Why Should You Use Rune Slices Instead of Strings When Working with Multibyte Characters in Go?

Understanding the Difference Between Ranging Over String and Rune Slice

When working with strings in Go, you may encounter two common scenarios: ranging over a string and ranging over a rune slice. While both approaches may seem similar in retrieving characters, there's a subtle difference that can impact program behavior.

Ranging Over a String

Consider the following code:

<code class="go">func main() {
    str := "123456"
    for _, s := range str {
        fmt.Printf("type of v: %s, value: %v, string v: %s \n", reflect.TypeOf(s), s, string(s))
    }
}</code>

This code iterates over each character in the string, but notice the data type of s is an int32, and string(s) is used to encode the rune as a string.

Ranging Over a Rune Slice

Now, let's examine a variation where we convert the string to a rune slice using []rune(str):

<code class="go">func main() {
    str := "123456"
    for _, s := range []rune(str) {
        fmt.Printf("type : %s, value: %v ,string : %s\n", reflect.TypeOf(s), s, string(s))
    }
}</code>

Here, s is a rune, and string(s) still provides the same string representation.

The Subtle Distinction

Although the results in both scenarios may appear identical at first glance, there's a crucial difference:

• When ranging over a string, the index (_) represents byte indices, and s stores the unicode code point.
• When ranging over a rune slice, the index also represents byte indices, but s stores the rune itself.

This subtle distinction becomes evident when working with multibyte characters (non-Latin characters like Chinese or Korean), as their UTF-8 representations consist of multiple bytes.

Practical Implications

When dealing with multibyte characters, using a rune slice instead of a string is more appropriate. This is because a rune represents a single logical character, while a string may contain multiple bytes representing that same character.

To avoid potential issues, it's generally recommended to prefer range loops over rune slices rather than strings, especially when dealing with non-Latin characters.

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