Go语言中使用fmt.Printf的小技巧

When printing numbers using the fmt package, we can control the radix and format with the %d, %o, and %x verbs, as shown in this example:

o := 0666
fmt.Printf(“%d %[1]o %#[1]o\n”, o) // “438 666 0666”
x := int64(0xdeadbeef)
fmt.Printf(“%d %[1]x %#[1]x %#[1]X\n”, x)
// Output:
// 3735928559 deadbeef 0xdeadbeef 0XDEADBEEF

Note the use of two fmt tricks. Usually a Printf format string containing multiple % verbs would require the same number of extra operands, but the [1] “adverbs” after % tell Printf to use the first operand over and over again. Second, the # adverb for %o or %x or %X tells Printf to emit a 0 or 0x or 0X prefix respectively.

Rune literals are written as a character within single quotes. The simplest example is an ASCII character like ‘a’, but it’s possible to write any Unicode code point either directly or with numeric escapes, as we will see shortly.

Runes are printed with %c, or with %q if quoting is desired: ascii := ‘a’
unicode := ‘ ‘
newline := ‘\n’
fmt.Printf(“%d %[1]c %[1]q\n”, ascii) // “97 a ‘a'”
fmt.Printf(“%d %[1]c %[1]q\n”, unicode) // “22269 ‘ ‘”
fmt.Printf(“%d %[1]q\n”, newline) // “10 ‘\n'”

总结一下，“%[1]”还是格式化第一个参数；“%#”会打印出数值的前缀；而“%q”会加上引号。举例如下：

package main

import "fmt"

func main() {
    var c rune = '楠'
    fmt.Printf("%c %[1]d %#[1]x %[1]q", c)
}

执行结果：

楠 26976 0x6960 '楠'

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