String

In Go, string is an immutable array of bytes. So if created, we can’t change its value. E.g.: 

  1. package main
  3. func main()  {
  4.     s := "Hello"
  5.     s[0] = 'h'
  6. }

The compiler will complain: 

  1. cannot assign to s[0]

To modify the content of a string, you could convert it to a byte array. But in fact, you do not operate on the original string, just a copy: 

  1. package main
  3. import "fmt"
  5. func main()  {
  6.     s := "Hello"
  7.     b := []byte(s)
  8.     b[0] = 'h'
  9.     fmt.Printf("%s\n", b)
  10. }

The result is like this: 

  1. hello

Since Go uses UTF-8 encoding, you must remember the len function will return the string’s byte number, not character number: 

  1. package main
  3. import "fmt"
  5. func main()  {
  6.     s := "日志log"
  7.     fmt.Println(len(s))
  8. }

The result is: 

  1. 9

Because each Chinese character occupied 3 bytes, s in the above example contains 5 characters and 9 bytes.

If you want to access every character, for ... range loop can give a help: 

  1. package main
  2. import "fmt"
  4. func main() {
  5.     s := "日志log"
  6.     for index, runeValue := range s {
  7.         fmt.Printf("%#U starts at byte position %d\n", runeValue, index)
  8.     }
  9. }

The result is: 

  1. U+65E5 '日' starts at byte position 0
  2. U+5FD7 '志' starts at byte position 3
  3. U+006C 'l' starts at byte position 6
  4. U+006F 'o' starts at byte position 7
  5. U+0067 'g' starts at byte position 8

Reference:
Strings, bytes, runes and characters in Go;
The Go Programming Language.