Logs, Logs, Logs!

The rune type in Go is an alias for int32. Given this underlying int32 type, the rune type holds a signed 32-bit integer value. However, unlike an int32 type, the integer value stored in a rune type represents a single Unicode character.

Unicode and Unicode Code Points

Unicode is a superset of ASCII that represents characters by assigning a unique number to every character. This unique number is called a Unicode code point. Unicode aims to represent all the world's characters including various alphabets, numbers, symbols, and even emoji as Unicode code points.

In Go, the rune type represents a single Unicode code point.

The following table contains example Unicode characters along with their Unicode code point and decimal values:

UTF-8

UTF-8 is a variable-width character encoding that is used to encode every Unicode code point as 1, 2, 3, or 4 bytes. Since a Unicode code point can be encoded as a maximum of 4 bytes, the rune type needs to be able to hold up to 4 bytes of data. That is why the rune type is an alias for int32 as an int32 type is capable of holding up to 4 bytes of data.

Go source code files are encoded using UTF-8.

Using Runes

Variables of type rune are declared by placing a character inside single quotes:

myRune := '¿'

Since rune is just an alias for int32, printing a rune's type will yield int32:

myRune := '¿'
fmt.Printf("myRune type: %T\n", myRune)
// Output: myRune type: int32

Similarly, printing a rune's value will yield its integer (decimal) value:

myRune := '¿'
fmt.Printf("myRune value: %v\n", myRune)
// Output: myRune value: 191

To print the Unicode character represented by the rune, use the %c formatting verb:

myRune := '¿'
fmt.Printf("myRune Unicode character: %c\n", myRune)
// Output: myRune Unicode character: ¿

To print the Unicode code point represented by the rune, use the %U formatting verb:

myRune := '¿'
fmt.Printf("myRune Unicode code point: %U\n", myRune)
// Output: myRune Unicode code point: U+00BF

Runes and Strings

Strings in Go are encoded using UTF-8 which means they contain Unicode characters. Since the rune type represents a Unicode character, a string in Go is often referred to as a sequence of runes. However, runes are stored as 1, 2, 3, or 4 bytes depending on the character. Due to this, strings are really just a sequence of bytes. In Go, slices are used to represent sequences and these slices can be iterated over using range.

Even though a string is just a slice of bytes, the range keyword iterates over a string's runes, not its bytes. In this example, the index variable represents the starting index of the current rune's byte sequence and the char variable represents the current rune:

myString := "❗hello"
for index, char := range myString {
  fmt.Printf("Index: %d\tCharacter: %c\t\tCode Point: %U\n", index, char, char)
}
// Output:
// Index: 0	Character: ❗		Code Point: U+2757
// Index: 3	Character: h		Code Point: U+0068
// Index: 4	Character: e		Code Point: U+0065
// Index: 5	Character: l		Code Point: U+006C
// Index: 6	Character: l		Code Point: U+006C
// Index: 7	Character: o		Code Point: U+006F

Since runes can be stored as 1, 2, 3, or 4 bytes, the length of a string may not always equal the number of characters in the string. Use the builtin len function to get the length of a string in bytes and the utf8.RuneCountInString function to get the number of runes in a string:

import "unicode/utf8"

myString := "❗hello"
stringLength := len(myString)
numberOfRunes := utf8.RuneCountInString(myString)

fmt.Printf("myString - Length: %d - Runes: %d\n", stringLength, numberOfRunes)
// Output: myString - Length: 8 - Runes: 6