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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" class = "active" > < 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" > < 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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< li > < em > KusonStack一站式可编程配置技术栈: < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / 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 >
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< h2 id = "12-命令行参数" > < a class = "header" href = "#12-命令行参数" > 1.2. 命令行参数< / a > < / h2 >
< p > 大多数的程序都是处理输入，产生输出；这也正是“计算”的定义。但是，程序如何获取要处理的输入数据呢？一些程序生成自己的数据，但通常情况下，输入来自于程序外部：文件、网络连接、其它程序的输出、敲键盘的用户、命令行参数或其它类似输入源。下面几个例子会讨论其中几个输入源，首先是命令行参数。< / p >
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< p > < code > os< / code > 包以跨平台的方式，提供了一些与操作系统交互的函数和变量。程序的命令行参数可从 < code > os< / code > 包的 < code > Args< / code > 变量获取；< code > os< / code > 包外部使用 < code > os.Args< / code > 访问该变量。< / p >
< p > < code > os.Args< / code > 变量是一个字符串（ ） < em > 切片< / em > （ ） （ ： < code > s< / code > 当作数组元素序列，序列的长度动态变化，用 < code > s[i]< / code > 访问单个元素，用 < code > s[m:n]< / code > 获取子序列（译注：和 Python 里的语法差不多）。序列的元素数目为 < code > len(s)< / code > 。和大多数编程语言类似， ， < code > a=[1,2,3,4,5]< / code > , < code > a[0:3]=[1,2,3]< / code > ，不包含最后一个元素）。比如 < code > s[m:n]< / code > 这个切片，< code > 0≤m≤n≤len(s)< / code > ，包含 < code > n-m< / code > 个元素。< / p >
< p > < code > os.Args< / code > 的第一个元素：< code > os.Args[0]< / code > ，是命令本身的名字；其它的元素则是程序启动时传给它的参数。< code > s[m:n]< / code > 形式的切片表达式，产生从第 < code > m< / code > 个元素到第 < code > n-1< / code > 个元素的切片，下个例子用到的元素包含在 < code > os.Args[1:len(os.Args)]< / code > 切片中。如果省略切片表达式的 < code > m< / code > 或 < code > n< / code > ，会默认传入 < code > 0< / code > 或 < code > len(s)< / code > ，因此前面的切片可以简写成 < code > os.Args[1:]< / code > 。< / p >
< p > 下面是 Unix 里 < code > echo< / code > 命令的一份实现，< code > echo< / code > 把它的命令行参数打印成一行。程序导入了两个包，用括号把它们括起来写成列表形式，而没有分开写成独立的 < code > import< / code > 声明。两种形式都合法，列表形式习惯上用得多。包导入顺序并不重要；< code > gofmt< / code > 工具格式化时按照字母顺序对包名排序。（示例有多个版本时，我们会对示例编号，这样可以明确当前正在讨论的是哪个。）< / p >
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< p > < u > < i > gopl.io/ch1/echo1< / i > < / u > < / p >
< pre > < code class = "language-go" > // Echo1 prints its command-line arguments.
package main
import (
" fmt"
" os"
)
func main() {
var s, sep string
for i := 1; i < len(os.Args); i++ {
s += sep + os.Args[i]
sep = " "
}
fmt.Println(s)
}
< / code > < / pre >
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< p > 注释语句以 < code > //< / code > 开头。对于程序员来说，< code > //< / code > 之后到行末之间所有的内容都是注释，被编译器忽略。按照惯例，我们在每个包的包声明前添加注释；对于 < code > main package< / code > ，注释包含一句或几句话，从整体角度对程序做个描述。< / p >
< p > < code > var< / code > 声明定义了两个 < code > string< / code > 类型的变量 < code > s< / code > 和 < code > sep< / code > 。变量会在声明时直接初始化。如果变量没有显式初始化，则被隐式地赋予其类型的 < em > 零值< / em > （ ） ， < code > 0< / code > ，字符串类型是空字符串 < code > " " < / code > 。这个例子里，声明把 < code > s< / code > 和 < code > sep< / code > 隐式地初始化成空字符串。第 2 章再来详细地讲解变量和声明。< / p >
< p > 对数值类型， < code > string< / code > 类型，< code > +< / code > 运算符连接字符串（译注：和 C++ 或者 JavaScript 是一样的）。所以表达式：< code > sep + os.Args[i]< / code > 表示连接字符串 < code > sep< / code > 和 < code > os.Args< / code > 。程序中使用的语句：< code > s+=sep+os.Args[i]< / code > 是一条 < em > 赋值语句< / em > ，将 < code > s< / code > 的旧值跟 < code > sep< / code > 与 < code > os.Args[i]< / code > 连接后赋值回 < code > s< / code > ，等价于：< code > s=s+sep+os.Args[i]< / code > 。< / p >
< p > 运算符 < code > +=< / code > 是赋值运算符（ ） ， ， < code > +< / code > 或 < code > *< / code > ，都有对应的赋值运算符。< / p >
< p > < code > echo< / code > 程序可以每循环一次输出一个参数，这个版本却是不断地把新文本追加到末尾来构造字符串。字符串 < code > s< / code > 开始为空，即值为 < code > " " < / code > ， ； ， ， （ ） ， ， ， < code > echo< / code > ，这种情形不大可能出现。本章会介绍 < code > echo< / code > 的若干改进版，下一章解决低效问题。< / p >
< p > 循环索引变量 < code > i< / code > 在 < code > for< / code > 循环的第一部分中定义。符号 < code > :=< / code > 是 < em > 短变量声明< / em > （ ） ， < / p >
< p > 自增语句 < code > i++< / code > 给 < code > i< / code > 加 < code > 1< / code > ；这和 < code > i+=1< / code > 以及 < code > i=i+1< / code > 都是等价的。对应的还有 < code > i--< / code > 给 < code > i< / code > 减 < code > 1< / code > 。它们是语句，而不像 C 系的其它语言那样是表达式。所以 < code > j=i++< / code > 非法，而且 < code > ++< / code > 和 < code > --< / code > 都只能放在变量名后面，因此 < code > --i< / code > 也非法。< / p >
< p > Go 语言只有 < code > for< / code > 循环这一种循环语句。< code > for< / code > 循环有多种形式，其中一种如下所示：< / p >
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< pre > < code class = "language-go" > for initialization; condition; post {
// zero or more statements
}
< / code > < / pre >
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< p > < code > for< / code > 循环三个部分不需括号包围。大括号强制要求，左大括号必须和 < em > < code > post< / code > < / em > 语句在同一行。< / p >
< p > < em > < code > initialization< / code > < / em > 语句是可选的，在循环开始前执行。< em > < code > initalization< / code > < / em > 如果存在，必须是一条 < em > 简单语句< / em > （ ） ， ， < code > condition< / code > 是一个布尔表达式（ ） ， < code > true< / code > 则执行循环体语句。< code > post< / code > 语句在循环体执行结束后执行，之后再次对 < code > condition< / code > 求值。< code > condition< / code > 值为 < code > false< / code > 时，循环结束。< / p >
< p > for 循环的这三个部分每个都可以省略，如果省略 < code > initialization< / code > 和 < code > post< / code > ，分号也可以省略：< / p >
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< pre > < code class = "language-go" > // a traditional " while" loop
for condition {
// ...
}
< / code > < / pre >
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< p > 如果连 < code > condition< / code > 也省略了，像下面这样：< / p >
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< pre > < code class = "language-go" > // a traditional infinite loop
for {
// ...
}
< / code > < / pre >
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< p > 这就变成一个无限循环，尽管如此，还可以用其他方式终止循环，如一条 < code > break< / code > 或 < code > return< / code > 语句。< / p >
< p > < code > for< / code > 循环的另一种形式， （ ） ， < code > echo< / code > 的第二版本展示了这种形式：< / p >
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< p > < u > < i > gopl.io/ch1/echo2< / i > < / u > < / p >
< pre > < code class = "language-go" > // Echo2 prints its command-line arguments.
package main
import (
" fmt"
" os"
)
func main() {
s, sep := " " , " "
for _, arg := range os.Args[1:] {
s += sep + arg
sep = " "
}
fmt.Println(s)
}
< / code > < / pre >
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< p > 每次循环迭代，< code > range< / code > 产生一对值；索引以及在该索引处的元素值。这个例子不需要索引，但 < code > range< / code > 的语法要求，要处理元素，必须处理索引。一种思路是把索引赋值给一个临时变量（如 < code > temp< / code > ）然后忽略它的值，但 Go 语言不允许使用无用的局部变量（ ） ， < / p >
< p > Go 语言中这种情况的解决方法是用 < em > 空标识符< / em > （ ） ， < code > _< / code > （也就是下划线）。空标识符可用于在任何语法需要变量名但程序逻辑不需要的时候（如：在循环里）丢弃不需要的循环索引，并保留元素值。大多数的 Go 程序员都会像上面这样使用 < code > range< / code > 和 < code > _< / code > 写 < code > echo< / code > 程序，因为隐式地而非显式地索引 < code > os.Args< / code > ，容易写对。< / p >
< p > < code > echo< / code > 的这个版本使用一条短变量声明来声明并初始化 < code > s< / code > 和 < code > seps< / code > ，也可以将这两个变量分开声明，声明一个变量有好几种方式，下面这些都等价：< / p >
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< pre > < code class = "language-go" > s := " "
var s string
var s = " "
var s string = " "
< / code > < / pre >
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< p > 用哪种不用哪种，为什么呢？第一种形式，是一条短变量声明，最简洁，但只能用在函数内部，而不能用于包变量。第二种形式依赖于字符串的默认初始化零值机制，被初始化为 < code > " " < / code > 。第三种形式用得很少，除非同时声明多个变量。第四种形式显式地标明变量的类型，当变量类型与初值类型相同时，类型冗余，但如果两者类型不同，变量类型就必须了。实践中一般使用前两种形式中的某个，初始值重要的话就显式地指定变量的类型，否则使用隐式初始化。< / p >
< p > 如前文所述，每次循环迭代字符串 < code > s< / code > 的内容都会更新。< code > +=< / code > 连接原字符串、空格和下个参数，产生新字符串，并把它赋值给 < code > s< / code > 。< code > s< / code > 原来的内容已经不再使用，将在适当时机对它进行垃圾回收。< / p >
< p > 如果连接涉及的数据量很大，这种方式代价高昂。一种简单且高效的解决方案是使用 < code > strings< / code > 包的 < code > Join< / code > 函数：< / p >
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< p > < u > < i > gopl.io/ch1/echo3< / i > < / u > < / p >
< pre > < code class = "language-go" > func main() {
fmt.Println(strings.Join(os.Args[1:], " " ))
}
< / code > < / pre >
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< p > 最后，如果不关心输出格式，只想看看输出值，或许只是为了调试，可以用 < code > Println< / code > 为我们格式化输出。< / p >
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< pre > < code class = "language-go" > fmt.Println(os.Args[1:])
< / code > < / pre >
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< p > 这条语句的输出结果跟 < code > strings.Join< / code > 得到的结果很像，只是被放到了一对方括号里。切片都会被打印成这种格式。< / p >
< hr / >
< p > < strong > 练习 1.1： < / strong > 修改 < code > echo< / code > 程序，使其能够打印 < code > os.Args[0]< / code > ，即被执行命令本身的名字。< / p >
< p > < strong > 练习 1.2： < / strong > 修改 < code > echo< / code > 程序，使其打印每个参数的索引和值，每个一行。< / p >
< p > < strong > 练习 1.3： < / strong > 做实验测量潜在低效的版本和使用了 < code > strings.Join< / code > 的版本的运行时间差异。（< a href = "./ch1-06.html" > 1.6 节< / a > 讲解了部分 < code > time< / code > 包，< a href = "../ch11/ch11-04.html" > 11.4 节< / a > 展示了如何写标准测试程序，以得到系统性的性能评测。）< / p >
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