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<li class="toctree-l3"><a class="reference internal" href="#id1">注意LTRIM命令和编程语言区间函数的区别</a></li>
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<div class="section" id="ltrim-key-start-stop">
<span id="ltrim"></span><h1>LTRIM key start stop<a class="headerlink" href="#ltrim-key-start-stop" title="Permalink to this headline">¶</a></h1>
<blockquote>
<div><div class="line-block">
<div class="line">可用版本： >= 1.0.0</div>
<div class="line">时间复杂度: O(N)， <code class="docutils literal notranslate"><span class="pre">N</span></code> 为被移除的元素的数量。</div>
</div>
</div></blockquote>
<p>对一个列表进行修剪(trim)，就是说，让列表只保留指定区间内的元素，不在指定区间之内的元素都将被删除。</p>
<p>举个例子，执行命令 <code class="docutils literal notranslate"><span class="pre">LTRIM</span> <span class="pre">list</span> <span class="pre">0</span> <span class="pre">2</span></code> ，表示只保留列表 <code class="docutils literal notranslate"><span class="pre">list</span></code> 的前三个元素，其余元素全部删除。</p>
<p>下标(index)参数 <code class="docutils literal notranslate"><span class="pre">start</span></code> 和 <code class="docutils literal notranslate"><span class="pre">stop</span></code> 都以 <code class="docutils literal notranslate"><span class="pre">0</span></code> 为底，也就是说，以 <code class="docutils literal notranslate"><span class="pre">0</span></code> 表示列表的第一个元素，以 <code class="docutils literal notranslate"><span class="pre">1</span></code> 表示列表的第二个元素，以此类推。</p>
<p>你也可以使用负数下标，以 <code class="docutils literal notranslate"><span class="pre">-1</span></code> 表示列表的最后一个元素， <code class="docutils literal notranslate"><span class="pre">-2</span></code> 表示列表的倒数第二个元素，以此类推。</p>
<p>当 <code class="docutils literal notranslate"><span class="pre">key</span></code> 不是列表类型时，返回一个错误。</p>
<p><a class="reference internal" href="#ltrim">LTRIM</a> 命令通常和 <a class="reference internal" href="lpush.html#lpush"><span class="std std-ref">LPUSH key value [value …]</span></a> 命令或 <a class="reference internal" href="rpush.html#rpush"><span class="std std-ref">RPUSH key value [value …]</span></a> 命令配合使用，举个例子：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">LPUSH</span> <span class="n">log</span> <span class="n">newest_log</span>
<span class="n">LTRIM</span> <span class="n">log</span> <span class="mi">0</span> <span class="mi">99</span>
</pre></div>
</div>
<p>这个例子模拟了一个日志程序，每次将最新日志 <code class="docutils literal notranslate"><span class="pre">newest_log</span></code> 放到 <code class="docutils literal notranslate"><span class="pre">log</span></code> 列表中，并且只保留最新的 <code class="docutils literal notranslate"><span class="pre">100</span></code> 项。注意当这样使用 <code class="docutils literal notranslate"><span class="pre">LTRIM</span></code> 命令时，时间复杂度是O(1)，因为平均情况下，每次只有一个元素被移除。</p>
<div class="section" id="id1">
<h2>注意LTRIM命令和编程语言区间函数的区别<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<p>假如你有一个包含一百个元素的列表 <code class="docutils literal notranslate"><span class="pre">list</span></code> ，对该列表执行 <code class="docutils literal notranslate"><span class="pre">LTRIM</span> <span class="pre">list</span> <span class="pre">0</span> <span class="pre">10</span></code> ，结果是一个包含11个元素的列表，这表明 <code class="docutils literal notranslate"><span class="pre">stop</span></code> 下标也在 <a class="reference internal" href="#ltrim">LTRIM</a> 命令的取值范围之内(闭区间)，这和某些语言的区间函数可能不一致，比如Ruby的 <code class="docutils literal notranslate"><span class="pre">Range.new</span></code> 、 <code class="docutils literal notranslate"><span class="pre">Array#slice</span></code> 和Python的 <code class="docutils literal notranslate"><span class="pre">range()</span></code> 函数。</p>
</div>
<div class="section" id="id2">
<h2>超出范围的下标<a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h2>
<p>超出范围的下标值不会引起错误。</p>
<p>如果 <code class="docutils literal notranslate"><span class="pre">start</span></code> 下标比列表的最大下标 <code class="docutils literal notranslate"><span class="pre">end</span></code> ( <code class="docutils literal notranslate"><span class="pre">LLEN</span> <span class="pre">list</span></code> 减去 <code class="docutils literal notranslate"><span class="pre">1</span></code> )还要大，或者 <code class="docutils literal notranslate"><span class="pre">start</span> <span class="pre">></span> <span class="pre">stop</span></code> ， <a class="reference internal" href="#ltrim">LTRIM</a> 返回一个空列表(因为 <a class="reference internal" href="#ltrim">LTRIM</a> 已经将整个列表清空)。</p>
<p>如果 <code class="docutils literal notranslate"><span class="pre">stop</span></code> 下标比 <code class="docutils literal notranslate"><span class="pre">end</span></code> 下标还要大，Redis将 <code class="docutils literal notranslate"><span class="pre">stop</span></code> 的值设置为 <code class="docutils literal notranslate"><span class="pre">end</span></code> 。</p>
</div>
<div class="section" id="id3">
<h2>返回值<a class="headerlink" href="#id3" title="Permalink to this headline">¶</a></h2>
<p>命令执行成功时，返回 <code class="docutils literal notranslate"><span class="pre">ok</span></code> 。</p>
</div>
<div class="section" id="id4">
<h2>代码示例<a class="headerlink" href="#id4" title="Permalink to this headline">¶</a></h2>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># 情况 1： 常见情况， start 和 stop 都在列表的索引范围之内</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LRANGE</span> <span class="n">alpha</span> <span class="mi">0</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># alpha 是一个包含 5 个字符串的列表</span>
<span class="mi">1</span><span class="p">)</span> <span class="s2">"h"</span>
<span class="mi">2</span><span class="p">)</span> <span class="s2">"e"</span>
<span class="mi">3</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">4</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">5</span><span class="p">)</span> <span class="s2">"o"</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LTRIM</span> <span class="n">alpha</span> <span class="mi">1</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># 删除 alpha 列表索引为 0 的元素</span>
<span class="n">OK</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LRANGE</span> <span class="n">alpha</span> <span class="mi">0</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># "h" 被删除了</span>
<span class="mi">1</span><span class="p">)</span> <span class="s2">"e"</span>
<span class="mi">2</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">3</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">4</span><span class="p">)</span> <span class="s2">"o"</span>
<span class="c1"># 情况 2： stop 比列表的最大下标还要大</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LTRIM</span> <span class="n">alpha</span> <span class="mi">1</span> <span class="mi">10086</span> <span class="c1"># 保留 alpha 列表索引 1 至索引 10086 上的元素</span>
<span class="n">OK</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LRANGE</span> <span class="n">alpha</span> <span class="mi">0</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># 只有索引 0 上的元素 "e" 被删除了，其他元素还在</span>
<span class="mi">1</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">2</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">3</span><span class="p">)</span> <span class="s2">"o"</span>
<span class="c1"># 情况 3： start 和 stop 都比列表的最大下标要大，并且 start < stop</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LTRIM</span> <span class="n">alpha</span> <span class="mi">10086</span> <span class="mi">123321</span>
<span class="n">OK</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LRANGE</span> <span class="n">alpha</span> <span class="mi">0</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># 列表被清空</span>
<span class="p">(</span><span class="n">empty</span> <span class="nb">list</span> <span class="ow">or</span> <span class="nb">set</span><span class="p">)</span>
<span class="c1"># 情况 4： start 和 stop 都比列表的最大下标要大，并且 start > stop</span>
<span class="n">redis</span><span class="o">></span> <span class="n">RPUSH</span> <span class="n">new</span><span class="o">-</span><span class="n">alpha</span> <span class="s2">"h"</span> <span class="s2">"e"</span> <span class="s2">"l"</span> <span class="s2">"l"</span> <span class="s2">"o"</span> <span class="c1"># 重新建立一个新列表</span>
<span class="p">(</span><span class="n">integer</span><span class="p">)</span> <span class="mi">5</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LRANGE</span> <span class="n">new</span><span class="o">-</span><span class="n">alpha</span> <span class="mi">0</span> <span class="o">-</span><span class="mi">1</span>
<span class="mi">1</span><span class="p">)</span> <span class="s2">"h"</span>
<span class="mi">2</span><span class="p">)</span> <span class="s2">"e"</span>
<span class="mi">3</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">4</span><span class="p">)</span> <span class="s2">"l"</span>
<span class="mi">5</span><span class="p">)</span> <span class="s2">"o"</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LTRIM</span> <span class="n">new</span><span class="o">-</span><span class="n">alpha</span> <span class="mi">123321</span> <span class="mi">10086</span> <span class="c1"># 执行 LTRIM</span>
<span class="n">OK</span>
<span class="n">redis</span><span class="o">></span> <span class="n">LRANGE</span> <span class="n">new</span><span class="o">-</span><span class="n">alpha</span> <span class="mi">0</span> <span class="o">-</span><span class="mi">1</span> <span class="c1"># 同样被清空</span>
<span class="p">(</span><span class="n">empty</span> <span class="nb">list</span> <span class="ow">or</span> <span class="nb">set</span><span class="p">)</span>
</pre></div>
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