什么是符文？

Question

What is a rune in Go?

I've been googling but Golang only says in one line: rune is an alias for int32.

But how come integers are used all around like swapping cases?

The following is a function swapcase. What is all the <= and -?

And why doesn't switch have any arguments?

&& should mean and but what is r <= 'z'?

func SwapRune(r rune) rune {
    switch {
    case 'a' <= r && r <= 'z':
        return r - 'a' + 'A'
    case 'A' <= r && r <= 'Z':
        return r - 'A' + 'a'
    default:
        return r
    }
}

Most of them are from http://play.golang.org/p/H6wjLZj6lW

func SwapCase(str string) string {
    return strings.Map(SwapRune, str)
}

I understand this is mapping rune to string so that it can return the swapped string. But I do not understand how exactly rune or byte works here.

Answer 1

Rune literals are just 32-bit integer values (however they're untyped constants, so their type can change). They represent unicode codepoints. For example, the rune literal 'a' is actually the number 97.

Therefore your program is pretty much equivalent to:

package main

import "fmt"

func SwapRune(r rune) rune {
    switch {
    case 97 <= r && r <= 122:
        return r - 32
    case 65 <= r && r <= 90:
        return r + 32
    default:
        return r
    }
}

func main() {
    fmt.Println(SwapRune('a'))
}

It should be obvious, if you were to look at the Unicode mapping, which is identical to ASCII in that range. Furthermore, 32 is in fact the offset between the uppercase and lowercase codepoint of the character. So by adding 32 to 'A', you get 'a' and vice versa.