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<title>过程 | 编程导引</title>
<meta name="description" content="程序的基本控制结构有哪几种？递归和循环的区别是什么？短路求值的作用是什么？">
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class="visually-hidden" id="sidebar-aria-label" data-v-af16598e> Sidebar Navigation </span><!--[--><!--]--><!--[--><div class="group" data-v-af16598e><section class="VPSidebarItem level-0 has-active" data-v-af16598e data-v-c4656e6d><!----><div class="items" data-v-c4656e6d><!--[--><div class="VPSidebarItem level-1 is-link" data-v-c4656e6d data-v-c4656e6d><div class="item" data-v-c4656e6d><div class="indicator" data-v-c4656e6d></div><a class="VPLink link link" href="/intro.html" data-v-c4656e6d data-v-8f4dc553><!--[--><p class="text" data-v-c4656e6d>引言</p><!--]--><!----></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-c4656e6d data-v-c4656e6d><div class="item" data-v-c4656e6d><div class="indicator" data-v-c4656e6d></div><a class="VPLink link link" href="/ch01_environment.html" data-v-c4656e6d data-v-8f4dc553><!--[--><p class="text" data-v-c4656e6d>环境</p><!--]--><!----></a><!----></div><!----></div><div class="VPSidebarItem level-1 is-link" data-v-c4656e6d 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class="outline-title" data-v-ff0f39c8>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-ff0f39c8><span class="visually-hidden" id="doc-outline-aria-label" data-v-ff0f39c8> Table of Contents for current page </span><ul class="root" data-v-ff0f39c8 data-v-9a431c33><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-3f215769></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-c4b0d3cf><div class="content-container" data-v-c4b0d3cf><!--[--><!--]--><!----><main class="main" data-v-c4b0d3cf><div style="position:relative;" class="vp-doc _ch03_procedure" data-v-c4b0d3cf><div><h1 id="过程" tabindex="-1">过程 <a class="header-anchor" href="#过程" aria-label="Permalink to "过程""></a></h1><p>什么是过程呢？</p><p>凭经验我们就能知道，实际的问题并不是像理想中那么简单，不是一个表达式就能描述的清楚的，而且也并不一定能通过组合来直接解决。</p><p>甚至是简单的数学问题，也不能用他们来解决。比如：</p><p><span class="katex-display"><span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:1.75em;"></span><span class="strut bottom" style="height:3.0000299999999998em;vertical-align:-1.25003em;"></span><span class="base displaystyle textstyle uncramped"><span class="mord mathrm">∣</span><span class="mord mathit">x</span><span class="mord mathrm">∣</span><span class="mrel">=</span><span class="mord mathit">a</span><span class="mord mathit">b</span><span class="mord mathit">s</span><span class="mopen">(</span><span class="mord mathit">x</span><span class="mclose">)</span><span class="mrel">=</span><span class="minner displaystyle textstyle uncramped"><span class="style-wrap reset-textstyle textstyle uncramped" style="top:0em;"><span class="delimsizing size4">{</span></span><span class="mord"><span class="mtable"><span class="col-align-l"><span class="vlist"><span style="top:-0.6819999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit">x</span><span class="mpunct">,</span></span></span><span style="top:0.7579999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord">−</span><span class="mord mathit">x</span><span class="mpunct">,</span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span></span></span></span><span class="arraycolsep" style="width:1em;"></span><span class="col-align-l"><span class="vlist"><span style="top:-0.6819999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit">x</span><span class="mrel">≥</span><span class="mord mathrm">0</span></span></span><span style="top:0.7579999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit">x</span><span class="mrel"><</span><span class="mord mathrm">0</span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span></span></span></span></span></span><span class="sizing reset-size5 size5 reset-textstyle textstyle uncramped nulldelimiter"></span></span></span></span></span></span></p><p>求绝对值，或者</p><p><span class="katex-display"><span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:1.6985050000000004em;"></span><span class="strut bottom" style="height:2.965618em;vertical-align:-1.267113em;"></span><span class="base displaystyle textstyle uncramped"><span class="mord mathit">p</span><span class="mord mathit">o</span><span class="mord mathit" style="margin-right:0.02691em;">w</span><span class="mopen">(</span><span class="mord mathit">x</span><span class="mpunct">,</span><span class="mord mathit" style="margin-right:0.03588em;">y</span><span class="mclose">)</span><span class="mrel">=</span><span class="mop op-limits"><span class="vlist"><span style="top:1.1671129999999998em;margin-left:0em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="reset-textstyle scriptstyle cramped"><span class="mord scriptstyle cramped"><span class="mord mathrm">1</span></span></span></span><span style="top:-0.000005000000000143778em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span><span class="op-symbol large-op mop">∏</span></span></span><span style="top:-1.2971130000000002em;margin-left:0em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="reset-textstyle scriptstyle uncramped"><span class="mord scriptstyle uncramped"><span class="mord mathit" style="margin-right:0.03588em;">y</span></span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span></span></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit">x</span></span></span></span></span></span></p><p>求积。</p><p>其中一个需要我们根据参数的值，来选择不同的计算方式，返回不同的结果；另外一个则是通过对特定的表达式做一个累乘（没错其实行为跟乘方是一样的，所以我把它叫 pow）。</p><p>我们先看累乘。</p><h2 id="累乘" tabindex="-1">累乘 <a class="header-anchor" href="#累乘" aria-label="Permalink to "累乘""></a></h2><p>如果我们已知指数的话，要写起来是很简单的：</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow1</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow2</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow1</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow3</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow2</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow4</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow3</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow5</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow4</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span></code></pre></div><p>这样写其实是符合pow的定义的。</p><p>然后其实我们可以把它变换一下：</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 伪代码，无法正确运行</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> 1</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> 2</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> 3</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">2</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> 4</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">3</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> 5</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">4</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span></code></pre></div><p>所以我们得到了pow函数的另一个形式：</p><p><span class="katex-display"><span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:1.75em;"></span><span class="strut bottom" style="height:3.0000299999999998em;vertical-align:-1.25003em;"></span><span class="base displaystyle textstyle uncramped"><span class="mord mathit">p</span><span class="mord mathit">o</span><span class="mord mathit" style="margin-right:0.02691em;">w</span><span class="mopen">(</span><span class="mord mathit">x</span><span class="mpunct">,</span><span class="mord mathit" style="margin-right:0.03588em;">y</span><span class="mclose">)</span><span class="mrel">=</span><span class="minner displaystyle textstyle uncramped"><span class="style-wrap reset-textstyle textstyle uncramped" style="top:0em;"><span class="delimsizing size4">{</span></span><span class="mord"><span class="mtable"><span class="col-align-l"><span class="vlist"><span style="top:-0.6819999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit">x</span><span class="mpunct">,</span></span></span><span style="top:0.7579999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit">p</span><span class="mord mathit">o</span><span class="mord mathit" style="margin-right:0.02691em;">w</span><span class="mopen">(</span><span class="mord mathit">x</span><span class="mpunct">,</span><span class="mord mathit" style="margin-right:0.03588em;">y</span><span class="mbin">−</span><span class="mord mathrm">1</span><span class="mclose">)</span><span class="mbin">×</span><span class="mord mathit">x</span><span class="mpunct">,</span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span></span></span></span><span class="arraycolsep" style="width:1em;"></span><span class="col-align-l"><span class="vlist"><span style="top:-0.6819999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit" style="margin-right:0.03588em;">y</span><span class="mrel">=</span><span class="mord mathrm">1</span></span></span><span style="top:0.7579999999999999em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span><span class="mord displaystyle textstyle uncramped"><span class="mord mathit" style="margin-right:0.03588em;">y</span><span class="mrel">></span><span class="mord mathrm">1</span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;"></span></span></span></span></span></span></span><span class="sizing reset-size5 size5 reset-textstyle textstyle uncramped nulldelimiter"></span></span></span></span></span></span></p><p>没错我们又归结到了选择结构上。后面我们会拿这个来讨论。现在只是简单提一提。</p><p>哦对了，现在pow的这个形式叫做递归定义，数学上叫递推关系式（好像又叫归纳定义？）。</p><p>好的，我们回到<span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:0.75em;"></span><span class="strut bottom" style="height:1.00001em;vertical-align:-0.25001em;"></span><span class="base textstyle uncramped"><span class="op-symbol small-op mop" style="top:-0.0000050000000000050004em;">∏</span></span></span></span>运算符上来。</p><p>根据前面的分析，我们可以知道，如果能够拿到前面一个乘积，然后和第y个相乘，，就可以得到我们要的结果。</p><p>于是我们假设那第<code>y-1</code>个乘积是<code>prod</code>，最终的结果就是<code>prod*x</code>。再往前推<code>prod</code>，这个关系是基本上不变的。</p><p>那么，我们只要有一个东西来保存前面的结果，再跟后面的相乘之后更新保存的结果就可以了。</p><p>这个能保存数据的东西我们把它叫做变量（Variable）。</p><p>比如最开始<code>prod = 1</code>，然后<code>prod = prod * x</code>，会把<code>prod</code>与<code>x</code>相乘的结果计算出来，再保存进<code>prod</code>里面。</p><p>所以我们可以简单地把这个计算看成</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> prod</span><span style="color:#89DDFF;">=</span><span style="color:#F78C6C;">1</span></span>
<span class="line"><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">...</span></span>
<span class="line"><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> </span><span style="color:#676E95;font-style:italic;">// 执行y次</span></span></code></pre></div><p>那么怎么让他能够只执行<code>y</code>次呢？</p><h2 id="循环" tabindex="-1">循环 <a class="header-anchor" href="#循环" aria-label="Permalink to "循环""></a></h2><p>JavaScript提供了一种东西叫做循环语句。</p><p>大概是长这个样子的</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> prod</span><span style="color:#89DDFF;">=</span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;"><=</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">+</span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>这个叫循环语句，意思就是，在满足指定条件的情况下，会一直去执行语句体（花括号之间）的内容。</p><p>上面那段代码的意思是，让<code>prod</code>初值为1，让<code>i</code>从1开始，每次增加1，如果<code>i</code>小于等于<code>y</code>的话，执行<code>prod = prod*x</code>，否则跳出循环。</p><p>确实很复杂，特别是比起上面那个递归定义来。</p><p>我们对它进行一个简单的改写。</p><p>一般来说，程序员是从 <span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:0.64444em;"></span><span class="strut bottom" style="height:0.64444em;vertical-align:0em;"></span><span class="base textstyle uncramped"><span class="mord mathrm">0</span></span></span></span> 开始计数的。于是从 <span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:0.64444em;"></span><span class="strut bottom" style="height:0.64444em;vertical-align:0em;"></span><span class="base textstyle uncramped"><span class="mord mathrm">1</span></span></span></span> 到 <span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:0.43056em;"></span><span class="strut bottom" style="height:0.625em;vertical-align:-0.19444em;"></span><span class="base textstyle uncramped"><span class="mord mathit" style="margin-right:0.03588em;">y</span></span></span></span>，就变成了从 <span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:0.64444em;"></span><span class="strut bottom" style="height:0.64444em;vertical-align:0em;"></span><span class="base textstyle uncramped"><span class="mord mathrm">0</span></span></span></span> 到 <span class="katex"><span class="katex-mathml"><!----></span><span class="katex-html" aria-hidden="true"><span class="strut" style="height:0.64444em;"></span><span class="strut bottom" style="height:0.8388800000000001em;vertical-align:-0.19444em;"></span><span class="base textstyle uncramped"><span class="mord mathit" style="margin-right:0.03588em;">y</span><span class="mbin">−</span><span class="mord mathrm">1</span></span></span></span>，进一步地<code>i <= y-1</code>又可以进一步地写成<code>i < y</code>（反正都是整数，证明我就不给了），这样条件就简化掉了一个符号。</p><p>另外，但凡类似<code>x=x+y</code>这种的，都可以写成<code>x+=y</code>，特别地，当运算符为<code>+</code>和<code>-</code>，同时<code>y</code>为1的时候，可以直接写成<code>x++</code>或者<code>x--</code>；</p><p>于是一个典型的循环就出来了：</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> prod</span><span style="color:#89DDFF;">=</span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;">(</span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;"><</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">*=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>那么我们的pow函数也有了：</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">=</span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#F07178;">(</span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;"><</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#89DDFF;">++</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">*=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><h2 id="选择" tabindex="-1">选择 <a class="header-anchor" href="#选择" aria-label="Permalink to "选择""></a></h2><p>好的，回头来看我们的abs函数。</p><p>我们要根据x的符号来决定时返回x还是他的相反数。</p><p>JavaScript让我们能通过选择语句<code>if...else...</code>做这件事。</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">abs</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">>=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">else</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">-</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>当然<code>if/else</code>还有一种简单的形式，可以让我们写起来更省力。</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">abs</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> (</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">>=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">?</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">:</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">-</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p><code>?:</code>运算符共有三部分，问好前面是判断条件，问号后面由冒号分成两部分，如果判断条件为真，那么返回冒号与问号之间的，否则返回冒号之后的。</p><h2 id="递归" tabindex="-1">递归 <a class="header-anchor" href="#递归" aria-label="Permalink to "递归""></a></h2><p>好的，这样子我们就能完善我们前面写的递归定义的pow了。</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> (</span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">==</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">?</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">:</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#89DDFF;">-</span><span style="color:#F78C6C;">1</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>不能比循环的明白更多。</p><h3 id="尾递归" tabindex="-1">尾递归 <a class="header-anchor" href="#尾递归" aria-label="Permalink to "尾递归""></a></h3><p>尾递归这个概念可以简单做个了解。主要是因为如果有了尾递归，编译器就可以更好的优化程序的执行过程。而且更重要的一点是，尾递归跟循环几乎是一一对应的关系。</p><p>我们先简单的把这个实现写出来吧。</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">prod</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> (</span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">==</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">?</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">:</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#89DDFF;">-</span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*</span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>怎么转换到循环的呢？</p><p>首先我们提取出变量来。</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">prod</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">if</span><span style="color:#F07178;"> (</span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">==</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">-</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*=</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">prow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;">)</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>正式写成循环</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">prod</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">!=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span><span style="color:#F07178;"> </span><span style="color:#676E95;font-style:italic;">// or while(y != 0)</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">-</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">*=</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>改写成递增的变量</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#F07178;">(</span><span style="color:#C792EA;">let</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;"><</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#89DDFF;">++</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">*=</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">prod</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><h2 id="短路求值" tabindex="-1">短路求值 <a class="header-anchor" href="#短路求值" aria-label="Permalink to "短路求值""></a></h2><p>试想一下我们的递归版pow代码</p><div class="language-javascript"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-theme-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">x</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">y</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;"> </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> (</span><span style="color:#A6ACCD;">y</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">==</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">1</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">?</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">:</span><span style="color:#F07178;"> </span><span style="color:#82AAFF;">pow</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">,</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">y</span><span style="color:#89DDFF;">-</span><span style="color:#F78C6C;">1</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">*</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span></code></pre></div><p>如果这里你把函数自身的调用写成了<code>pow(x, y) * x</code>，结果会是怎么样的呢？</p><p>如果这样写，y作为一个计数器并没有递减，那么每回调用都会跟当前调用一样传递相同的参数进去。</p><p>于是会一在重复递归地调用<code>pow(x, y)</code>，直到不能再调用导致栈溢出。</p><p>然而一定是这样吗？你可以调用一下<code>pow(4, 1)</code>试试，会正常输出<code>4</code>。</p><p>为什么会有这种不同呢？</p><p>上一讲，我们提到了组合的概念，说，通过组合会强制求值每一个传递给函数的参数，但是当我们把这个东西忘<code>?:</code>操作符上套的时候，并没有效果。</p><p>这种特性叫做短路求值（short-circuit evaluation），对于<code>?:</code>来说，只有当条件为真的时候才去求值真分支，为假的时候才去求值假分支。</p><p>所以如果写错的那个pow，当<code>y == 1</code>的时候，还是能够正常执行的。</p><h2 id="过程-1" tabindex="-1">过程 <a class="header-anchor" href="#过程-1" aria-label="Permalink to "过程""></a></h2><p>一个再复杂的流程，最终都会由这样的三部分组成：</p><ul><li>顺序</li><li>分支（选择）</li><li>循环</li></ul><p>根据我们前面提到的组合的思想，再复杂的过程都能通过这些来组合解决。</p><h2 id="练习" tabindex="-1">练习 <a class="header-anchor" href="#练习" aria-label="Permalink to "练习""></a></h2><ul><li>试写出求阶乘、求斐波那契数等函数的循环、递归和尾递归版本的实现。</li><li>用以上代码解释短路求值</li><li>请使用顺序、分支、循环来描述现实中的一些过程。</li></ul></div></div></main><footer class="VPDocFooter" data-v-c4b0d3cf data-v-face870a><!--[--><!--]--><!----><div class="prev-next" data-v-face870a><div class="pager" data-v-face870a><a class="pager-link prev" href="/ch02_computation.html" data-v-face870a><span class="desc" data-v-face870a>Previous page</span><span class="title" data-v-face870a>计算</span></a></div><div class="has-prev pager" data-v-face870a><a class="pager-link next" href="/ch04_encoding.html" data-v-face870a><span class="desc" data-v-face870a>Next page</span><span class="title" data-v-face870a>编码</span></a></div></div></footer><!--[--><!--]--></div></div></div><!--[--><!--]--></div></div><footer class="VPFooter has-sidebar" data-v-b2cf3e0b data-v-2f86ebd2><div class="container" data-v-2f86ebd2><!----><p class="copyright" data-v-2f86ebd2>CC-BY 4.0 Licensed | Copyright © 2015-present Kimmy Leo</p></div></footer><!--[--><!--]--></div></div>
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