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2018-06-08
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<p>今天要说的是最简单的 for 循环，一个简单的 for 循环看似没有任何优化的意义，但实质上优化前后差距挺大的，那么该如何优化呢？</p>
<hr>
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<p>从最简单的遍历数组说起。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 定义一个数组arr（假设是从后台返回的数据）</span></span><br><span class="line"><span class="keyword">let</span> i = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">let</span> arr = [];</span><br><span class="line"><span class="keyword">while</span> (i < <span class="number">50</span>) {</span><br><span class="line"> arr.push(i);</span><br><span class="line"> i++;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>如果我们想从数组 arr 中取出数据，就必须要进行遍历，普遍的做法是：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i < arr.length; i++) {</span><br><span class="line"> <span class="comment">// arr[i]</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>但其实这样的写法遍历是最慢的，他要经过两次迭代，第一次是 i 的迭代，每次都要判断 i 是否小于 arr.length，第二次是 arr 的迭代，每次循环 arr 都会调用底层的迭代器，对长度进行计算，这样循环的效率非常低，时间空间复杂度为 O[n^2]。</p>
<p>下面进行优化，看看两者到底有什么区别：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>, len = arr.length; i < len; i++) {</span><br><span class="line"> <span class="comment">// arr[i]</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>区别就是，整个循环当中，我们预存了 len 来保存数组的长度，这样不需要每次循环都调用底层迭代器，调用一次即可，这样的时间空间复杂度为 O[n+1]。</p>
<p>但是这并不是最完美的，因为会多了一次迭代操作，那么该如何进行优化呢？</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>, item; item = arr[i++];) {</span><br><span class="line"> <span class="comment">// item</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这次迭代的时间空间复杂度为 O[n] ，完美做到了每次一迭代没有通过长度进行判断，而是直接通过下标进行取值的方式映射到了循环体内部。</p>
<p>最后用5万条数据进行测试各种方式的循环时间：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 定义一个数组arr（假设是从后台返回的数据）</span></span><br><span class="line"><span class="keyword">let</span> index = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">let</span> arr = [];</span><br><span class="line"><span class="keyword">while</span> (index < <span class="number">50000</span>) {</span><br><span class="line"> arr.push(index);</span><br><span class="line"> index++;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.time(<span class="string">'one'</span>);</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i < arr.length; i++) {</span><br><span class="line"></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.timeEnd(<span class="string">'one'</span>);</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.time(<span class="string">'two'</span>);</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>, len = arr.length; i < len; i++) {</span><br><span class="line"></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.timeEnd(<span class="string">'two'</span>);</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.time(<span class="string">'three'</span>);</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>, item; item = arr[i++];) {</span><br><span class="line"></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.timeEnd(<span class="string">'three'</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// es6的数组遍历</span></span><br><span class="line"><span class="built_in">console</span>.time(<span class="string">'four'</span>);</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i <span class="keyword">of</span> arr) {</span><br><span class="line"></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.timeEnd(<span class="string">'four'</span>);</span><br><span class="line"><span class="comment">// 会访问可枚举属性和原型的遍历，数组不推荐使用</span></span><br><span class="line"><span class="built_in">console</span>.time(<span class="string">'five'</span>);</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i <span class="keyword">in</span> arr) {</span><br><span class="line"></span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.timeEnd(<span class="string">'five'</span>);</span><br><span class="line"><span class="comment">// one: 0.711ms</span></span><br><span class="line"><span class="comment">// two: 4.508ms</span></span><br><span class="line"><span class="comment">// three: 0.006ms</span></span><br><span class="line"><span class="comment">// four: 3.255ms</span></span><br><span class="line"><span class="comment">// five: 11.144ms</span></span><br></pre></td></tr></table></figure>
<p>在数据量大的情况下，第三种循环方式效果显而易见。</p>
<h2 id="为什么普通-for-循环的性能远远高于-forEach-的性能"><a href="#为什么普通-for-循环的性能远远高于-forEach-的性能" class="headerlink" title="为什么普通 for 循环的性能远远高于 forEach 的性能"></a>为什么普通 for 循环的性能远远高于 forEach 的性能</h2><ul>
<li>区别：</li>
</ul>
<p>一个按顺序遍历，一个使用iterator迭代器遍历；</p>
<ul>
<li>从数据结构来说：</li>
</ul>
<p>for循环是随机访问元素，foreach是顺序链表访问元素；</p>
<ul>
<li>性能上：</li>
</ul>
<p>对于arraylist，是顺序表，使用for循环可以顺序访问，速度较快；使用foreach会比for循环稍慢一些。
对于linkedlist，是单链表，使用for循环每次都要从第一个元素读取next域来读取，速度非常慢；使用foreach可以直接读取当前结点，数据较快；</p>
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