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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example 中文版: Switch 分支结构</title>
<link rel=stylesheet href="site.css">
</head>
<script>
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<body>
<div class="example" id="switch">
<h2><a href="./">Go by Example 中文版</a>: Switch 分支结构</h2>
<table>
<tr>
<td class="docs">
<p><em>switch</em> 是多分支情况时快捷的条件语句。</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="https://play.studygolang.com/p/MmQyn9nsEJv"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"fmt"</span>
<span class="s">"time"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>一个基本的 <code>switch</code>。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">i</span> <span class="o">:=</span> <span class="mi">2</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Print</span><span class="p">(</span><span class="s">"write "</span><span class="p">,</span> <span class="nx">i</span><span class="p">,</span> <span class="s">" as "</span><span class="p">)</span>
<span class="k">switch</span> <span class="nx">i</span> <span class="p">{</span>
<span class="k">case</span> <span class="mi">1</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"one"</span><span class="p">)</span>
<span class="k">case</span> <span class="mi">2</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"two"</span><span class="p">)</span>
<span class="k">case</span> <span class="mi">3</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"three"</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>在同一个 <code>case</code> 语句中，你可以使用逗号来分隔多个表达式。
在这个例子中，我们还使用了可选的 <code>default</code> 分支。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="k">switch</span> <span class="nx">time</span><span class="p">.</span><span class="nf">Now</span><span class="p">().</span><span class="nf">Weekday</span><span class="p">()</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Saturday</span><span class="p">,</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Sunday</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's the weekend"</span><span class="p">)</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's a weekday"</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>不带表达式的 <code>switch</code> 是实现 if/else 逻辑的另一种方式。
这里还展示了 <code>case</code> 表达式也可以不使用常量。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">t</span> <span class="o">:=</span> <span class="nx">time</span><span class="p">.</span><span class="nf">Now</span><span class="p">()</span>
<span class="k">switch</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">t</span><span class="p">.</span><span class="nf">Hour</span><span class="p">()</span> <span class="p"><</span> <span class="mi">12</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's before noon"</span><span class="p">)</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"It's after noon"</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>类型开关 (<code>type switch</code>) 比较类型而非值。可以用来发现一个接口值的类型。
在这个例子中，变量 <code>t</code> 在每个分支中会有相应的类型。</p>
</td>
<td class="code">
<pre class="chroma">
<span class="nx">whatAmI</span> <span class="o">:=</span> <span class="kd">func</span><span class="p">(</span><span class="nx">i</span> <span class="kd">interface</span><span class="p">{})</span> <span class="p">{</span>
<span class="k">switch</span> <span class="nx">t</span> <span class="o">:=</span> <span class="nx">i</span><span class="p">.(</span><span class="kd">type</span><span class="p">)</span> <span class="p">{</span>
<span class="k">case</span> <span class="kt">bool</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"I'm a bool"</span><span class="p">)</span>
<span class="k">case</span> <span class="kt">int</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"I'm an int"</span><span class="p">)</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"Don't know type %T\n"</span><span class="p">,</span> <span class="nx">t</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nf">whatAmI</span><span class="p">(</span><span class="kc">true</span><span class="p">)</span>
<span class="nf">whatAmI</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="nf">whatAmI</span><span class="p">(</span><span class="s">"hey"</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code">
<pre class="chroma"><span class="gp">$</span> go run switch.go
<span class="go">Write 2 as two
</span><span class="go">It's a weekday
</span><span class="go">It's after noon
</span><span class="go">I'm a bool
</span><span class="go">I'm an int
</span><span class="go">Don't know type string</span></pre>
</td>
</tr>
</table>
<p class="next">
下一个例子: <a href="arrays">数组</a>
</p>
<p class="footer">
<a href="https://twitter.com/mmcgrana">@mmcgrana</a> 和<a href="https://eli.thegreenplace.net">Eli Bendersky</a>编写 | <a href="https://github.com/gobyexample-cn">gobyexample-cn</a> 翻译 | <a href="https://github.com/gobyexample-cn/gobyexample/issues">反馈</a> | <a href="https://github.com/gobyexample-cn/gobyexample">源码</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a> </p>
</p>
</div>
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