String and byte

Learn the byte slice behind a string.

Strings and bytes

A Go string is an immutable sequence of bytes. You can't change a character in place; s[0] = 'G' is a compile error. To change it, convert to []byte (or []rune for Turkish letters) and build a new string:

s := "merhaba"
b := []byte(s)
b[0] = 'M'
fmt.Println(string(b))  // Merhaba

s[i] gives a byte, not a string.

Why immutability helps

Because strings are immutable they can be shared safely, and substrings (s[2:5]) point at the same bytes without copying. The cost: s += "x" in a loop allocates a new string every time. When building text from many pieces, use strings.Builder (you'll see it in the concatenation lesson).

Exercise Capitalize the first letter

Declare s := "golang". Convert it to []byte, set the first byte to 'G' and print the new string.

Show example solution

package main

import "fmt"

func main() {
    s := "golang"
    b := []byte(s)
    b[0] = 'G'
    fmt.Println(string(b))
}

Test yourself

What does <code>fmt.Println("Go"[0])</code> print?
- "G"
- An error
- 71 (the byte value of G)
- G
Show answer

71 (the byte value of G) — Indexing returns a byte (uint8); to see it as a letter use string("Go"[0:1]) or %c.

Convert to a byte slice

b := []____(s)
b[0] = 'M'

Show answer

b := []byte(s)
b[0] = 'M'

Expression and type

"Go"

string
"Go"[0]

byte (71)
"Go"[0:1]

string ("G")
[]byte("Go")

[71 111]

Next lesson: String concatenation

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