The Go programming language is easy to learn, but there are some tricky traps. This article series is trying to show these booby traps so that you avoid them.

We would like to print the length of a string. So we write the following code:

package main

func main() {
	s := "échec"
	println("length of", s, "is", len(s))
}

On the Playground

But when we run it, we get:

$ go run broken.go
length of échec is 6

This is probably not what you expected! Why? How could you fix this code?

Explanation

The term “character string” is inaccurate for the Go string type. We should name it bytes array. To be precise, a Go string is an array of bytes resulting from the encoding of a string in UTF-8. As our string contains an accented character, it won’t result in a single byte encoded in UTF-8 but in two. Thus the size of the bytes array is not the same as the string length.

The question now is how to get the number of characters (or runes in Go) in this string…

First solution is to convert the string into a Runes array and get its size:

package main

func main() {
	s := "échec"
	println("length of", s, "is", len([]rune(s)))
}

On the Playground

Another solution is to use the dedicated function RuneCountInString() from the utf8 package:

package main

import "unicode/utf8"

func main() {
	s := "échec"
	println("length of", s, "is", utf8.RuneCountInString(s))
}

On the Playground

In both cases, we get the correct size:

$ go run fixed.go
length of échec is 5

What you need to remember here is that len(string) returns the length of a string only if it contains ASCII characters. Thus you should not use the len(string) function in the general case.

Enjoy!