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          <blockquote>
<center><font color="#1BC3FB" size="4">Python3 基础学习笔记第四章 —— 【if语句】</font></center>
</blockquote>
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<h1 id="4-1-一个简单的数列"><a href="#4-1-一个简单的数列" class="headerlink" title=" - 4.1 一个简单的数列 "></a><font color="#FF0000"> - 4.1 一个简单的数列 </font></h1><p> 给定一个汽车列表，将其中每一辆汽车的名称打印出来，要求打印 ‘bmw’ 时所有字母都要大写，其余名称只需要首字母大写：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">cars = [<span class="string">'audi'</span> , <span class="string">'bmw'</span> , <span class="string">'subaru'</span> , <span class="string">'toyota'</span>]</span><br><span class="line"><span class="keyword">for</span> car <span class="keyword">in</span> cars:</span><br><span class="line">	<span class="keyword">if</span> car == <span class="string">'bmw'</span>:</span><br><span class="line">	print(car.upper())</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line">	print(car.title())</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">Audi</span><br><span class="line">BMW</span><br><span class="line">Subaru</span><br><span class="line">Toyota</span><br></pre></td></tr></table></figure>
<h1 id="4-1-1-检查特定值是否包含在列表当中"><a href="#4-1-1-检查特定值是否包含在列表当中" class="headerlink" title=" - 4.1.1 检查特定值是否包含在列表当中 "></a><font color="#FF0000"> - 4.1.1 检查特定值是否包含在列表当中 </font></h1><blockquote>
<p> 要判断特定的值是否已包含在列表当中，可使用关键字 <font color="#FF0000">in</font></p>
</blockquote>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">user_names = [<span class="string">'andia'</span> , <span class="string">'david'</span> , <span class="string">'liwa'</span>]</span><br><span class="line">user = <span class="string">'andia'</span></span><br><span class="line"><span class="keyword">if</span> user <span class="keyword">in</span> user_names:</span><br><span class="line">	print(user.title() + <span class="string">"is in user_name."</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Andiais <span class="keyword">in</span> user_name.</span><br></pre></td></tr></table></figure>
<blockquote>
<p>要判断特定的值是否不包含在列表当中，可使用关键字 <font color="#FF0000"> not in</font></p>
</blockquote>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">user_names = [<span class="string">'andia'</span> , <span class="string">'david'</span> , <span class="string">'liwa'</span>]</span><br><span class="line">user = <span class="string">'kivle'</span></span><br><span class="line"><span class="keyword">if</span> user <span class="keyword">not</span> <span class="keyword">in</span> user_names:</span><br><span class="line">	print(user.title() + <span class="string">"is not in user_name."</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Kivleis <span class="keyword">not</span> <span class="keyword">in</span> user_name.</span><br></pre></td></tr></table></figure>
<h1 id="4-2-if-else-语句"><a href="#4-2-if-else-语句" class="headerlink" title=" - 4.2 if-else 语句 "></a><font color="#FF0000"> - 4.2 if-else 语句 </font></h1><figure class="highlight python"><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">age = input(<span class="string">"请输入你的年龄查看是否可以去网吧："</span>)</span><br><span class="line"><span class="keyword">if</span> int(age) &gt;= <span class="number">18</span>:</span><br><span class="line">    print(<span class="string">"You are old enough to go to the net bar!"</span>)</span><br><span class="line">    print(<span class="string">"You should go to net bar less,study more!"</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line">    print(<span class="string">"You are too young to go to the net bar!"</span>)</span><br><span class="line">    print(<span class="string">"Wait until you are 18 to go to the net bar!"</span>)</span><br></pre></td></tr></table></figure>
<p>分别输入19和15，输出结果如下：</p>
<figure class="highlight python"><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">请输入你的年龄查看是否可以去网吧：<span class="number">19</span></span><br><span class="line">You are old enough to go to the net bar!</span><br><span class="line">You should go to net bar less,study more!</span><br></pre></td></tr></table></figure>
<figure class="highlight python"><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">请输入你的年龄查看是否可以去网吧：<span class="number">15</span></span><br><span class="line">You are too young to go to the net bar!</span><br><span class="line">Wait until you are <span class="number">18</span> to go to the net bar!</span><br></pre></td></tr></table></figure>
<h1 id="4-3-if-elif-else-结构"><a href="#4-3-if-elif-else-结构" class="headerlink" title=" - 4.3 if-elif-else 结构 "></a><font color="#FF0000"> - 4.3 if-elif-else 结构 </font></h1><figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">age = <span class="number">12</span></span><br><span class="line"><span class="keyword">if</span> age &lt; <span class="number">4</span>:</span><br><span class="line">	price = <span class="number">0</span></span><br><span class="line"><span class="keyword">elif</span> age &lt; <span class="number">18</span>:</span><br><span class="line">	price = <span class="number">5</span></span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line">	price = <span class="number">10</span></span><br><span class="line">print(<span class="string">"Your admission cost is $"</span> + str(price) + <span class="string">"."</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Your admission cost <span class="keyword">is</span> $<span class="number">5.</span></span><br></pre></td></tr></table></figure>
<h1 id="4-3-1-使用多个-elif-代码块"><a href="#4-3-1-使用多个-elif-代码块" class="headerlink" title=" - 4.3.1 使用多个 elif 代码块 "></a><font color="#FF0000"> - 4.3.1 使用多个 elif 代码块 </font></h1> <figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">age = <span class="number">20</span></span><br><span class="line"><span class="keyword">if</span> age &lt; <span class="number">4</span>:</span><br><span class="line">	price = <span class="number">0</span></span><br><span class="line"><span class="keyword">elif</span> age &lt; <span class="number">18</span>:</span><br><span class="line">	price = <span class="number">5</span></span><br><span class="line"><span class="keyword">elif</span> age &lt; <span class="number">65</span>:</span><br><span class="line">	price = <span class="number">15</span></span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line">	price = <span class="number">10</span></span><br><span class="line">print(<span class="string">"Your admission cost is $"</span> + str(price) + <span class="string">"."</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Your admission cost <span class="keyword">is</span> $<span class="number">15.</span></span><br></pre></td></tr></table></figure>
<h1 id="4-3-2-省略-else-代码块"><a href="#4-3-2-省略-else-代码块" class="headerlink" title=" - 4.3.2 省略 else 代码块 "></a><font color="#FF0000"> - 4.3.2 省略 else 代码块 </font></h1><p> Python并不要求 if-elif 结构后面必须有 else 代码块：<br> <figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">age = <span class="number">20</span></span><br><span class="line"><span class="keyword">if</span> age &lt; <span class="number">4</span>:</span><br><span class="line">	price = <span class="number">0</span></span><br><span class="line"><span class="keyword">elif</span> age &lt; <span class="number">18</span>:</span><br><span class="line">	price = <span class="number">5</span></span><br><span class="line"><span class="keyword">elif</span> age &lt; <span class="number">65</span>:</span><br><span class="line">	price = <span class="number">15</span></span><br><span class="line"><span class="keyword">elif</span> age &gt;= <span class="number">65</span>:</span><br><span class="line">	price = <span class="number">10</span></span><br><span class="line">print(<span class="string">"Your admission cost is $"</span> + str(price) + <span class="string">"."</span>)</span><br></pre></td></tr></table></figure></p>
<p>输出结果仍与3.3.1一样</p>
<h1 id="4-4-测试多个条件"><a href="#4-4-测试多个条件" class="headerlink" title=" - 4.4 测试多个条件 "></a><font color="#FF0000"> - 4.4 测试多个条件 </font></h1><p> if-elif-else结构功能强大，但仅适用于只有一个条件满足的情况：遇到通过了的测试后，Python就会跳过余下的测试：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line"> names = [<span class="string">'Zhangshan'</span> , <span class="string">'Wanger'</span>]</span><br><span class="line"><span class="keyword">if</span> <span class="string">'Zhangshan'</span> <span class="keyword">in</span> names:</span><br><span class="line">    print(<span class="string">"Zhangshan is here!"</span>)</span><br><span class="line"><span class="keyword">if</span> <span class="string">'Wanger'</span> <span class="keyword">in</span> names:</span><br><span class="line">    print(<span class="string">"Wanger is here!"</span>)</span><br><span class="line"><span class="keyword">if</span> <span class="string">'Xiaoming'</span> <span class="keyword">in</span> names:</span><br><span class="line">    print(<span class="string">"Xiaoming is here!"</span>)</span><br><span class="line">print(<span class="string">"All the students are here!"</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><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">Zhangshan <span class="keyword">is</span> here!</span><br><span class="line">Wanger <span class="keyword">is</span> here!</span><br><span class="line">All the students are here!</span><br></pre></td></tr></table></figure>
<p>相同的程序，如果使用 if-elif-else 结构，代码将不能正确运行：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">names = [<span class="string">'Zhangshan'</span> , <span class="string">'Wanger'</span>]</span><br><span class="line"><span class="keyword">if</span> <span class="string">'Zhangshan'</span> <span class="keyword">in</span> names:</span><br><span class="line">    print(<span class="string">"Zhangshan is here!"</span>)</span><br><span class="line"><span class="keyword">elif</span> <span class="string">'Wanger'</span> <span class="keyword">in</span> names:</span><br><span class="line">    print(<span class="string">"Wanger is here!"</span>)</span><br><span class="line"><span class="keyword">elif</span> <span class="string">'Xiaoming'</span> <span class="keyword">in</span> names:</span><br><span class="line">    print(<span class="string">"Xiaoming is here!"</span>)</span><br><span class="line">print(<span class="string">"All the students are here!"</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：<br><figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Zhangshan <span class="keyword">is</span> here!</span><br><span class="line">All the students are here!</span><br></pre></td></tr></table></figure></p>
<font color="#FF000">总之，如果我们只想执行一个代码块，就使用 if-elif-else 结构；如果要运行多个代码块，就必须使用一系列独立的 if 语句！</font>

<h1 id="4-5-使用-if-语句处理列表"><a href="#4-5-使用-if-语句处理列表" class="headerlink" title=" - 4.5 使用 if 语句处理列表 "></a><font color="#FF0000"> - 4.5 使用 if 语句处理列表 </font></h1><h1 id="4-5-1-检查特殊元素"><a href="#4-5-1-检查特殊元素" class="headerlink" title=" - 4.5.1 检查特殊元素"></a><font color="#FF0000"> - 4.5.1 检查特殊元素</font></h1><p>对3.4例子改版，加入姓名 ‘Xiaoming’，当检索到Xiaoming时告诉他，他妈妈叫他回家吃饭<br><figure class="highlight python"><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">names = [<span class="string">'Zhangshan'</span> , <span class="string">'Wanger'</span> , <span class="string">'Xiaoming'</span>]</span><br><span class="line"><span class="keyword">for</span> name <span class="keyword">in</span> names:</span><br><span class="line">    <span class="keyword">if</span> name == <span class="string">'Xiaoming'</span>:</span><br><span class="line">        print(<span class="string">"Xiaoming,Your mother told you to go home for dinner!"</span>)</span><br><span class="line">    <span class="keyword">else</span>:</span><br><span class="line">        print(name +<span class="string">"is here!"</span>)</span><br><span class="line">print(<span class="string">"All the students are here!"</span>)</span><br></pre></td></tr></table></figure></p>
<p>输出结果如下：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">Zhangshanis here!</span><br><span class="line">Wangeris here!</span><br><span class="line">Xiaoming,Your mother told you to go home <span class="keyword">for</span> dinner!</span><br><span class="line">All the students are here!</span><br></pre></td></tr></table></figure>
<h1 id="4-5-2-确定列表不是空的"><a href="#4-5-2-确定列表不是空的" class="headerlink" title=" - 4.5.2 确定列表不是空的"></a><font color="#FF0000"> - 4.5.2 确定列表不是空的</font></h1><p>  在检索姓名前检查姓名是否为空，不为空则打印出所有姓名，为空则提示没有姓名：</p>
<figure class="highlight python"><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">names = []</span><br><span class="line"><span class="keyword">if</span> names:</span><br><span class="line">    <span class="keyword">for</span> name <span class="keyword">in</span> names:</span><br><span class="line">        print(name +<span class="string">" is here!"</span>)</span><br><span class="line">    print(<span class="string">"All the students are here!"</span>)</span><br><span class="line"><span class="keyword">else</span>:</span><br><span class="line">    print(<span class="string">"There is no students!"</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">There <span class="keyword">is</span> no students!</span><br></pre></td></tr></table></figure>
<font color="#FF000">在if语句中将列表名用在条件表达式中时，Python将在列表至少包含一个元素时返回Ture，并在列表为空时返回False</font>

<h1 id="4-5-3-使用多个列表"><a href="#4-5-3-使用多个列表" class="headerlink" title=" - 4.5.3 使用多个列表"></a><font color="#FF0000"> - 4.5.3 使用多个列表</font></h1><p>  两个列表names_1和names_2，要求输出既在names_2中又在names_1中的元素：</p>
<figure class="highlight python"><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></pre></td><td class="code"><pre><span class="line">names_1 = [<span class="string">'Zhangshan'</span> , <span class="string">'Liyang'</span>  , <span class="string">'Wanger'</span> , <span class="string">'Tangyang'</span> , <span class="string">'Xiaoming'</span>]</span><br><span class="line">names_2 = [<span class="string">'Liyang'</span> , <span class="string">'Zhangwei'</span> , <span class="string">'Tangyang'</span>]</span><br><span class="line"><span class="keyword">for</span> names <span class="keyword">in</span> names_2:</span><br><span class="line">    <span class="keyword">if</span> names <span class="keyword">in</span> names_1:</span><br><span class="line">        print(names +<span class="string">" is here!"</span>)</span><br><span class="line">print(<span class="string">"All the students are here!"</span>)</span><br></pre></td></tr></table></figure>
<p>输出结果如下：</p>
<figure class="highlight python"><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">Liyang <span class="keyword">is</span> here!</span><br><span class="line">Tangyang <span class="keyword">is</span> here!</span><br><span class="line">All the students are here!</span><br></pre></td></tr></table></figure>
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