Print Rune

Rune is codepoint, thus is a integer.

The following format works with integer:

• %c → the character represented by the corresponding Unicode code point
• %q → a single-quoted character literal safely escaped with Go syntax.
• %U → Unicode format. e.g. U+03B1.
• %b → base 2
• %o → base 8
• %d → base 10
• %x → base 16, with lower-case letters for a-f

package main

import "fmt"

func main() {
    // var x = 'A'
    // var x = '♥'
    var x = '😂'

    fmt.Printf("%c\n", x) // 😂
    fmt.Printf("%q\n", x) // '😂'
    fmt.Printf("%U\n", x) // U+1F602

    fmt.Printf("%b\n", x) // 11111011000000010
    fmt.Printf("%o\n", x) // 373002
    fmt.Printf("%d\n", x) // 128514
    fmt.Printf("%x\n", x) // 1f602
}

Reference

The Go Programming Language Specification - The Go Programming Language#String_literals