深度学习与计算机视觉系列(1)_基础介绍
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转载自：
深度学习与计算机视觉系列(1)_基础介绍 - 龙心尘 - 博客频道 - CSDN.NET
http://blog.csdn.net/longxinchen_ml/article/details/49963327
作者： 寒小阳 &&龙心尘
时间：2015年11月。
出处：
http://blog.csdn.net/han_xiaoyang/article/details/49876119
http://blog.csdn.net/longxinchen_ml/article/details/49963327
声明：版权所有，转载请注明出处，谢谢。
1.背景
计算机视觉/computer vision是一个火了N年的topic。持续化升温的原因也非常简单：在搜索/影像内容理解/医学应用/地图识别等等领域应用太多，大家都有一个愿景『让计算机能够像人一样去”看”一张图片，甚至”读懂”一张图片』。
有几个比较重要的计算机视觉任务，比如图片的分类,物体识别，物体定位于检测等等。而近年来的神经网络/深度学习使得上述任务的准确度有了非常大的提升。加之最近做了几个不大不小的计算机视觉上的项目，爱凑热闹的博主自然不打算放过此领域，也稍微写点东西介绍一下，写的不正确的地方，欢迎大家提出和指正。
2.基础知识
为了简单易读易懂，这个系列中绝大多数的代码都使用python完成。这里稍微介绍一下python和Numpy/Scipy(python中的科学计算包)的一些基础。
2.1 python基础
python是一种长得像伪代码，具备高可读性的编程语言。
优点挺多：可读性相当好，写起来也简单，所想立马可以转为实现代码，且社区即为活跃，可用的package相当多；缺点：效率一般。
2.1.1 基本数据类型
最常用的有数值型(Numbers),布尔型(Booleans)和字符串(String)三种。
- 数值型(Numbers)
可进行简单的运算，如下：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">x = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> type(x) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "<type 'int'>"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "5"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 加; prints "6"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x - <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 减; prints "4"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 乘; prints "10"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x ** <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 幂; prints "25"</span>x += <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#自加</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "6"</span>x *= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#自乘</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "12"</span>y = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2.5</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> type(y) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "<type 'float'>"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> y, y + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, y * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, y ** <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "2.5 3.5 5.0 6.25"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>
PS：python中没有x++ 和 x– 操作
- 布尔型(Booleans)
包含True False和常见的与或非操作
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">t = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">True</span>f = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">False</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> type(t) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "<type 'bool'>"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> t <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> f <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 逻辑与; prints "False"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> t <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">or</span> f <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 逻辑或; prints "True"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">not</span> t <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 逻辑非; prints "False"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> t != f <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># XOR; prints "True" </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>
- 字符串型(String)
字符串可以用单引号/双引号/三引号声明
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">hello = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello'</span> world = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"world"</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> hello <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "hello"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> len(hello) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 字符串长度; prints "5"</span>hw = hello + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">' '</span> + world <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 字符串连接</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> hw <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># prints "hello world"</span>hw2015 = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'%s %s %d'</span> % (hello, world, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2015</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 格式化字符串</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> hw2015 <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># prints "hello world 2015"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>
字符串对象有很有有用的函数：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">s = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"hello"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> s.capitalize() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 首字母大写; prints "Hello"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> s.upper() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 全大写; prints "HELLO"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> s.rjust(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 以7为长度右对齐，左边补空格; prints " hello"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> s.center(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 居中补空格; prints " hello "</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> s.replace(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'l'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'(ell)'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 字串替换;prints "he(ell)(ell)o"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">' world '</span>.strip() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 去首位空格; prints "world"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>
2.1.2 基本容器
- 列表/List
和数组类似的一个东东，不过可以包含不同类型的元素，同时大小也是可以调整的。
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">xs = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 创建</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> xs, xs[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[3, 1, 2] 2"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> xs[-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 第-1个元素，即最后一个</span>xs[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'foo'</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 下标从0开始，这是第3个元素</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> xs <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 可以有不同类型，Prints "[3, 1, 'foo']"</span>xs.append(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'bar'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 尾部添加一个元素</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> xs <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints </span>x = xs.pop() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 去掉尾部的元素</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x, xs <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "bar [3, 1, 'foo']"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>
列表最常用的操作有：
切片/slicing
即取子序列/一部分元素，如下：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">nums = range(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 从1到5的序列</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[0, 1, 2, 3, 4]"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 下标从2到4-1的元素 prints "[2, 3]"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>:] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 下标从2到结尾的元素</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums[:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 从开头到下标为2-1的元素 [0, 1]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums[:] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 恩，就是全取出来了</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums[:-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 从开始到第-1个元素(最后的元素)</span>nums[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>] = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 对子序列赋值</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> nums <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[0, 1, 8, 8, 4]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>
循环/loops
即遍历整个list，做一些操作，如下：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">animals = [<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'dog'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'monkey'</span>]<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> animal <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> animals: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> animal<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 依次输出 "cat", "dog", "monkey"，每个一行.</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>
可以用enumerate取出元素的同时带出下标
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">animals = [<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'dog'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'monkey'</span>]<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> idx, animal <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> enumerate(animals): <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'#%d: %s'</span> % (idx + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, animal)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 输出 "#1: cat", "#2: dog", "#3: monkey"，一个一行。</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>
List comprehension
这个相当相当相当有用，在很长的list生成过程中，效率完爆for循环：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># for 循环</span>nums = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]squares = []<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> nums: squares.append(x ** <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> squares <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints [0, 1, 4, 9, 16]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># list comprehension</span>nums = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]squares = [x ** <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> nums]<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> squares <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints [0, 1, 4, 9, 16]</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>
你猜怎么着，list comprehension也是可以加多重条件的：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">nums = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]even_squares = [x ** <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> nums <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> x % <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> even_squares <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[0, 4, 16]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>
- 字典/Dict
和Java中的Map一样的东东，用于存储key-value对：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">d = {<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cute'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'dog'</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'furry'</span>} <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 创建</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d[<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 根据key取出value</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> d <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 判断是否有'cat'这个key</span>d[<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fish'</span>] = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'wet'</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 添加元素</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d[<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fish'</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "wet"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># print d['monkey'] # KeyError: 'monkey'非本字典的key</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d.get(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'monkey'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'N/A'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 有key返回value，无key返回"N/A"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d.get(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fish'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'N/A'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># prints "wet"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">del</span> d[<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fish'</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 删除某个key以及对应的value</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d.get(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'fish'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'N/A'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># prints "N/A"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>对应list的那些操作，你在dict里面也能找得到：
循环/loops
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># for循环</span>d = {<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'person'</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'spider'</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> animal <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> d: legs = d[animal] <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'A %s has %d legs'</span> % (animal, legs)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "A person has 2 legs", "A spider has 8 legs", "A cat has 4 legs"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 通过iteritems</span>d = {<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'person'</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cat'</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'spider'</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> animal, legs <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> d.iteritems(): <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'A %s has %d legs'</span> % (animal, legs)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "A person has 2 legs", "A spider has 8 legs", "A cat has 4 legs"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Dictionary comprehension</span>nums = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]even_num_to_square = {x: x ** <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> nums <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> x % <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> even_num_to_square <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "{0: 0, 2: 4, 4: 16}"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>- 元组/turple
本质上说，还是一个list，只不过里面的每个元素都是一个两元组对。
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">d = {(x, x + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>): x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> range(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>)} <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 创建</span>t = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Create a tuple</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> type(t) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "<type 'tuple'>"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d[t] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "5"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d[(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "1"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>2.1.3 函数
用def可以定义一个函数：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">sign</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(x)</span>:</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> x > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'positive'</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">elif</span> x < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'negative'</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'zero'</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> x <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> [-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> sign(x)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "negative", "zero", "positive"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">hello</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(name, loud=False)</span>:</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> loud: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'HELLO, %s'</span> % name.upper() <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Hello, %s!'</span> % namehello(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Bob'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "Hello, Bob"</span>hello(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Fred'</span>, loud=<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">True</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "HELLO, FRED!"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>
- 类/Class
python里面的类定义非常的直接和简洁：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Greeter</span>:</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Constructor</span> <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">__init__</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(self, name)</span>:</span> self.name = name <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Create an instance variable</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Instance method</span> <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">def</span> <span class="hljs-title" style="box-sizing: border-box;">greet</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(self, loud=False)</span>:</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> loud: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'HELLO, %s!'</span> % self.name.upper() <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Hello, %s'</span> % self.nameg = Greeter(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Fred'</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Construct an instance of the Greeter class</span>g.greet() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Call an instance method; prints "Hello, Fred"</span>g.greet(loud=<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">True</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Call an instance method; prints "HELLO, FRED!"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>
2.2.NumPy基础
NumPy是Python的科学计算的一个核心库。它提供了一个高性能的多维数组(矩阵)对象，可以完成在其之上的很多操作。很多机器学习中的计算问题，把数据vectorize之后可以进行非常高效的运算。
2.2.1 数组
一个NumPy数组是一些类型相同的元素组成的类矩阵数据。用list或者层叠的list可以初始化：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npa = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>]) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 一维Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> type(a) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "<type 'numpy.ndarray'>"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a.shape <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "(3,)"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "1 2 3"</span>a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>] = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 重赋值</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[5, 2, 3]"</span>b = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>],[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>]]) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 二维Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> b.shape <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "(2, 3)"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> b[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], b[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], b[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "1 2 4"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>
生成一些特殊的Numpy数组(矩阵)时，我们有特定的函数可以调用：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npa = np.zeros((<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 全0的2*2 Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 0. 0.]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 0. 0.]]"</span>b = np.ones((<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 全1 Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> b <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 1. 1.]]"</span>c = np.full((<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>), <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 固定值Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> c <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 7. 7.]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 7. 7.]]"</span>d = np.eye(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 2*2 对角Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 1. 0.]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 0. 1.]]"</span>e = np.random.random((<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 2*2 的随机Numpy数组</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> e <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 随机输出</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>
2.2.2 Numpy数组索引与取值
可以通过像list一样的分片/slicing操作取出需要的数值部分。
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> np<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 创建如下的3*4 Numpy数组</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 1 2 3 4]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 5 6 7 8]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 9 10 11 12]]</span>a = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>]])<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 通过slicing取出前两行的2到3列:</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[2 3]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [6 7]]</span>b = a[:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>]<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 需要注意的是取出的b中的数据实际上和a的这部分数据是同一份数据.</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "2"</span>b[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>] = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">77</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># b[0, 0] 和 a[0, 1] 是同一份数据</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># a也被修改了，Prints "77"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npa = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>]])row_r1 = a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, :] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># a 的第二行 </span>row_r2 = a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, :] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 同上</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> row_r1, row_r1.shape <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[5 6 7 8] (4,)"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> row_r2, row_r2.shape <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[5 6 7 8]] (1, 4)"</span>col_r1 = a[:, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]col_r2 = a[:, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>]<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> col_r1, col_r1.shape <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[ 2 6 10] (3,)"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> col_r2, col_r2.shape <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 2]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 6]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [10]] (3, 1)"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>
还可以这么着取：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npa = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>]])<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 取出(0,0) (1,1) (2,0)三个位置的值</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[1 4 5]"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 和上面一样</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.array([a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]]) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[1 4 5]"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 取出(0,1) (0,1) 两个位置的值</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[2 2]"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 同上</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.array([a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], a[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]]) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[2 2]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>
我们还可以通过条件得到bool型的Numpy数组结果，再通过这个数组取出符合条件的值，如下：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npa = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>]])bool_idx = (a > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 判定a大于2的结果矩阵</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> bool_idx <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[False False]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ True True]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ True True]]"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 再通过bool_idx取出我们要的值</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[bool_idx] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[3 4 5 6]"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 放在一起我们可以这么写</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> a[a > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>] <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[3 4 5 6]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>
Numpy数组的类型
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>]) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x.dtype <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "int64"</span>x = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2.0</span>]) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x.dtype <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "float64"</span>x = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>], dtype=np.int64) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 强制使用某个type</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x.dtype <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "int64"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>
2.2.3 Numpy数组的运算
矩阵的加减开方和(元素对元素)乘除如下：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>],[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]], dtype=np.float64)y = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>],[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>]], dtype=np.float64)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 6.0 8.0]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [10.0 12.0]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x + y<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.add(x, y)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[-4.0 -4.0]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [-4.0 -4.0]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x - y<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.subtract(x, y)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 元素对元素，点对点的乘积</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 5.0 12.0]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [21.0 32.0]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x * y<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.multiply(x, y)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 元素对元素，点对点的除法</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 0.2 0.33333333]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 0.42857143 0.5 ]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x / y<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.divide(x, y)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 开方</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 1. 1.41421356]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 1.73205081 2. ]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.sqrt(x)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li></ul>
矩阵的内积是通过下列方法计算的：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>],[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]])y = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>],[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>]])v = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>])w = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>])<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 向量内积，得到 219</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> v.dot(w)<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.dot(v, w)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 矩阵乘法，得到 [29 67]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x.dot(v)<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.dot(x, v)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 矩阵乘法</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[19 22]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [43 50]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x.dot(y)<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.dot(x, y)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>
特别特别有用的一个操作是，sum/求和(对某个维度)：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>],[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]])<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.sum(x) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 整个矩阵的和，得到 "10"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.sum(x, axis=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 每一列的和 得到 "[4 6]"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.sum(x, axis=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 每一行的和 得到 "[3 7]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>
还有一个经常会用到操作是矩阵的转置，在Numpy数组里用.T实现：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>]])<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[1 2]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [3 4]]"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x.T <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[1 3]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [2 4]]"</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 1*n的Numpy数组，用.T之后其实啥也没做:</span>v = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>])<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> v <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[1 2 3]"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> v.T <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[1 2 3]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>
2.2.4 Broadcasting
Numpy还有一个非常牛逼的机制，你想想，如果你现在有一大一小俩矩阵，你想使用小矩阵在大矩阵上做多次操作。额，举个例子好了，假如你想将一个1*n的矩阵，加到m*n的矩阵的每一行上：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#你如果要用for循环实现是酱紫的(下面用y的原因是，你不想改变原来的x)</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>]])v = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>])y = np.empty_like(x) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 设置一个和x一样维度的Numpy数组y</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 逐行相加</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> i <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> range(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>): y[i, :] = x[i, :] + v<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 恩，y就是你想要的了</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 2 2 4]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 5 5 7]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 8 8 10]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [11 11 13]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> y</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#上一种方法如果for的次数非常多，会很慢，于是我们改进了一下</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>]])v = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>])vv = np.tile(v, (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 变形，重复然后叠起来</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> vv <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[1 0 1]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [1 0 1]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [1 0 1]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [1 0 1]]"</span>y = x + vv <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 相加</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> y <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 2 2 4</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 5 5 7]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 8 8 10]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [11 11 13]]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#其实因为Numpy的Broadcasting，你可以直接酱紫操作</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npx = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>]])v = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>])y = x + v <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 直接加！！！</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> y <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># Prints "[[ 2 2 4]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 5 5 7]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 8 8 10]</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [11 11 13]]"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>
更多Broadcasting的例子请看下面：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> npv = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>]) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># v has shape (3,)</span>w = np.array([<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>]) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># w has shape (2,)</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 首先把v变成一个列向量</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># v现在的形状是(3, 1);</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 作用在w上得到的结果形状是(3, 2)，如下</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 4 5]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 8 10]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [12 15]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> np.reshape(v, (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)) * w<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 逐行相加</span>x = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>]])<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 得到如下结果:</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[2 4 6]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [5 7 9]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x + v<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 先逐行相加再转置，得到以下结果:</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 5 6 7]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 9 10 11]]</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> (x.T + w).T<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 恩，也可以这么做</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x + np.reshape(w, (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>))</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul>
2.3 SciPy
Numpy提供了一个非常方便操作和计算的高维向量对象，并提供基本的操作方法，而Scipy是在Numpy的基础上，提供很多很多的函数和方法去直接完成你需要的矩阵操作。有兴趣可以浏览Scipy方法索引查看具体的方法，函数略多，要都记下来有点困难，随用随查吧。
向量距离计算
需要特别拎出来说一下的是，向量之间的距离计算，这个Scipy提供了很好的接口scipy.spatial.distance.pdist：
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> np<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> scipy.spatial.distance <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> pdist, squareform<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[0 1]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [1 0]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [2 0]]</span>x = np.array([[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]])<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> x<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 计算矩阵每一行和每一行之间的欧氏距离</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># d[i, j] 是 x[i, :] 和 x[j, :] 之间的距离,</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 结果如下：</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [[ 0. 1.41421356 2.23606798]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 1.41421356 0. 1. ]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># [ 2.23606798 1. 0. ]]</span>d = squareform(pdist(x, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'euclidean'</span>))<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">print</span> d</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>
2.4 Matplotlib
这是python中的一个作图工具包。如果你熟悉matlab的语法的话，应该会用得挺顺手。可以通过matplotlib.pyplot.plot了解更多绘图相关的设置和参数。
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> np<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> matplotlib.pyplot <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> plt<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 计算x和对应的sin值作为y</span>x = np.arange(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> * np.pi, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.1</span>)y = np.sin(x)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 用matplotlib绘出点的变化曲线</span>plt.plot(x, y)plt.show() <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 只有调用plt.show()之后才能显示</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>
结果如下： 
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 在一个图中画出2条曲线</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> np<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> matplotlib.pyplot <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> plt<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 计算x对应的sin和cos值</span>x = np.arange(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> * np.pi, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.1</span>)y_sin = np.sin(x)y_cos = np.cos(x)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 用matplotlib作图</span>plt.plot(x, y_sin)plt.plot(x, y_cos)plt.xlabel(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'x axis label'</span>)plt.ylabel(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'y axis label'</span>)plt.title(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Sine and Cosine'</span>)plt.legend([<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Sine'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Cosine'</span>])plt.show()</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 用subplot分到子图里</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> np<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> matplotlib.pyplot <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> plt<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 得到x对应的sin和cos值</span>x = np.arange(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> * np.pi, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.1</span>)y_sin = np.sin(x)y_cos = np.cos(x)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 2*1个子图，第一个位置.</span>plt.subplot(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 画第一个子图</span>plt.plot(x, y_sin)plt.title(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Sine'</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 画第2个子图</span>plt.subplot(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)plt.plot(x, y_cos)plt.title(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Cosine'</span>)plt.show()</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul>
2.5 简单图片读写
可以使用imshow来显示图片。
<code class="language-python hljs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> numpy <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> np<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> scipy.misc <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> imread, imresize<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">import</span> matplotlib.pyplot <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> pltimg = imread(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'/Users/HanXiaoyang/Comuter_vision/computer_vision.jpg'</span>)img_tinted = img * [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.95</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.9</span>]<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 显示原始图片</span>plt.subplot(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)plt.imshow(img)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># 显示调色后的图片</span>plt.subplot(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)plt.imshow(np.uint8(img_tinted))plt.show()</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>
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