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<div class="example" id="select">
<h2><a href="./">Go by Example</a>: Select</h2>
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<p>Go&rsquo;s <em>select</em> lets you wait on multiple channel
operations. Combining goroutines and channels with
select is powerful feature of Go.</p>
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<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;time&quot;</span>
<span class="kn">import</span> <span class="s">&quot;fmt&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>For our example we&rsquo;ll select across two channels.</p>
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<div class="highlight"><pre> <span class="nx">c1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
<span class="nx">c2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
</pre></div>
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<p>Each channel will receive a value after some amount
of time, to simulate e.g. blocking RPC operations
executing in concurrent goroutines.</p>
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<div class="highlight"><pre> <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">1</span><span class="p">)</span>
<span class="nx">c1</span> <span class="o">&lt;-</span> <span class="s">&quot;one&quot;</span>
<span class="p">}()</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span>
<span class="nx">c2</span> <span class="o">&lt;-</span> <span class="s">&quot;two&quot;</span>
<span class="p">}()</span>
</pre></div>
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<p>We&rsquo;ll use <code>select</code> to await both of these values
simultaneously, printing each one as it arrives.</p>
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p">&lt;</span> <span class="mi">2</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
<span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">msg1</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">c1</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;received&quot;</span><span class="p">,</span> <span class="nx">msg1</span><span class="p">)</span>
<span class="k">case</span> <span class="nx">msg2</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">c2</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;received&quot;</span><span class="p">,</span> <span class="nx">msg2</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<p>We receive the values <code>&quot;one&quot;</code> and then <code>&quot;two&quot;</code> as
expected.</p>
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<div class="highlight"><pre><span class="gp">$</span> <span class="nb">time </span>go run <span class="k">select</span>.go
<span class="go">received one</span>
<span class="go">received two</span>
</pre></div>
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<p>Note that the total execution time is only ~2 seconds
since both the 1 and 2 second <code>Sleeps</code> execute
concurrently.</p>
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<div class="highlight"><pre><span class="go">real 0m2.245s</span>
</pre></div>
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<p class="next">
Next example: <a href="timeouts">Timeouts</a>.
</p>
<p class="footer">
<a href="https://twitter.com/gobyexample">@gobyexample</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/select">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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