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<h1 id="33">3.3. &nbsp; 数字编码 *<a class="headerlink" href="#33" title="Permanent link">&para;</a></h1>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>在本书中，标题带有的 * 符号的是选读章节。如果你时间有限或感到理解困难，建议先跳过，等学完必读章节后再单独攻克。</p>
</div>
<h2 id="331">3.3.1. &nbsp; 原码、反码和补码<a class="headerlink" href="#331" title="Permanent link">&para;</a></h2>
<p>从上一节的表格中我们发现，所有整数类型能够表示的负数都比正数多一个。例如，<code>byte</code> 的取值范围是 <span class="arithmatex">\([-128, 127]\)</span> 。这个现象比较反直觉，它的内在原因涉及到原码、反码、补码的相关知识。在展开分析之前，我们首先给出三者的定义：</p>
<ul>
<li><strong>原码</strong>：我们将数字的二进制表示的最高位视为符号位，其中 <span class="arithmatex">\(0\)</span> 表示正数，<span class="arithmatex">\(1\)</span> 表示负数，其余位表示数字的值。</li>
<li><strong>反码</strong>：正数的反码与其原码相同，负数的反码是对其原码除符号位外的所有位取反。</li>
<li><strong>补码</strong>：正数的补码与其原码相同，负数的补码是在其反码的基础上加 <span class="arithmatex">\(1\)</span> 。</li>
</ul>
<p><img alt="原码、反码与补码之间的相互转换" src="../number_encoding.assets/1s_2s_complement.png" /></p>
<p align="center"> Fig. 原码、反码与补码之间的相互转换 </p>
<p>显然，「原码」最为直观，<strong>然而数字却是以「补码」的形式存储在计算机中的</strong>。这是因为原码存在一些局限性。</p>
<p>一方面，<strong>负数的原码不能直接用于运算</strong>。例如，我们在原码下计算 <span class="arithmatex">\(1 + (-2)\)</span> ，得到的结果是 <span class="arithmatex">\(-3\)</span> ，这显然是不对的。</p>
<div class="arithmatex">\[
\begin{aligned}
&amp; 1 + (-2) \newline
&amp; = 0000 \space 0001 + 1000 \space 0010 \newline
&amp; = 1000 \space 0011 \newline
&amp; = -3
\end{aligned}
\]</div>
<p>为了解决此问题，计算机引入了「反码」。例如，我们先将原码转换为反码，并在反码下计算 <span class="arithmatex">\(1 + (-2)\)</span> ，并将结果从反码转化回原码，则可得到正确结果 <span class="arithmatex">\(-1\)</span> 。</p>
<div class="arithmatex">\[
\begin{aligned}
&amp; 1 + (-2) \newline
&amp; = 0000 \space 0001 \space \text{(原码)} + 1000 \space 0010 \space \text{(原码)} \newline
&amp; = 0000 \space 0001 \space \text{(反码)} + 1111 \space 1101 \space \text{(反码)} \newline
&amp; = 1111 \space 1110 \space \text{(反码)} \newline
&amp; = 1000 \space 0001 \space \text{(原码)} \newline
&amp; = -1
\end{aligned}
\]</div>
<p>另一方面，<strong>数字零的原码有 <span class="arithmatex">\(+0\)</span> 和 <span class="arithmatex">\(-0\)</span> 两种表示方式</strong>。这意味着数字零对应着两个不同的二进制编码，而这可能会带来歧义问题。例如，在条件判断中，如果没有区分正零和负零，可能会导致错误的判断结果。如果我们想要处理正零和负零歧义，则需要引入额外的判断操作，其可能会降低计算机的运算效率。</p>
<div class="arithmatex">\[
+0 = 0000 \space 0000 \newline
-0 = 1000 \space 0000
\]</div>
<p>与原码一样，反码也存在正负零歧义问题。为此，计算机进一步引入了「补码」。那么，补码有什么作用呢？我们先来分析一下负零的补码的计算过程：</p>
<div class="arithmatex">\[
\begin{aligned}
-0 = \space &amp; 1000 \space 0000 \space \text{(原码)} \newline
= \space &amp; 1111 \space 1111 \space \text{(反码)} \newline
= 1 \space &amp; 0000 \space 0000 \space \text{(补码)} \newline
\end{aligned}
\]</div>
<p>在负零的反码基础上加 <span class="arithmatex">\(1\)</span> 会产生进位，而由于 byte 的长度只有 8 位，因此溢出到第 9 位的 <span class="arithmatex">\(1\)</span> 会被舍弃。<strong>从而得到负零的补码为 <span class="arithmatex">\(0000 \space 0000\)</span> ，与正零的补码相同</strong>。这意味着在补码表示中只存在一个零，从而解决了正负零歧义问题。</p>
<p>还剩余最后一个疑惑：byte 的取值范围是 <span class="arithmatex">\([-128, 127]\)</span> ，多出来的一个负数 <span class="arithmatex">\(-128\)</span> 是如何得到的呢？我们注意到，区间 <span class="arithmatex">\([-127, +127]\)</span> 内的所有整数都有对应的原码、反码和补码，并且原码和补码之间是可以互相转换的。</p>
<p>然而，<strong>补码 <span class="arithmatex">\(1000 \space 0000\)</span> 是一个例外，它并没有对应的原码</strong>。根据转换方法，我们得到该补码的原码为 <span class="arithmatex">\(0000 \space 0000\)</span> 。这显然是矛盾的，因为该原码表示数字 <span class="arithmatex">\(0\)</span> ，它的补码应该是自身。计算机规定这个特殊的补码 <span class="arithmatex">\(1000 \space 0000\)</span> 代表 <span class="arithmatex">\(-128\)</span> 。实际上，<span class="arithmatex">\((-1) + (-127)\)</span> 在补码下的计算结果就是 <span class="arithmatex">\(-128\)</span> 。</p>
<div class="arithmatex">\[
\begin{aligned}
&amp; (-127) + (-1) \newline
&amp; = 1111 \space 1111 \space \text{(原码)} + 1000 \space 0001 \space \text{(原码)} \newline
&amp; = 1000 \space 0000 \space \text{(反码)} + 1111 \space 1110 \space \text{(反码)} \newline
&amp; = 1000 \space 0001 \space \text{(补码)} + 1111 \space 1111 \space \text{(补码)} \newline
&amp; = 1000 \space 0000 \space \text{(补码)} \newline
&amp; = -128
\end{aligned}
\]</div>
<p>你可能已经发现，上述的所有计算都是加法运算。这暗示着一个重要事实：<strong>计算机内部的硬件电路主要是基于加法运算设计的</strong>。这是因为加法运算相对于其他运算（比如乘法、除法和减法）来说，硬件实现起来更简单，更容易进行并行化处理，从而提高运算速度。</p>
<p>然而，这并不意味着计算机只能做加法。<strong>通过将加法与一些基本逻辑运算结合，计算机能够实现各种其他的数学运算</strong>。例如，计算减法 <span class="arithmatex">\(a - b\)</span> 可以转换为计算加法 <span class="arithmatex">\(a + (-b)\)</span> ；计算乘法和除法可以转换为计算多次加法或减法。</p>
<p>现在，我们可以总结出计算机使用补码的原因：基于补码表示，计算机可以用同样的电路和操作来处理正数和负数的加法，不需要设计特殊的硬件电路来处理减法，并且无需特别处理正负零的歧义问题。这大大简化了硬件设计，并提高了运算效率。</p>
<p>补码的设计非常精妙，由于篇幅关系我们先介绍到这里。建议有兴趣的读者进一步深度了解。</p>
<h2 id="332">3.3.2. &nbsp; 浮点数编码<a class="headerlink" href="#332" title="Permanent link">&para;</a></h2>
<p>细心的你可能会发现：<code>int</code> 和 <code>float</code> 长度相同，都是 4 bytes，但为什么 <code>float</code> 的取值范围远大于 <code>int</code> ？这非常反直觉，因为按理说 <code>float</code> 需要表示小数，取值范围应该变小才对。</p>
<p>实际上，这是因为浮点数 <code>float</code> 采用了不同的表示方式。根据 IEEE 754 标准，32-bit 长度的 <code>float</code> 由以下部分构成：</p>
<ul>
<li>符号位 <span class="arithmatex">\(\mathrm{S}\)</span> ：占 1 bit ；</li>
<li>指数位 <span class="arithmatex">\(\mathrm{E}\)</span> ：占 8 bits ；</li>
<li>分数位 <span class="arithmatex">\(\mathrm{N}\)</span> ：占 24 bits ，其中 23 位显式存储；</li>
</ul>
<p>设 32-bit 二进制数的第 <span class="arithmatex">\(i\)</span> 位为 <span class="arithmatex">\(b_i\)</span> ，则 <code>float</code> 值的计算方法定义为：</p>
<div class="arithmatex">\[
\text { val } = (-1)^{b_{31}} \times 2^{\left(b_{30} b_{29} \ldots b_{23}\right)_2-127} \times\left(1 . b_{22} b_{21} \ldots b_0\right)_2
\]</div>
<p>转化到十进制下的计算公式为</p>
<div class="arithmatex">\[
\text { val }=(-1)^{\mathrm{S}} \times 2^{\mathrm{E} -127} \times (1 + \mathrm{N})
\]</div>
<p>其中各项的取值范围为</p>
<div class="arithmatex">\[
\begin{aligned}
\mathrm{S} \in &amp; \{ 0, 1\} , \quad \mathrm{E} \in \{ 1, 2, \dots, 254 \} \newline
(1 + \mathrm{N}) = &amp; (1 + \sum_{i=1}^{23} b_{23-i} 2^{-i}) \subset [1, 2 - 2^{-23}]
\end{aligned}
\]</div>
<p><img alt="IEEE 754 标准下的 float 表示方式" src="../number_encoding.assets/ieee_754_float.png" /></p>
<p align="center"> Fig. IEEE 754 标准下的 float 表示方式 </p>
<p>以上图为例，<span class="arithmatex">\(\mathrm{S} = 0\)</span> ， <span class="arithmatex">\(\mathrm{E} = 124\)</span> ，<span class="arithmatex">\(\mathrm{N} = 2^{-2} + 2^{-3} = 0.375\)</span> ，易得</p>
<div class="arithmatex">\[
\text { val } = (-1)^0 \times 2^{124 - 127} \times (1 + 0.375) = 0.171875
\]</div>
<p>现在我们可以回答最初的问题：<strong><code>float</code> 的表示方式包含指数位，导致其取值范围远大于 <code>int</code></strong> <code>。根据以上计算，float</code> 可表示的最大正数为 <span class="arithmatex">\(2^{254 - 127} \times (2 - 2^{-23}) \approx 3.4 \times 10^{38}\)</span> ，切换符号位便可得到最小负数。</p>
<p><strong>尽管浮点数 <code>float</code> 扩展了取值范围，但其副作用是牺牲了精度</strong>。整数类型 <code>int</code> 将全部 32 位用于表示数字，数字是均匀分布的；而由于指数位的存在，浮点数 <code>float</code> 的数值越大，相邻两个数字之间的差值就会趋向越大。</p>
<p>进一步地，指数位 <span class="arithmatex">\(E = 0\)</span> 和 <span class="arithmatex">\(E = 255\)</span> 具有特殊含义，<strong>用于表示零、无穷大、<span class="arithmatex">\(\mathrm{NaN}\)</span> 等</strong>。</p>
<div class="center-table">
<table>
<thead>
<tr>
<th>指数位 E</th>
<th>分数位 <span class="arithmatex">\(\mathrm{N} = 0\)</span></th>
<th>分数位 <span class="arithmatex">\(\mathrm{N} \ne 0\)</span></th>
<th>计算公式</th>
</tr>
</thead>
<tbody>
<tr>
<td><span class="arithmatex">\(0\)</span></td>
<td><span class="arithmatex">\(\pm 0\)</span></td>
<td>次正规数</td>
<td><span class="arithmatex">\((-1)^{\mathrm{S}} \times 2^{-126} \times (0.\mathrm{N})\)</span></td>
</tr>
<tr>
<td><span class="arithmatex">\(1, 2, \dots, 254\)</span></td>
<td>正规数</td>
<td>正规数</td>
<td><span class="arithmatex">\((-1)^{\mathrm{S}} \times 2^{(\mathrm{E} -127)} \times (1.\mathrm{N})\)</span></td>
</tr>
<tr>
<td><span class="arithmatex">\(255\)</span></td>
<td><span class="arithmatex">\(\pm \infty\)</span></td>
<td><span class="arithmatex">\(\mathrm{NaN}\)</span></td>
<td></td>
</tr>
</tbody>
</table>
</div>
<p>特别地，次正规数显著提升了浮点数的精度，这是因为：</p>
<ul>
<li>最小正正规数为 <span class="arithmatex">\(2^{-126} \approx 1.18 \times 10^{-38}\)</span> ；</li>
<li>最小正次正规数为 <span class="arithmatex">\(2^{-126} \times 2^{-23} \approx 1.4 \times 10^{-45}\)</span> ；</li>
</ul>
<p>双精度 <code>double</code> 也采用类似 <code>float</code> 的表示方法，此处不再详述。</p>
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