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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-zh.html">译者序</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" class="active"><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"><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a></li></ol></li><li class="chapter-item expanded "><a href="../ch5/ch5.html"><strong aria-hidden="true">5.</strong> 函数</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch5/ch5-01.html"><strong aria-hidden="true">5.1.</strong> 函数声明</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-02.html"><strong aria-hidden="true">5.2.</strong> 递归</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-03.html"><strong aria-hidden="true">5.3.</strong> 多返回值</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-04.html"><strong aria-hidden="true">5.4.</strong> 错误</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-05.html"><strong aria-hidden="true">5.5.</strong> 函数值</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-06.html"><strong aria-hidden="true">5.6.</strong> 匿名函数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-07.html"><strong aria-hidden="true">5.7.</strong> 可变参数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-08.html"><strong aria-hidden="true">5.8.</strong> Deferred函数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-09.html"><strong aria-hidden="true">5.9.</strong> Panic异常</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-10.html"><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a></li></ol></li><li class="chapter-item expanded "><a href="../ch6/ch6.html"><strong aria-hidden="true">6.</strong> 方法</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch6/ch6-01.html"><strong aria-hidden="true">6.1.</strong> 方法声明</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-02.html"><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-03.html"><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-04.html"><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-05.html"><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-06.html"><strong aria-hidden="true">6.6.</strong> 封装</a></li></ol></li><li class="chapter-item expanded "><a href="../ch7/ch7.html"><strong aria-hidden="true">7.</strong> 接口</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch7/ch7-01.html"><strong aria-hidden="true">7.1.</strong> 接口是合约</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-02.html"><strong aria-hidden="true">7.2.</strong> 接口类型</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-03.html"><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-04.html"><strong aria-hidden="true">7.4.</strong> flag.Value接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-05.html"><strong aria-hidden="true">7.5.</strong> 接口值</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-06.html"><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-07.html"><strong aria-hidden="true">7.7.</strong> http.Handler接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-08.html"><strong aria-hidden="true">7.8.</strong> error接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-09.html"><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-10.html"><strong aria-hidden="true">7.10.</strong> 类型断言</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-11.html"><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-12.html"><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-13.html"><strong aria-hidden="true">7.13.</strong> 类型分支</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-14.html"><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-15.html"><strong aria-hidden="true">7.15.</strong> 补充几点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch8/ch8.html"><strong aria-hidden="true">8.</strong> Goroutines和Channels</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch8/ch8-01.html"><strong aria-hidden="true">8.1.</strong> Goroutines</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-02.html"><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-03.html"><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-04.html"><strong aria-hidden="true">8.4.</strong> Channels</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-05.html"><strong aria-hidden="true">8.5.</strong> 并发的循环</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-06.html"><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-07.html"><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-08.html"><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-09.html"><strong aria-hidden="true">8.9.</strong> 并发的退出</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-10.html"><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a></li></ol></li><li class="chapter-item expanded "><a href="../ch9/ch9.html"><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch9/ch9-01.html"><strong aria-hidden="true">9.1.</strong> 竞争条件</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-02.html"><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-03.html"><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-04.html"><strong aria-hidden="true">9.4.</strong> 内存同步</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-05.html"><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-06.html"><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-07.html"><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-08.html"><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a></li></ol></li><li class="chapter-item expanded "><a href="../ch10/ch10.html"><strong aria-hidden="true">10.</strong> 包和工具</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch10/ch10-01.html"><strong aria-hidden="true">10.1.</strong> 包简介</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-02.html"><strong aria-hidden="true">10.2.</strong> 导入路径</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-03.html"><strong aria-hidden="true">10.3.</strong> 包声明</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-04.html"><strong aria-hidden="true">10.4.</strong> 导入声明</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-05.html"><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-06.html"><strong aria-hidden="true">10.6.</strong> 包和命名</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-07.html"><strong aria-hidden="true">10.7.</strong> 工具</a></li></ol></li><li class="chapter-item expanded "><a href="../ch11/ch11.html"><strong aria-hidden="true">11.</strong> 测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch11/ch11-01.html"><strong aria-hidden="true">11.1.</strong> go test</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-02.html"><strong aria-hidden="true">11.2.</strong> 测试函数</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-03.html"><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-04.html"><strong aria-hidden="true">11.4.</strong> 基准测试</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-05.html"><strong aria-hidden="true">11.5.</strong> 剖析</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-06.html"><strong aria-hidden="true">11.6.</strong> 示例函数</a></li></ol></li><li class="chapter-item expanded "><a href="../ch12/ch12.html"><strong aria-hidden="true">12.</strong> 反射</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch12/ch12-01.html"><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-02.html"><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-03.html"><strong aria-hidden="true">12.3.</strong> Display递归打印</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-04.html"><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-05.html"><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-06.html"><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-07.html"><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-08.html"><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-09.html"><strong aria-hidden="true">12.9.</strong> 几点忠告</a></li></ol></li><li class="chapter-item expanded "><a href="../ch13/ch13.html"><strong aria-hidden="true">13.</strong> 底层编程</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch13/ch13-01.html"><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-02.html"><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-03.html"><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-04.html"><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-05.html"><strong aria-hidden="true">13.5.</strong> 几点忠告</a></li></ol></li><li class="chapter-item expanded "><a href="../appendix/appendix.html"><strong 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<ul dir="auto">
<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
<li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
</ul>
<hr>
<h2 id="13-查找重复的行"><a class="header" href="#13-查找重复的行">1.3. 查找重复的行</a></h2>
<p>对文件做拷贝、打印、搜索、排序、统计或类似事情的程序都有一个差不多的程序结构：一个处理输入的循环，在每个元素上执行计算处理，在处理的同时或最后产生输出。我们会展示一个名为 <code>dup</code> 的程序的三个版本；灵感来自于 Unix 的 <code>uniq</code> 命令，其寻找相邻的重复行。该程序使用的结构和包是个参考范例，可以方便地修改。</p>
<p><code>dup</code> 的第一个版本打印标准输入中多次出现的行，以重复次数开头。该程序将引入 <code>if</code> 语句，<code>map</code> 数据类型以及 <code>bufio</code> 包。</p>
<p><u><i>gopl.io/ch1/dup1</i></u></p>
<pre><code class="language-go">// Dup1 prints the text of each line that appears more than
// once in the standard input, preceded by its count.
package main
import (
&quot;bufio&quot;
&quot;fmt&quot;
&quot;os&quot;
)
func main() {
counts := make(map[string]int)
input := bufio.NewScanner(os.Stdin)
for input.Scan() {
counts[input.Text()]++
}
// NOTE: ignoring potential errors from input.Err()
for line, n := range counts {
if n &gt; 1 {
fmt.Printf(&quot;%d\t%s\n&quot;, n, line)
}
}
}
</code></pre>
<p>正如 <code>for</code> 循环一样，<code>if</code> 语句条件两边也不加括号，但是主体部分需要加。<code>if</code> 语句的 <code>else</code> 部分是可选的，在 <code>if</code> 的条件为 <code>false</code> 时执行。</p>
<p><strong>map</strong> 存储了键/值（key/value）的集合，对集合元素，提供常数时间的存、取或测试操作。键可以是任意类型，只要其值能用 <code>==</code> 运算符比较，最常见的例子是字符串；值则可以是任意类型。这个例子中的键是字符串，值是整数。内置函数 <code>make</code> 创建空 <code>map</code>，此外，它还有别的作用。4.3 节讨论 <code>map</code>。</p>
<p>（译注：从功能和实现上说，<code>Go</code> 的 <code>map</code> 类似于 <code>Java</code> 语言中的 <code>HashMap</code>，Python 语言中的 <code>dict</code>，<code>Lua</code> 语言中的 <code>table</code>，通常使用 <code>hash</code> 实现。遗憾的是，对于该词的翻译并不统一，数学界术语为<em>映射</em>，而计算机界众说纷纭莫衷一是。为了防止对读者造成误解，保留不译。）</p>
<p>每次 <code>dup</code> 读取一行输入，该行被当做键存入 <code>map</code>，其对应的值递增。<code>counts[input.Text()]++</code> 语句等价下面两句：</p>
<pre><code class="language-go">line := input.Text()
counts[line] = counts[line] + 1
</code></pre>
<p><code>map</code> 中不含某个键时不用担心，首次读到新行时，等号右边的表达式 <code>counts[line]</code> 的值将被计算为其类型的零值，对于 <code>int</code> 即 <code>0</code>。</p>
<p>为了打印结果，我们使用了基于 <code>range</code> 的循环，并在 <code>counts</code> 这个 <code>map</code> 上迭代。跟之前类似，每次迭代得到两个结果，键和其在 <code>map</code> 中对应的值。<code>map</code> 的迭代顺序并不确定，从实践来看，该顺序随机，每次运行都会变化。这种设计是有意为之的，因为能防止程序依赖特定遍历顺序，而这是无法保证的。（译注：具体可以参见这里<a href="https://stackoverflow.com/questions/11853396/google-go-lang-assignment-order">https://stackoverflow.com/questions/11853396/google-go-lang-assignment-order</a>）</p>
<p>继续来看 <code>bufio</code> 包，它使处理输入和输出方便又高效。<code>Scanner</code> 类型是该包最有用的特性之一，它读取输入并将其拆成行或单词；通常是处理行形式的输入最简单的方法。</p>
<p>程序使用短变量声明创建 <code>bufio.Scanner</code> 类型的变量 <code>input</code>。</p>
<pre><code class="language-go">input := bufio.NewScanner(os.Stdin)
</code></pre>
<p>该变量从程序的标准输入中读取内容。每次调用 <code>input.Scan()</code>，即读入下一行，并移除行末的换行符；读取的内容可以调用 <code>input.Text()</code> 得到。<code>Scan</code> 函数在读到一行时返回 <code>true</code>，不再有输入时返回 <code>false</code>。</p>
<p>类似于 C 或其它语言里的 <code>printf</code> 函数，<code>fmt.Printf</code> 函数对一些表达式产生格式化输出。该函数的首个参数是个格式字符串，指定后续参数被如何格式化。各个参数的格式取决于“转换字符”（conversion character），形式为百分号后跟一个字母。举个例子，<code>%d</code> 表示以十进制形式打印一个整型操作数，而 <code>%s</code> 则表示把字符串型操作数的值展开。</p>
<p><code>Printf</code> 有一大堆这种转换，Go程序员称之为<em>动词（verb）</em>。下面的表格虽然远不是完整的规范，但展示了可用的很多特性：</p>
<pre><code class="language-text">%d 十进制整数
%x, %o, %b 十六进制，八进制，二进制整数。
%f, %g, %e 浮点数： 3.141593 3.141592653589793 3.141593e+00
%t 布尔：true或false
%c 字符（rune） (Unicode码点)
%s 字符串
%q 带双引号的字符串&quot;abc&quot;或带单引号的字符'c'
%v 变量的自然形式（natural format）
%T 变量的类型
%% 字面上的百分号标志（无操作数）
</code></pre>
<p><code>dup1</code> 的格式字符串中还含有制表符<code>\t</code>和换行符<code>\n</code>。字符串字面上可能含有这些代表不可见字符的<strong>转义字符（escape sequences）</strong>。默认情况下，<code>Printf</code> 不会换行。按照惯例，以字母 <code>f</code> 结尾的格式化函数，如 <code>log.Printf</code> 和 <code>fmt.Errorf</code>，都采用 <code>fmt.Printf</code> 的格式化准则。而以 <code>ln</code> 结尾的格式化函数，则遵循 <code>Println</code> 的方式，以跟 <code>%v</code> 差不多的方式格式化参数，并在最后添加一个换行符。（译注：后缀 <code>f</code> 指 <code>format</code>，<code>ln</code> 指 <code>line</code>。）</p>
<p>很多程序要么从标准输入中读取数据，如上面的例子所示，要么从一系列具名文件中读取数据。<code>dup</code> 程序的下个版本读取标准输入或是使用 <code>os.Open</code> 打开各个具名文件，并操作它们。</p>
<p><u><i>gopl.io/ch1/dup2</i></u></p>
<pre><code class="language-go">// Dup2 prints the count and text of lines that appear more than once
// in the input. It reads from stdin or from a list of named files.
package main
import (
&quot;bufio&quot;
&quot;fmt&quot;
&quot;os&quot;
)
func main() {
counts := make(map[string]int)
files := os.Args[1:]
if len(files) == 0 {
countLines(os.Stdin, counts)
} else {
for _, arg := range files {
f, err := os.Open(arg)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;dup2: %v\n&quot;, err)
continue
}
countLines(f, counts)
f.Close()
}
}
for line, n := range counts {
if n &gt; 1 {
fmt.Printf(&quot;%d\t%s\n&quot;, n, line)
}
}
}
func countLines(f *os.File, counts map[string]int) {
input := bufio.NewScanner(f)
for input.Scan() {
counts[input.Text()]++
}
// NOTE: ignoring potential errors from input.Err()
}
</code></pre>
<p><code>os.Open</code> 函数返回两个值。第一个值是被打开的文件（<code>*os.File</code>），其后被 <code>Scanner</code> 读取。</p>
<p><code>os.Open</code> 返回的第二个值是内置 <code>error</code> 类型的值。如果 <code>err</code> 等于内置值<code>nil</code>（译注：相当于其它语言里的 <code>NULL</code>），那么文件被成功打开。读取文件，直到文件结束，然后调用 <code>Close</code> 关闭该文件，并释放占用的所有资源。相反的话，如果 <code>err</code> 的值不是 <code>nil</code>，说明打开文件时出错了。这种情况下，错误值描述了所遇到的问题。我们的错误处理非常简单，只是使用 <code>Fprintf</code> 与表示任意类型默认格式值的动词 <code>%v</code>，向标准错误流打印一条信息，然后 <code>dup</code> 继续处理下一个文件；<code>continue</code> 语句直接跳到 <code>for</code> 循环的下个迭代开始执行。</p>
<p>为了使示例代码保持合理的大小，本书开始的一些示例有意简化了错误处理，显而易见的是，应该检查 <code>os.Open</code> 返回的错误值，然而，使用 <code>input.Scan</code> 读取文件过程中，不大可能出现错误，因此我们忽略了错误处理。我们会在跳过错误检查的地方做说明。5.4 节中深入介绍错误处理。</p>
<p>注意 <code>countLines</code> 函数在其声明前被调用。函数和包级别的变量（package-level entities）可以任意顺序声明，并不影响其被调用。（译注：最好还是遵循一定的规范）</p>
<p><code>map</code> 是一个由 <code>make</code> 函数创建的数据结构的引用。<code>map</code> 作为参数传递给某函数时，该函数接收这个引用的一份拷贝（copy，或译为副本），被调用函数对 <code>map</code> 底层数据结构的任何修改，调用者函数都可以通过持有的 <code>map</code> 引用看到。在我们的例子中，<code>countLines</code> 函数向 <code>counts</code> 插入的值，也会被 <code>main</code> 函数看到。（译注：类似于 C++ 里的引用传递，实际上指针是另一个指针了，但内部存的值指向同一块内存）</p>
<p><code>dup</code> 的前两个版本以&quot;流”模式读取输入，并根据需要拆分成多个行。理论上，这些程序可以处理任意数量的输入数据。还有另一个方法，就是一口气把全部输入数据读到内存中，一次分割为多行，然后处理它们。下面这个版本，<code>dup3</code>，就是这么操作的。这个例子引入了 <code>ReadFile</code> 函数（来自于<code>io/ioutil</code>包），其读取指定文件的全部内容，<code>strings.Split</code> 函数把字符串分割成子串的切片。（<code>Split</code> 的作用与前文提到的 <code>strings.Join</code> 相反。请注意如果在Windows下测试注意换行是否为\r\n，否则最后一行是否有空行将会影响结果。）</p>
<p>我们略微简化了 <code>dup3</code>。首先，由于 <code>ReadFile</code> 函数需要文件名作为参数，因此只读指定文件，不读标准输入。其次，由于行计数代码只在一处用到，故将其移回 <code>main</code> 函数。</p>
<p><u><i>gopl.io/ch1/dup3</i></u></p>
<pre><code class="language-go">package main
import (
&quot;fmt&quot;
&quot;io/ioutil&quot;
&quot;os&quot;
&quot;strings&quot;
)
func main() {
counts := make(map[string]int)
for _, filename := range os.Args[1:] {
data, err := ioutil.ReadFile(filename)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;dup3: %v\n&quot;, err)
continue
}
for _, line := range strings.Split(string(data), &quot;\n&quot;) {
counts[line]++
}
}
for line, n := range counts {
if n &gt; 1 {
fmt.Printf(&quot;%d\t%s\n&quot;, n, line)
}
}
}
</code></pre>
<p><code>ReadFile</code> 函数返回一个字节切片（byte slice），必须把它转换为 <code>string</code>，才能用 <code>strings.Split</code> 分割。我们会在3.5.4 节详细讲解字符串和字节切片。</p>
<p>实现上，<code>bufio.Scanner</code>、<code>ioutil.ReadFile</code> 和 <code>ioutil.WriteFile</code> 都使用 <code>*os.File</code> 的 <code>Read</code> 和 <code>Write</code> 方法，但是，大多数程序员很少需要直接调用那些低级（lower-level）函数。高级（higher-level）函数，像 <code>bufio</code> 和 <code>io/ioutil</code> 包中所提供的那些，用起来要容易点。</p>
<hr />
<p><strong>练习 1.4：</strong> 修改 <code>dup2</code>，出现重复的行时打印文件名称。</p>
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