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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" class = "active" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-06.html" > < s
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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
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< hr >
< h2 id = "26-包和文件" > < a class = "header" href = "#26-包和文件" > 2.6. 包和文件< / a > < / h2 >
< p > Go语言中的包和其他语言的库或模块的概念类似， ， ； < / p >
< p > 每个包都对应一个独立的名字空间。例如， ， < / p >
< p > 包还可以让我们通过控制哪些名字是外部可见的来隐藏内部实现信息。在Go语言中， ： ， （ ： ， ） < / p >
< p > 为了演示包基本的用法， ， ？ < / p >
< p > 让我们创建一个名为gopl.io/ch2/tempconv的包， （ ， ） ， ； ， < / p >
< p > 我们把变量的声明、对应的常量， ： < / p >
< p > < u > < i > gopl.io/ch2/tempconv< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Package tempconv performs Celsius and Fahrenheit conversions.
package tempconv
import " fmt"
type Celsius float64
type Fahrenheit float64
const (
AbsoluteZeroC Celsius = -273.15
FreezingC Celsius = 0
BoilingC Celsius = 100
)
func (c Celsius) String() string { return fmt.Sprintf(" %g°C" , c) }
func (f Fahrenheit) String() string { return fmt.Sprintf(" %g°F" , f) }
< / code > < / pre >
< p > 转换函数则放在另一个conv.go源文件中： < / p >
< pre > < code class = "language-Go" > package tempconv
// CToF converts a Celsius temperature to Fahrenheit.
func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }
// FToC converts a Fahrenheit temperature to Celsius.
func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }
< / code > < / pre >
< p > 每个源文件都是以包的声明语句开始， ， ， ， ， ， ， < / p >
< p > 因为包级别的常量名都是以大写字母开头， ： < / p >
< pre > < code class = "language-Go" > fmt.Printf(" Brrrr! %v\n" , tempconv.AbsoluteZeroC) // " Brrrr! -273.15°C"
< / code > < / pre >
< p > 要将摄氏温度转换为华氏温度， ， ： < / p >
< pre > < code class = "language-Go" > fmt.Println(tempconv.CToF(tempconv.BoilingC)) // " 212°F"
< / code > < / pre >
< p > 在每个源文件的包声明前紧跟着的注释是包注释（ ） ， （ ： ， ） ， < / p >
< p > < strong > 练习 2.1： < / strong > 向tempconv包添加类型、常量和函数用来处理Kelvin绝对温度的转换， − ， < / p >
< h3 id = "261-导入包" > < a class = "header" href = "#261-导入包" > 2.6.1. 导入包< / a > < / h3 >
< p > 在Go语言程序中， " gopl.io/ch2/tempconv" 的字符串对应包的导入路径。Go语言的规范并没有定义这些字符串的具体含义或包来自哪里， （ ） ， < / p >
< p > 除了包的导入路径， ， （ ） ， ， ， < / p >
< p > 要使用gopl.io/ch2/tempconv包， ： < / p >
< p > < u > < i > gopl.io/ch2/cf< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Cf converts its numeric argument to Celsius and Fahrenheit.
package main
import (
" fmt"
" os"
" strconv"
" gopl.io/ch2/tempconv"
)
func main() {
for _, arg := range os.Args[1:] {
t, err := strconv.ParseFloat(arg, 64)
if err != nil {
fmt.Fprintf(os.Stderr, " cf: %v\n" , err)
os.Exit(1)
}
f := tempconv.Fahrenheit(t)
c := tempconv.Celsius(t)
fmt.Printf(" %s = %s, %s = %s\n" ,
f, tempconv.FToC(f), c, tempconv.CToF(c))
}
}
< / code > < / pre >
< p > 导入语句将导入的包绑定到一个短小的名字， ， （ ： ） ， ， （ < / p >
< p > cf程序将命令行输入的一个温度在Celsius和Fahrenheit温度单位之间转换： < / p >
< pre > < code > $ go build gopl.io/ch2/cf
$ ./cf 32
32°F = 0°C, 32°C = 89.6°F
$ ./cf 212
212°F = 100°C, 212°C = 413.6°F
$ ./cf -40
-40°F = -40°C, -40°C = -40°F
< / code > < / pre >
< p > 如果导入了一个包， ， ， " got here!" )的打印语句可能导致需要同时删除log包导入声明， ， ， < / p >
< p > 不过有更好的解决方案， ， ； ， ， < / p >
< p > < strong > 练习 2.2： < / strong > 写一个通用的单位转换程序， ， ， ， ， < / p >
< h3 id = "262-包的初始化" > < a class = "header" href = "#262-包的初始化" > 2.6.2. 包的初始化< / a > < / h3 >
< p > 包的初始化首先是解决包级变量的依赖顺序，然后按照包级变量声明出现的顺序依次初始化：< / p >
< pre > < code class = "language-Go" > var a = b + c // a 第三个初始化, 为 3
var b = f() // b 第二个初始化, 为 2, 通过调用 f (依赖c)
var c = 1 // c 第一个初始化, 为 1
func f() int { return c + 1 }
< / code > < / pre >
< p > 如果包中含有多个.go源文件， ， ， < / p >
< p > 对于在包级别声明的变量， ， ， ， < / p >
< pre > < code class = "language-Go" > func init() { /* ... */ }
< / code > < / pre >
< p > 这样的init初始化函数除了不能被调用或引用外， ， < / p >
< p > 每个包在解决依赖的前提下， ， ， ， ， ， ， < / p >
< p > 下面的代码定义了一个PopCount函数， ， ， ， （ ， ， ， ） < / p >
< p > < u > < i > gopl.io/ch2/popcount< / i > < / u > < / p >
< pre > < code class = "language-Go" > package popcount
// pc[i] is the population count of i.
var pc [256]byte
func init() {
for i := range pc {
pc[i] = pc[i/2] + byte(i& 1)
}
}
// PopCount returns the population count (number of set bits) of x.
func PopCount(x uint64) int {
return int(pc[byte(x> > (0*8))] +
pc[byte(x> > (1*8))] +
pc[byte(x> > (2*8))] +
pc[byte(x> > (3*8))] +
pc[byte(x> > (4*8))] +
pc[byte(x> > (5*8))] +
pc[byte(x> > (6*8))] +
pc[byte(x> > (7*8))])
}
< / code > < / pre >
< p > 译注： ， ， ： < / p >
< pre > < code class = "language-Go" > // pc[i] is the population count of i.
var pc [256]byte = func() (pc [256]byte) {
for i := range pc {
pc[i] = pc[i/2] + byte(i& 1)
}
return
}()
< / code > < / pre >
< p > 要注意的是在init函数中， ， ： < / p >
< pre > < code class = "language-Go" > for i, _ := range pc {
< / code > < / pre >
< p > 我们在下一节和10.5节还将看到其它使用init函数的地方。< / p >
< p > < strong > 练习 2.3： < / strong > 重写PopCount函数， （ < / p >
< p > < strong > 练习 2.4： < / strong > 用移位算法重写PopCount函数， ， < / p >
< p > < strong > 练习 2.5： < / strong > 表达式< code > x& (x-1)< / code > 用于将x的最低的一个非零的bit位清零。使用这个算法重写PopCount函数， < / p >
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