如何在Go中将新字符分配给字符串？

Question

I'm trying to alter an existing string in Go but I keep getting this error "cannot assign to new_str[i]"

package main
import "fmt"

func ToUpper(str string) string {
    new_str := str
    for i:=0; i<len(str); i++{
        if str[i]>='a' && str[i]<='z'{
            chr:=uint8(rune(str[i])-'a'+'A')
            new_str[i]=chr
        }
    }
    return new_str
}

func main() {
    fmt.Println(ToUpper("cdsrgGDH7865fxgh"))
}

This is my code, I wish to change lowercase to uppercase but I cant alter the string. Why? How can I alter it?

P.S I wish to use ONLY the fmt package!

Thanks in advance.

Answer 1

You can't... they are immutable. From the Golang Language Specification:

Strings are immutable: once created, it is impossible to change the contents of a string.

You can however, cast it to a []byte slice and alter that:

func ToUpper(str string) string {
    new_str := []byte(str)
    for i := 0; i < len(str); i++ {
        if str[i] >= 'a' && str[i] <= 'z' {
            chr := uint8(rune(str[i]) - 'a' + 'A')
            new_str[i] = chr
        }
    }
    return string(new_str)
}

Working sample: http://play.golang.org/p/uZ_Gui7cYl