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Go Strings and Runes

In Go, strings are read-only slices of bytes. When we talk about "characters," Go uses a concept called a rune.

What is a Rune?

A rune is an alias for int32. It represents a single Unicode character (code point). Since some characters (like '世' or emojis) take up multiple bytes, a simple index like s[i] might only give you a fraction of a character.

Basic Example

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package main

import (
    "fmt"
    "unicode/utf8"
)

func main() {
    // A string with a multi-byte character
    const s = "Hello, 世界"

    // len() returns the number of BYTES
    fmt.Println("Len:", len(s)) // 13

    // RuneCountInString returns the number of CHARACTERS
    fmt.Println("Rune count:", utf8.RuneCountInString(s)) // 9

    // To access characters correctly, we use a range loop
    for i, runeValue := range s {
        fmt.Printf("%#U starts at byte position %d\n", runeValue, i)
    }
}

Output:

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U+0048 'H' starts at byte position 0
U+0065 'e' starts at byte position 1
U+006C 'l' starts at byte position 2
U+006C 'l' starts at byte position 3
U+006F 'o' starts at byte position 4
U+002C ',' starts at byte position 5
U+0020 ' ' starts at byte position 6
U+4E16 '世' starts at byte position 7
U+754C '界' starts at byte position 10

Key Differences

Feature String Byte (s[i]) Rune (rune)
Represents A single byte (8 bits) A Unicode character (32 bits)
Best for Raw data, ASCII text International text, Emojis

String Operations

Go provides a powerful strings package for common tasks:

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package main

import (
    "fmt"
    "strings"
)

func main() {
    s := "test"

    fmt.Println(strings.Contains(s, "es")) // true
    fmt.Println(strings.Count(s, "t"))    // 2
    fmt.Println(strings.HasPrefix(s, "te")) // true
    fmt.Println(strings.ToUpper(s))         // TEST
}

Important Rule: Immutability

Strings in Go are immutable. You cannot change a character inside a string once it is created.

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s := "hello"
// s[0] = 'H' // This will cause an error

To modify a string, you must convert it to a slice of runes or bytes, change it, and convert it back:

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runes := []rune(s)
runes[0] = 'H'
s = string(runes) // "Hello"