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gobyexample/public/switch
Mark McGranaghan a3b3bff8cb Edits and generation for type switch update
2016-12-27 12:41:21 -08:00

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<title>Go by Example: Switch</title>
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<div class="example" id="switch">
<h2><a href="./">Go by Example</a>: Switch</h2>
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<p><em>Switch statements</em> express conditionals across many
branches.</p>
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<a href="http://play.golang.org/p/kxkBPpY_ue"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;time&quot;</span>
</pre></div>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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<p>Here&rsquo;s a basic <code>switch</code>.</p>
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<div class="highlight"><pre> <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="nx">Print</span><span class="p">(</span><span class="s">&quot;Write &quot;</span><span class="p">,</span> <span class="nx">i</span><span class="p">,</span> <span class="s">&quot; as &quot;</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="nx">Println</span><span class="p">(</span><span class="s">&quot;one&quot;</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="nx">Println</span><span class="p">(</span><span class="s">&quot;two&quot;</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="nx">Println</span><span class="p">(</span><span class="s">&quot;three&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<p>You can use commas to separate multiple expressions
in the same <code>case</code> statement. We use the optional
<code>default</code> case in this example as well.</p>
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<div class="highlight"><pre> <span class="k">switch</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">().</span><span class="nx">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="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s the weekend&quot;</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="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s a weekday&quot;</span><span class="p">)</span>
<span class="p">}</span>
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<p><code>switch</code> without an expression is an alternate way
to express if/else logic. Here we also show how the
<code>case</code> expressions can be non-constants.</p>
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<div class="highlight"><pre> <span class="nx">t</span> <span class="o">:=</span> <span class="nx">time</span><span class="p">.</span><span class="nx">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="nx">Hour</span><span class="p">()</span> <span class="p">&lt;</span> <span class="mi">12</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s before noon&quot;</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="nx">Println</span><span class="p">(</span><span class="s">&quot;It&#39;s after noon&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<p>A type <code>switch</code> compares types instead of values. You
can use this to discover the the type of an interface
value. In this example, the variable <code>t</code> will have the
type corresponding to its clause.</p>
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<div class="highlight"><pre> <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="nx">Println</span><span class="p">(</span><span class="s">&quot;I&#39;m a bool&quot;</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="nx">Println</span><span class="p">(</span><span class="s">&quot;I&#39;m an int&quot;</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="nx">Printf</span><span class="p">(</span><span class="s">&quot;Don&#39;t know type %T\n&quot;</span><span class="p">,</span> <span class="nx">t</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">whatAmI</span><span class="p">(</span><span class="kc">true</span><span class="p">)</span>
<span class="nx">whatAmI</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="nx">whatAmI</span><span class="p">(</span><span class="s">&quot;hey&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<div class="highlight"><pre><span class="gp">$</span> go run switch.go
<span class="go">Write 2 as two</span>
<span class="go">It&#39;s a weekday</span>
<span class="go">It&#39;s after noon</span>
<span class="go">I&#39;m a bool</span>
<span class="go">I&#39;m an int</span>
<span class="go">Don&#39;t know type string</span>
</pre></div>
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<p class="next">
Next example: <a href="arrays">Arrays</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/switch">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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