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<h2><a href="./">Go by Example 中文版</a>: 常量</h2>

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<p>Go 支持字符、字符串、布尔和数值 <em>常量</em> 。</p>

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<a href="https://play.studygolang.com/p/lPnuXwPXXQu"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />

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<span class="kn">package</span> <span class="nx">main</span>

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<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>

<span class="s">&#34;fmt&#34;</span>

<span class="s">&#34;math&#34;</span>

<span class="p">)</span>

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<p><code>const</code> 用于声明一个常量。</p>

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<span class="kd">const</span> <span class="nx">s</span> <span class="kt">string</span> <span class="p">=</span> <span class="s">&#34;constant&#34;</span>

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<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</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="nx">s</span><span class="p">)</span>

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<p><code>const</code> 语句可以出现在任何 <code>var</code> 语句可以出现的地方</p>

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<span class="kd">const</span> <span class="nx">n</span> <span class="p">=</span> <span class="mi">500000000</span>

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<p>常数表达式可以执行任意精度的运算</p>

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<pre class="chroma">

<span class="kd">const</span> <span class="nx">d</span> <span class="p">=</span> <span class="mf">3e20</span> <span class="o">/</span> <span class="nx">n</span>

<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">d</span><span class="p">)</span>

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<p>数值型常量没有确定的类型，直到被给定某个类型，比如显式类型转化。</p>

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<pre class="chroma">

<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nb">int64</span><span class="p">(</span><span class="nx">d</span><span class="p">))</span>

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<p>一个数字可以根据上下文的需要（比如变量赋值、函数调用）自动确定类型。

举个例子，这里的 <code>math.Sin</code> 函数需要一个 <code>float64</code> 的参数，<code>n</code> 会自动确定类型。</p>

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<pre class="chroma">

<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">math</span><span class="p">.</span><span class="nf">Sin</span><span class="p">(</span><span class="nx">n</span><span class="p">))</span>

<span class="p">}</span>

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<pre class="chroma"><span class="gp">$</span> go run constant.go

<span class="go">constant

</span><span class="go">6e+11

</span><span class="go">600000000000

</span><span class="go">-0.28470407323754404</span></pre>

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下一个例子: <a href="for">For 循环</a>

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<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>

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