Repository navigation
Expand file tree
/
Copy path11.html
More file actions
223 lines (211 loc) · 19.7 KB
/
Copy path11.html
File metadata and controls
223 lines (211 loc) · 19.7 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="description" content="CodeClub - 5.4. 条件判断">
<meta name="author" content="Jackson">
<title>5.4. 条件判断 - CodeClub</title>
<link rel="icon" type="image/x-icon" href="../../img/logo.png">
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&display=swap" rel="stylesheet">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css" crossorigin="anonymous" referrerpolicy="no-referrer" />
<link rel="stylesheet" href="../../css/main.css">
<link rel="stylesheet" href="../../css/dark-mode.css">
<link rel="stylesheet" href="../../css/mobile-optimizations.css">
<link rel="stylesheet" href="../../css/tutorial.css">
<style>
.tutorial-container { max-width: 960px; margin: 70px auto 40px; padding: 0 20px; }
.tutorial-sidebar { position: fixed; top: 70px; left: 20px; width: 260px; max-height: calc(100vh - 90px); overflow-y: auto; background: var(--bg-secondary, #fff); border-radius: 8px; padding: 16px; box-shadow: 0 2px 8px rgba(0,0,0,0.08); }
.tutorial-sidebar h3 { font-size: 16px; margin-bottom: 12px; color: var(--text-primary, #1a1a2e); }
.tutorial-sidebar a { display: block; padding: 6px 10px; color: var(--text-secondary, #666); text-decoration: none; font-size: 14px; border-radius: 4px; }
.tutorial-sidebar a:hover, .tutorial-sidebar a.active { background: var(--primary, #3b82f6); color: #fff; }
.tutorial-sidebar .sub-link { padding-left: 24px; font-size: 13px; }
.tutorial-content { margin-left: 300px; background: var(--bg-secondary, #fff); border-radius: 8px; padding: 32px; box-shadow: 0 2px 8px rgba(0,0,0,0.08); }
.tutorial-content h1 { font-size: 28px; margin-bottom: 20px; color: var(--text-primary, #1a1a2e); border-bottom: 2px solid var(--primary, #3b82f6); padding-bottom: 12px; }
.tutorial-content h2 { font-size: 22px; margin: 24px 0 16px; color: var(--text-primary, #1a1a2e); }
.tutorial-content h3 { font-size: 18px; margin: 20px 0 12px; }
.tutorial-content p { line-height: 1.8; margin: 12px 0; color: var(--text-secondary, #333); }
.tutorial-content pre { background: #1e1e2e; color: #cdd6f4; padding: 16px; border-radius: 8px; overflow-x: auto; margin: 16px 0; }
.tutorial-content code { font-family: 'Fira Code', 'Cascadia Code', monospace; font-size: 14px; }
.tutorial-content p code { background: #f1f5f9; padding: 2px 6px; border-radius: 4px; color: #e83e8c; }
.tutorial-content blockquote { border-left: 4px solid var(--primary, #3b82f6); padding: 12px 20px; margin: 16px 0; background: #f8fafc; border-radius: 0 8px 8px 0; }
.tutorial-content ul, .tutorial-content ol { padding-left: 24px; margin: 12px 0; }
.tutorial-content li { margin: 6px 0; line-height: 1.7; }
.tutorial-content img { max-width: 100%; border-radius: 8px; margin: 16px 0; }
.tutorial-nav { display: flex; justify-content: space-between; margin-top: 32px; padding-top: 20px; border-top: 1px solid var(--border, #e5e7eb); }
.tutorial-nav a { color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500; }
.tutorial-nav a:hover { text-decoration: underline; }
.tutorial-index { max-width: 960px; margin: 70px auto 40px; padding: 0 20px; }
.tutorial-card { background: var(--bg-secondary, #fff); border-radius: 12px; padding: 24px; margin: 16px 0; box-shadow: 0 2px 8px rgba(0,0,0,0.08); transition: transform 0.2s; }
.tutorial-card:hover { transform: translateY(-2px); }
.tutorial-card h3 { margin: 0 0 8px; color: var(--text-primary, #1a1a2e); }
.tutorial-card p { color: var(--text-secondary, #666); margin: 0 0 12px; }
.tutorial-card .badge { display: inline-block; padding: 4px 10px; border-radius: 20px; font-size: 12px; font-weight: 600; margin-right: 8px; }
.badge-python { background: #3b82f6; color: #fff; }
.badge-git { background: #f97316; color: #fff; }
.badge-ai { background: #8b5cf6; color: #fff; }
@media (max-width: 768px) {
.tutorial-sidebar { display: none; }
.tutorial-content { margin-left: 0; }
}
</style>
</head>
<body>
<nav class="navbar">
<div class="navbar-container">
<a class="navbar-brand" href="../../index.html">
<img src="../../img/logo.png" alt="CodeClub" width="32" height="32">
<span>CodeClub</span>
</a>
<button class="mobile-menu-toggle" aria-label="切换导航菜单">
<span></span><span></span><span></span>
</button>
<ul class="navbar-nav">
<li><a class="nav-link" href="../../index.html">首页</a></li>
<li><a class="nav-link" href="../../nav.html">网址导航</a></li>
<li><a class="nav-link" href="../../blog/">博客</a></li>
<li><a class="nav-link" href="../../wiki/index.html" target="_blank">WiKi</a></li>
<li><a class="nav-link active" href="../../tutorials/index.html">教程</a></li>
<li><a class="nav-link" href="../../book.html">图书</a></li>
<li><a class="nav-link" href="../../contact.html">联系我</a></li>
<li><a class="nav-link" href="../../about.html">关于我</a></li>
</ul>
</div>
</nav>
<div class="tutorial-container" style="max-width: 960px; margin: 80px auto 40px; padding: 0 20px;"><div class="tutorial-sidebar"><h3>目录</h3><a href="0.html">1. 简介</a><a href="1.html">2. Python历史</a><a href="2.html">3. 安装Python</a><a href="3.html">3.1. Python解释器</a><a href="4.html">4. 第一个Python程序</a><a href="5.html">4.1. 使用文本编辑器</a><a href="6.html">4.2. 输入和输出</a><a href="7.html">5. Python基础</a><a href="8.html">5.1. 数据类型和变量</a><a href="9.html">5.2. 字符串和编码</a><a href="10.html">5.3. 使用list和tuple</a><a href="11.html">5.4. 条件判断</a><a href="12.html">5.5. 模式匹配</a><a href="13.html">5.6. 循环</a><a href="14.html">5.7. 使用dict和set</a><a href="15.html">6. 函数</a><a href="16.html">6.1. 调用函数</a><a href="17.html">6.2. 定义函数</a><a href="18.html">6.3. 函数的参数</a><a href="19.html">6.4. 递归函数</a><a href="20.html">7. 高级特性</a><a href="21.html">7.1. 切片</a><a href="22.html">7.2. 迭代</a><a href="23.html">7.3. 列表生成式</a><a href="24.html">7.4. 生成器</a><a href="25.html">7.5. 迭代器</a><a href="26.html">8. 函数式编程</a><a href="27.html">8.1. 高阶函数</a><a href="28.html">8.1.1. map/reduce</a><a href="29.html">8.1.2. filter</a><a href="30.html">8.1.3. sorted</a><a href="31.html">8.2. 返回函数</a><a href="32.html">8.3. 匿名函数</a><a href="33.html">8.4. 装饰器</a><a href="34.html">8.5. 偏函数</a><a href="35.html">9. 模块</a><a href="36.html">9.1. 使用模块</a><a href="37.html">9.2. 安装第三方模块</a><a href="38.html">10. 面向对象编程</a><a href="39.html">10.1. 类和实例</a><a href="40.html">10.2. 访问限制</a><a href="41.html">10.3. 继承和多态</a><a href="42.html">10.4. 获取对象信息</a><a href="43.html">10.5. 实例属性和类属性</a><a href="44.html">11. 面向对象高级编程</a><a href="45.html">11.1. 使用__slots__</a><a href="46.html">11.2. 使用@property</a><a href="47.html">11.3. 多重继承</a><a href="48.html">11.4. 定制类</a><a href="49.html">11.5. 使用枚举类</a><a href="50.html">11.6. 使用元类</a><a href="51.html">12. 错误、调试和测试</a><a href="52.html">12.1. 错误处理</a><a href="53.html">12.2. 调试</a><a href="54.html">12.3. 单元测试</a><a href="55.html">12.4. 文档测试</a><a href="56.html">13. IO编程</a><a href="57.html">13.1. 文件读写</a><a href="58.html">13.2. StringIO和BytesIO</a><a href="59.html">13.3. 操作文件和目录</a><a href="60.html">13.4. 序列化</a><a href="61.html">14. 进程和线程</a><a href="62.html">14.1. 多进程</a><a href="63.html">14.2. 多线程</a><a href="64.html">14.3. ThreadLocal</a><a href="65.html">14.4. 进程 vs. 线程</a><a href="66.html">14.5. 分布式进程</a><a href="67.html">15. 正则表达式</a><a href="68.html">16. 常用内建模块</a><a href="69.html">16.1. datetime</a><a href="70.html">16.2. collections</a><a href="71.html">16.3. argparse</a><a href="72.html">16.4. base64</a><a href="73.html">16.5. struct</a><a href="74.html">16.6. hashlib</a><a href="75.html">16.7. hmac</a><a href="76.html">16.8. itertools</a><a href="77.html">16.9. contextlib</a><a href="78.html">16.10. urllib</a><a href="79.html">16.11. XML</a><a href="80.html">16.12. HTMLParser</a><a href="81.html">16.13. venv</a><a href="82.html">17. 常用第三方模块</a><a href="83.html">17.1. Pillow</a><a href="84.html">17.2. requests</a><a href="85.html">17.3. chardet</a><a href="86.html">17.4. psutil</a><a href="87.html">18. 图形界面</a><a href="88.html">18.1. 海龟绘图</a><a href="89.html">19. 网络编程</a><a href="90.html">19.1. TCP/IP简介</a><a href="91.html">19.2. TCP编程</a><a href="92.html">19.3. UDP编程</a><a href="93.html">20. 电子邮件</a><a href="94.html">20.1. SMTP发送邮件</a><a href="95.html">20.2. POP3收取邮件</a><a href="96.html">21. 访问数据库</a><a href="97.html">21.1. 使用SQLite</a><a href="98.html">21.2. 使用MySQL</a><a href="99.html">21.3. 使用SQLAlchemy</a><a href="100.html">22. Web开发</a><a href="101.html">22.1. HTTP协议简介</a><a href="102.html">22.2. HTML简介</a><a href="103.html">22.3. WSGI接口</a><a href="104.html">22.4. 使用Web框架</a><a href="105.html">22.5. 使用模板</a><a href="106.html">23. 异步IO</a><a href="107.html">23.1. 协程</a><a href="108.html">23.2. 使用asyncio</a><a href="109.html">23.3. 使用aiohttp</a><a href="110.html">24. FAQ</a><a href="111.html">25. 期末总结</a></div><div class="tutorial-content"><h1>5.4. 条件判断</h1><div class="tutorial-body"><p>比如，输入用户年龄，根据年龄打印不同的内容，在Python程序中，用<code>if</code>语句实现：</p>
<pre class="hljs"><code class="language-python">age = <span class="hljs-number">20</span>
<span class="hljs-keyword">if</span> age >= <span class="hljs-number">18</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'your age is'</span>, age)
<span class="hljs-built_in">print</span>(<span class="hljs-string">'adult'</span>)
</code></pre>
<p>根据Python的缩进规则，如果<code>if</code>语句判断是<code>True</code>，就把缩进的两行print语句执行了，否则，什么也不做。</p>
<p>也可以给<code>if</code>添加一个<code>else</code>语句，意思是，如果<code>if</code>判断是<code>False</code>，不要执行<code>if</code>的内容，去把<code>else</code>执行了：</p>
<pre class="hljs"><code class="language-python">age = <span class="hljs-number">3</span>
<span class="hljs-keyword">if</span> age >= <span class="hljs-number">18</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'your age is'</span>, age)
<span class="hljs-built_in">print</span>(<span class="hljs-string">'adult'</span>)
<span class="hljs-keyword">else</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'your age is'</span>, age)
<span class="hljs-built_in">print</span>(<span class="hljs-string">'teenager'</span>)
</code></pre>
<p>注意不要少写了冒号<code>:</code>。</p>
<p>当然上面的判断是很粗略的，完全可以用<code>elif</code>做更细致的判断：</p>
<pre class="hljs"><code class="language-python">age = <span class="hljs-number">3</span>
<span class="hljs-keyword">if</span> age >= <span class="hljs-number">18</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'adult'</span>)
<span class="hljs-keyword">elif</span> age >= <span class="hljs-number">6</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'teenager'</span>)
<span class="hljs-keyword">else</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'kid'</span>)
</code></pre>
<p><code>elif</code>是<code>else if</code>的缩写，完全可以有多个<code>elif</code>，所以<code>if</code>语句的完整形式就是：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">if</span> <条件判断<span class="hljs-number">1</span>>:
<执行<span class="hljs-number">1</span>>
<span class="hljs-keyword">elif</span> <条件判断<span class="hljs-number">2</span>>:
<执行<span class="hljs-number">2</span>>
<span class="hljs-keyword">elif</span> <条件判断<span class="hljs-number">3</span>>:
<执行<span class="hljs-number">3</span>>
<span class="hljs-keyword">else</span>:
<执行<span class="hljs-number">4</span>>
</code></pre>
<p><code>if</code>语句执行有个特点，它是从上往下判断，如果在某个判断上是<code>True</code>，把该判断对应的语句执行后，就忽略掉剩下的<code>elif</code>和<code>else</code>，所以，请测试并解释为什么下面的程序打印的是<code>teenager</code>：</p>
<pre class="hljs"><code class="language-python">age = <span class="hljs-number">20</span>
<span class="hljs-keyword">if</span> age >= <span class="hljs-number">6</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'teenager'</span>)
<span class="hljs-keyword">elif</span> age >= <span class="hljs-number">18</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'adult'</span>)
<span class="hljs-keyword">else</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'kid'</span>)
</code></pre>
<p><code>if</code>判断条件还可以简写，比如写：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">if</span> x:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'True'</span>)
</code></pre>
<p>只要<code>x</code>是非零数值、非空字符串、非空list等，就判断为<code>True</code>，否则为<code>False</code>。</p>
<h3>再议input</h3>
<p>最后看一个有问题的条件判断。很多同学会用<code>input()</code>读取用户的输入，这样可以自己输入，程序运行得更有意思：</p>
<pre class="hljs"><code class="language-python">birth = <span class="hljs-built_in">input</span>(<span class="hljs-string">'birth: '</span>)
<span class="hljs-keyword">if</span> birth < <span class="hljs-number">2000</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'00前'</span>)
<span class="hljs-keyword">else</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'00后'</span>)
</code></pre>
<p>输入<code>1982</code>，结果报错：</p>
<pre class="hljs"><code class="language-plain">Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unorderable types: str() > int()
</code></pre>
<p>这是因为<code>input()</code>返回的数据类型是<code>str</code>，<code>str</code>不能直接和整数比较，必须先把<code>str</code>转换成整数。Python提供了<code>int()</code>函数来完成这件事情：</p>
<pre class="hljs"><code class="language-python">s = <span class="hljs-built_in">input</span>(<span class="hljs-string">'birth: '</span>)
birth = <span class="hljs-built_in">int</span>(s)
<span class="hljs-keyword">if</span> birth < <span class="hljs-number">2000</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'00前'</span>)
<span class="hljs-keyword">else</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'00后'</span>)
</code></pre>
<p>再次运行，就可以得到正确地结果。但是，如果输入<code>abc</code>呢？又会得到一个错误信息：</p>
<pre class="hljs"><code class="language-plain">Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'abc'
</code></pre>
<p>原来<code>int()</code>函数发现一个字符串并不是合法的数字时就会报错，程序就退出了。</p>
<p>如何检查并捕获程序运行期的错误呢？后面的错误和调试会讲到。</p>
<h3>练习</h3>
<p>小明身高1.75，体重80.5kg。请根据BMI公式（体重除以身高的平方）帮小明计算他的BMI指数，并根据BMI指数：</p>
<ul>
<li>低于18.5：过轻</li>
<li>18.5-25：正常</li>
<li>25-28：过重</li>
<li>28-32：肥胖</li>
<li>高于32：严重肥胖</li>
</ul>
<p>用<code>if-elif</code>判断并打印结果：</p>
<pre class="hljs"><code class="language-python">height = <span class="hljs-number">1.75</span>
weight = <span class="hljs-number">80.5</span>
bmi = ???
<span class="hljs-keyword">if</span> ???:
<span class="hljs-keyword">pass</span>
</code></pre>
<h3>参考源码</h3>
<p><a href="do_if.py">do_if.py</a></p>
<h3>小结</h3>
<p>条件判断可以让计算机自己做选择，Python的if...elif...else很灵活。</p>
<p>条件判断从上向下匹配，当满足条件时执行对应的块内语句，后续的elif和else都不再执行。</p>
<p><img src="if-elif-else.png" alt="python-if" loading="lazy"></p>
</div>
<hr class="gsc-border">
<div id="gsi-chapter-prev-next" class="flex flex-row mt-4">
<div id="gsi-chapter-prev" class="basis-1/2 mr-2 text-left">
<a href="/books/python/basic/list-tuple/index.html"
onclick="gs_load_chapter('/books/python/basic/list-tuple/index.html',true);return false;"
class="inline-block my-2">
<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5"
stroke="currentColor">
<path stroke-linecap="round" stroke-linejoin="round"
d="M18.75 19.5l-7.5-7.5 7.5-7.5m-6 15L5.25 12l7.5-7.5" />
</svg>
使用list和tuple
</a>
</div>
<div id="gsi-chapter-next" class="basis-1/2 ml-2 text-right">
<a href="/books/python/basic/match/index.html"
onclick="gs_load_chapter('/books/python/basic/match/index.html',true);return false;"
class="inline-block my-2">
模式匹配
<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5"
stroke="currentColor">
<path stroke-linecap="round" stroke-linejoin="round"
d="M11.25 4.5l7.5 7.5-7.5 7.5m-6-15l7.5 7.5-7.5 7.5" />
</svg>
</a></div><div style="display: flex; justify-content: space-between; margin-top: 32px; padding-top: 20px; border-top: 1px solid var(--border, #e5e7eb);"><a href="10.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">← 5.3. 使用list和tuple</a><a href="12.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">5.5. 模式匹配 →</a></div><p style="margin-top: 16px; font-size: 13px; color: var(--text-secondary, #999);">原文：<a href="https://liaoxuefeng.com/books/python/basic/if/index.html" style="color: var(--primary, #3b82f6);">廖雪峰 - 5.4. 条件判断</a></p></div></div>
<footer class="footer">
<div class="footer-content">
<p>© 2024 CodeClub. All rights reserved.</p>
</div>
</footer>
<script src="../../js/dark-mode.js"></script>
<script src="../../js/mobile-menu.js"></script>
</body>
</html>