Important Go Topics

Go Runes, Bytes and strings.Builder

Why len("héllo") is 6, how to work with characters, and how to build long strings fast.

Intermediate
4 min read
1 runnable example

What you'll learn

Bytes vs runes
[]rune
strings.Builder

Run code →

héllo

é1-2

5 characters, 6 bytes.

Bytes are not characters

A Go string is a sequence of bytes, and a character outside ASCII takes more than one byte. A rune is one Unicode character. Converting a string to []rune lets you work with characters instead of bytes.

héllo has 5 characters but 6 bytes, because é takes two bytes; that is why the positions printed by range jump from 1 to 3. []rune makes the characters editable. strings.Builder joins many pieces without creating a new string each time.

Key points

é takes two bytes in UTF-8.
range gives runes, not bytes.
[]rune makes characters editable.

Runnable Go example

Run code →

main.go

package main

import (
	"fmt"
	"strings"
	"unicode/utf8"
)

func main() {
	word := "héllo"
	fmt.Println(len(word), utf8.RuneCountInString(word))

	parts := []string{}
	for index, r := range word {
		parts = append(parts, fmt.Sprintf("%d:%c", index, r))
	}
	fmt.Println(strings.Join(parts, " "))

	runes := []rune(word)
	runes[1] = 'e'
	fmt.Println(string(runes), len(runes))

	bytes := []byte("Go")
	fmt.Println(bytes, string(bytes[0]))

	var builder strings.Builder
	for i := 3; i > 0; i-- {
		fmt.Fprintf(&builder, "%d...", i)
	}
	builder.WriteString("go!")
	fmt.Println(builder.String())
}

Get a Run button for this example Result of running the code

6 5
0:h 1:é 3:l 4:l 5:o
hello 5
[71 111] G
3...2...1...go!

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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