Assigning a String to a Byte Array in Go

The Problem

You don’t know how to assign the value of a string to a byte array in Go.

The Solution

If you only need a byte slice, not an array, use the following one-liner:

[]byte("example string")

This works because strings in Go are effectively read-only slices of bytes.

If you need to assign a string to a fixed-length array, use copy:

package main

import "fmt"

func main() {
	var array [8]byte
	example := "example"

	copy(array[:], example)
	fmt.Println(array)
}

This prints:

[101 120 97 109 112 108 101 0]

The copy function fills the target array with the bytes contained in the source string.

It’s important to remember that if the string is longer than the array, copy will drop the remaining bytes.

Remember That Strings Are UTF-8

In the above example, we copied ASCII-compatible characters into a byte array and each character took one byte. However, UTF-8 strings can have up to 4 bytes per character.

For example, the ✨ emoji uses three bytes. If the example string contains a single ✨ emoji, all bytes from the source string will be assigned to the array:

package main

import "fmt"

func main() {
	var array [8]byte
	example := "✨"

	copy(array[:], example)
	fmt.Println(array)
}

Which prints:

[226 156 168 0 0 0 0 0]

However, if you set the variable to have three sparkles, the last byte (168) will be dropped from the target array:

func main() {
	var array [8]byte
	example := "✨✨✨"

	copy(array[:], example)
	fmt.Println(array)
}

Which prints:

[226 156 168 226 156 168 226 156]

Compare this to a byte slice, which retains all of the information from the three-sparkle string due to its variable length:

func main() {
	var array [8]byte
	example := "✨✨✨"

	fmt.Println([]byte(example))
}

Which prints:

[226 156 168 226 156 168 226 156 168]

When copying string values to an array, it’s important to consider your target array length and the bytes per character.

Unless you require an array, it’s easier to use a byte slice.

Further Reading

• The Go slices package documentation
• The Strings, bytes, runes and characters in Go blog post
• The UTF-8 Wikipedia article