How to modify a specific character in a string

Question

In C, We define the character of a string as Char. So I want to change a character of a string in Go.

if sum%11 != int(str[strlen-1]) {
    str[strlen-1] = byte(sum % 11)
    //fmt.Printf("%T %T\n", str[strlen-1], byte(sum%11))
} else {
    fmt.Println(sum)
}

But it will report an error "cannot assign to str[strlen - 1]". Both str[strlen-1] and byte(sum%11) are uint8. Why is it wrong? How should I convert?

Strings are immutable, but slices are not. If you are dealing with binary data convert the string to []byte or []uint8 first. If you are dealing with UTF-8 encoded text convert to []rune (rune is an alias for uint32). If you're dealing with any other text encoding you're on your own. — Peter
– Peter, Commented Sep 19, 2019 at 6:48

Burak Serdar · Accepted Answer · 2019-09-19 03:15:27Z

1

Unlike in C, Go strings are immutable, so you cannot modify individual bytes in a string. However, you can convert the string to a byte array, change that, and convert that byte array to string.

arr:=[]byte(str)
arr[strlen-1]=byte(sum%11)
str=string(arr)

Note that Go strings are UTF-8 encoded. A rune may be represented as multiple bytes. By modifying strings like this you may end up with an invalid string.

edited Sep 19, 2019 at 3:15

answered Sep 19, 2019 at 2:50

Burak Serdar

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6 Comments

Burak Serdar Over a year ago

Yes, but the are still byte arrays.

peterSO Over a year ago

UTF-8 is a variable-length encoding with one to four bytes per character. A byte is not a character.

Burak Serdar Over a year ago

If the underlying string uses 1-byte runes, then you can treat the string as a byte array, can you not?

colm.anseo Over a year ago

Yes, the pedantic solution is go by last rune (1-4 bytes) and replace the rune with a 1 byte. But since the OP referenced C & most likely an ASCII string, this solution is probably fine.

peterSO Over a year ago

An 8-bit Unicode codepoint is UTF-8 encoded as one or two bytes.

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Nikita Petrov · Accepted Answer · 2022-10-20 04:39:15Z

In general, you should do it this way:

s := "some UTF8® string"
r := []rune(s)
fmt.Println("The last character is", r[len(r)-1])
result := string(r)
// Output: g

In your case I am not sure what you mean by a "sum" variable. Let's assume the sum is int and want to compare the end of the string with it. The issue here is that %11 could be a two digits string (10) also:

str := "anyUTF8®string9"
sum := 10
remainderDevision := strconv.Itoa(sum % 11) // It is "10"

rInput := []rune(str)
rDivision := []rune(remainderDevision)

// Compare the last 1 or two characters:
if string(rInput[len(rInput)-len(rDivision):]) != string(rDivision) {
    fmt.Println(string(rInput[len(rInput)-len(rDivision):])) // Output: g9

    if len(rDivision) > 1 {
        secondCharFromEnd := rInput[len(rInput)-2]
        _, errNotInt := strconv.Atoi(string(secondCharFromEnd))
        if errNotInt != nil {
            // Replace only the last char with two new chars
            rInput[len(rInput)-1] = rDivision[0]
            rInput = append(rInput, rDivision[1])
        } else {
            // Replace the last 2 chars
            rInput[len(rInput)-2] = rDivision[0]
            rInput[len(rInput)-1] = rDivision[1]
        }
    } else {
        // Just replace the last char with it
        rInput[len(rInput)-1] = rDivision[0]
    }
}
fmt.Println("Result:", string(rInput))
// Output: anyUTF8®string10

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