PHP中的循环和它们的类型

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css-old1by"><style data-emotion="css h8g7xa">.css-h8g7xa{margin:.5em 0;}</style><div class="MuiBox-root css-h8g7xa"><ol style="margin-bottom:0;line-height:1.75"><li><a href="/php-zhong-de-xun-huan-he-ta-men-de-lei-xing#xun-huan-de-yi-ban-shu-xing">循环的一般属性</a></li><li><a href="/php-zhong-de-xun-huan-he-ta-men-de-lei-xing#dui-yu-xun-huan">对于循环</a></li><li><a href="/php-zhong-de-xun-huan-he-ta-men-de-lei-xing#dang-yi-gedo-while-xun-huan">当一个do-while循环</a></li><li><a href="/php-zhong-de-xun-huan-he-ta-men-de-lei-xing#yu-lan-qi">预览器</a></li><li><a href="/php-zhong-de-xun-huan-he-ta-men-de-lei-xing#di-gui">递归</a></li></ol></div><style data-emotion="css 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print{.css-zgnaae{-webkit-print-color-adjust:exact;color-adjust:exact;}}.css-zgnaae:hover{background:#161b22;}</style><button class="MuiButtonBase-root css-zgnaae" tabindex="0" type="button">Zobrazit více<svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="KeyboardArrowDownIcon"><path d="M7.41 8.59 12 13.17l4.59-4.58L18 10l-6 6-6-6 1.41-1.41z"/></svg></button></div></div><style data-emotion="css 9l3uo3">.css-9l3uo3{margin:0;font-family:"Roboto","Helvetica","Arial",sans-serif;font-weight:400;font-size:1rem;line-height:1.5;letter-spacing:0.00938em;}</style><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3">循环一般用于重复相同的代码部分（通常是在一个数据集上）对于每一种类型的任务，都有不同类型的循环适合，其主要区别在于含义和语法。还需要注意的是，所有类型的循环都是可以相互转换的，只是在代码的复杂性和计算时间上可能并不总是值得的。 </p><p>循环的基本划分是以元素类型来划分的。</p> <code>for</code>：一个通用的循环，我们事先知道重复的次数，或者可以简单地事先计算出来。</li> <code>while</code>和<code>until-while</code>：一个一般的循环，我们事先不知道重复的次数。</li> <code>foreach</code>：浏览数组和对象。</li> <li>递归"：将一个复杂的问题分解成一组较小的问题。</li> <div id="xun-huan-de-yi-ban-shu-xing" class="content-anchor"/><h2>循环的一般属性</h2> <p>循环通常是通过重复一段有标记的代码来实现的，<strong>重复</strong>，<strong>只要终止条件</strong>成立。循环的停止是由于不能满足终止条件，或者通过调用<code>break;</code>手动停止。</p> <p>如果没有任何东西停止循环，那么就不能阻止它无限期地运行，或直到应用程序被手动终止，或处理PHP脚本的时间限制用完（通常是30秒）。</p> <p><strong>重要的术语：</strong>。</p> <li>循环"--一种语言表达方式，允许你重复代码的某一部分。</li> <li>迭代"--循环体的一个具体执行。</li> <li>初始化"--开始执行一个循环（在第一次迭代开始之前）。</li> <li>递增"--将一个迭代变量递增1。</li> <li>退出条件"--在每个迭代中被验证的条件（验证的位置因循环类型而异）。只要条件成立，该循环就会运行</li> <p><code>For</code>对于我们事先知道重复次数（或可以很容易地计算出来）的情况很有用。例如，它非常适合于区间的线性遍历。</p> <p>它一般是这样写的（3部分）。</p> <style data-emotion="css h773az">.css-h773az{margin-bottom:1.5em;}</style><div class="MuiBox-root css-h773az"><style data-emotion="css 1uwjw6e">.css-1uwjw6e{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:right;-ms-flex-pack:right;-webkit-justify-content:right;justify-content:right;}</style><div 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0, 0.2, 1) 0ms;}.css-1yxmbwk::-moz-focus-inner{border-style:none;}.css-1yxmbwk.Mui-disabled{pointer-events:none;cursor:default;}@media print{.css-1yxmbwk{-webkit-print-color-adjust:exact;color-adjust:exact;}}.css-1yxmbwk:hover{background-color:rgba(0, 0, 0, 0.04);}@media (hover: none){.css-1yxmbwk:hover{background-color:transparent;}}.css-1yxmbwk.Mui-disabled{background-color:transparent;color:rgba(0, 0, 0, 0.26);}</style><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">for (inicializace; výraz; inkrementace) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> // 循环的主体</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> <code>初始化</code>：定义循环的初始状态（我们需要哪些变量）。</li> <li>`表达式': 条件。只要它是有效的，该循环就会执行。</li> <code>increment</code>：改变上一次循环的状态（<code>increment</code>-增量为1）。</li> <p>一个例子是输出一系列从<code>0</code>到<code>10</code>的数字。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">for ($i = 0; $i &lt;= 10; $i++) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> echo $i . '&lt;br&gt;';</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p>或者从<code>2</code>到<code>100</code>的2的倍数。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">for ($i = 2; $i &lt;= 100; $i += 2) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> echo $i . '&lt;br&gt;';</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p>for循环对各种技巧很有用，例如<a href="https://zh.php.brj.cz/abeceda-cisla-intervals">获取字母表、数字数组和间隔</a>。</p> <div id="dang-yi-gedo-while-xun-huan" class="content-anchor"/><h2>当一个do-while循环</h2> <p>只要条件成立，<code>While</code>循环就会运行。while "和 "do-while "的区别在于何时评估条件。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">while (výraz) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> // 循环的主体</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">do {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> // 循环的主体</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">} while(výraz);</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> <code>while</code>首先评估条件，然后运行内循环。</li> <code>do-while</code>首先处理内循环，然后评估条件。</li> <p>当我们不能事先知道循环是否会运行，而想保证它总是至少运行一次时，这一点尤其有利（这时我们使用<code>do-while</code>）。</p> <p>我们有一个数字存储在变量<code>$x = 2</code>中。我们还想在变量<code>$y</code>中生成一个区间<code>&lt;0; 10&gt;</code>的数字，使其与变量<code>$x</code>中的数字不同。简单地说：在<code>0</code>和<code>10</code>之间生成一个非<code>2</code>的随机数。</p> <p>在这种情况下，使用 "do-while "循环很方便。我们事先不知道这个循环要运行多少次（我们可能不走运，一直碰到同一个数字，在这种情况下，这个循环会运行很长时间），而且我们还想保证在条件评估之前至少运行一次，因为我们必须先生成数字。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">$x = 2;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">$y = $x;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">do {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> $y = rand(0, 10);</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">} while($x == $y);</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">echo 'X :' . $x . '&lt;br&gt;';</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">echo 'Y:' . $y;</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> <p><code>foreach</code>是浏览字段和对象的完美选择。我们不仅可以得到具体的值，还可以得到键。</p> <p>如果我们只想获得特定的值。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">foreach ($data as $value) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> // 数据处理</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p>另外，我们可以拿到钥匙。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">foreach ($data as $key =&gt; $value) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> // 数据处理</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p><code>foreach</code>是浏览真实数据的完美选择--例如，来自数据库的结果，或者有键的字段。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">$countries = [</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> 'en' =&gt; '捷克共和国',</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> '见' =&gt; '斯洛伐克',</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">];</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">foreach ($countries as $shortcut =&gt; $name) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> echo $shortcut . ':' . $name . '&lt;br&gt;';</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> <p>递归发生在一个函数或方法调用自己的时候。它的作用是将一个复杂的问题分解成一组小问题。</p> <p>对于初学者来说，理解递归可能是一个挑战，因为它是基于责任传递的理念。</p> <p>该函数实际上说的是 "我不能解决这个问题，但我知道有人可以......"，所以它给他打电话，他给别人打电话，......。直到最后一个成员被调用，它就能解决这个问题。</p> <p>当然值得注意的是，任何递归算法都可以被改写成在不需要递归的地方使用循环（反之亦然）。递归是一个好的仆人，但却是一个坏的主人。它有助于简单和非常有效地解决某些类型的问题，而通过周期循环则对其他事情很有用。</p> <p>一般来说，递归是内存密集型的，因为它一直在创建新的函数和方法的实例和语境。然而，对于遍历树形结构来说，例如，这是解决问题的唯一合理方式。</p> <p>我们需要计算数字<code>5</code>的阶乘，这就是数学中的计算方法。</p> <p><code>5!= 5 * 4 * 3 * 2 * 1</code></p> <p>也就是说，从 "1 "到我们感兴趣的 "阶乘 "的一系列数字有一个连续的乘法。</p> <p>递归地，这可以非常优雅地解决。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">function factorial(int $n): int</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">{</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> if ($n === 0) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> return 1;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> }</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> return $n * factorial($n - 1);</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p>或者，使用三元运算符的更短实现。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">function factorial(int $n): int</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">{</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> return $n === 0 ? 1 : $n * factorial($n - 1);</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p>同样可以改写为使用循环。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">function factorial(int $n): int</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">{</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> $result = 1;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> for ($i = $n; $i &gt; 0; $i--) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> $result *= $i;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> }</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> return $result;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> }</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3"> </p><p>用周期写的时间稍长，但同样具有明显的内存占用率。我们只使用一个变量<code>$result</code>进行计算，在这里我们不断地改变数值，不必创建新的<code>factorial()</code>的实例。</p> <div id="fei-chang-jin-shen-de-bian-xie-wu-xian-xun-huan" class="content-anchor"/><h2>非常谨慎地编写无限循环</h2> <p>循环很容易变得无限大（我们通过永不满足终止条件来实现这一点）。你应该考虑到这一点，并可能自己用<code>break;</code>来停止循环。</p> <p>例如，掷骰子直到掷出数字<code>6</code>。该实施方案倾向于使用<code>while</code>循环。</p> <div class="MuiBox-root css-h773az"><div class="MuiBox-root css-1uwjw6e"><div class="MuiBox-root css-d24fs7"><button class="MuiButtonBase-root MuiIconButton-root MuiIconButton-sizeMedium css-1yxmbwk" tabindex="0" type="button"><svg class="MuiSvgIcon-root MuiSvgIcon-fontSizeMedium css-vubbuv" focusable="false" aria-hidden="true" viewbox="0 0 24 24" data-testid="ContentCopyIcon"><path d="M16 1H4c-1.1 0-2 .9-2 2v14h2V3h12V1zm3 4H8c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h11c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm0 16H8V7h11v14z"/></svg></button></div></div><pre class="prism-code language-php" style="background-color:#2a2734;color:#9a86fd;overflow:auto"><div class="token-line" style="color:#9a86fd"><span class="token plain">while (true) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> $value = rand(1, 6);</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain" style="display:inline-block"> </span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> if ($value === 6) {</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> echo '价值已经下降' . $value;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> break;</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain"> }</span></div><div class="token-line" style="color:#9a86fd"><span class="token plain">}</span></div></pre></div><p class="MuiTypography-root MuiTypography-body1 markdown css-9l3uo3">