Golang: Rune

What's Rune

“Rune” means Unicode codepoint. (think of it as a character.) 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 from 0 to 2^32-1, but not all are valid unicode codepoint.)
• A golang type, with keyword rune. It is alias to the type int32
• A unicode codepoint.

[see Golang: Basic Types]

Purpose of Rune

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

What's Codepoint

Unicode gives each character a integer id. This integer id is called codepoint.

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 Standard Notation for Codepoint

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

• space → U+20
• A → U+41
• ♥ → U+2665
• 😂 → U+1F602

Rune Literal

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

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

All of the following are rune values.

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

To see all backslash escapes, see Golang: String Backslash Escape

Print a rune in decimal, hex, and standard unicode notations

package main

import "fmt"

func main() {

	// print a rune in decimal, hex, and standard unicode notations

	var capA = 'A'

	fmt.Printf("%d %x %U\n", capA, capA, capA) // 65 41 U+0041
}

Print Rune

Rune is codepoint, thus is a integer.

The following Printf formats work with integer:

• %c → the character represented by the corresponding Unicode code point
• %q → a single-quoted character literal safely escaped with Go syntax.
• %U → standard Unicode notation. 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"

// print rune in different formats

func main() {
    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