machinelearning_notebook/0_python/3_Data_Structure_1.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 数据结构 - 1"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "数据结构是计算机存储、组织数据的方式，简单来说是指相互之间存在一种或多种特定关系的数据元素的集合。\n",
    "\n",
    "Python中的数据结构设计的非常巧妙，使用起来非常方便，几乎绝大多数的数据结构都可以通过`list`, `tuple`, `dict`, `string`, `set`等表示，因此用户几乎不需要自己定义数据结构，仅仅使用Python内置的数据结构即可实现非常复杂的算法和操作。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 1. 列表"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "列表是最常用的数据结构。可以把它看作用方括号括起来的数据序列，数据之间用逗号分隔。这些数据都可以通过调用其索引值来访问。\n",
    "\n",
    "`list`的声明只需将变量等同于`[ ]`或`list`即可。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "a = []"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "<class 'list'>\n"
     ]
    }
   ],
   "source": [
    "print(type(a))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以直接将数据序列分配给列表x，如下所示。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['apple', 'orange', 'peach']\n"
     ]
    }
   ],
   "source": [
    "x = ['apple', 'orange', 'peach']\n",
    "print(x)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 1.1 索引"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在Python中，索引从`0`开始。因此，现在包含两个元素的列表`x`的apple索引值为`0`,orange索引值为`1`。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'apple'"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x[0]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "索引也可以按照相反的顺序进行。这是最后一个可以被首先访问的元素。这里，索引从`-1`开始。因此，索引`-1`对应是橙色，索引`-2`对应的是苹果。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'peach'"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x[-1]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "正如你可能猜到的一样，x[0] = x[-2], x[1] = x[-1]。这个概念可以扩展到更多包含元素的列表。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [],
   "source": [
    "y = ['carrot','potato']"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在这里我们已经声明过两个列表`x`和`y`每一个包含自己的数据。现在，这两个列表可以再一次被放入另一个也具有自己的数据的列表`z`中。列表中的这个列表被称为`嵌套列表`，这就是数组的声明方式，我们将在后面看到。**这是和很多其他计算机语言不同的地方，不要求列表的元素是相同类型，因此编程的时候会非常方便，这也是为什么Python对人类比较友好**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[['apple', 'orange', 'peach'], ['carrot', 'potato'], 'Test']\n"
     ]
    }
   ],
   "source": [
    "z  = [x,y, 'Test']\n",
    "print(z)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'orange'"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "z[0][1]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "如何获得嵌套列表中的某个元素？让我们在上述嵌套列表中获得数据'apple'为例。\n",
    "* 首先在索引为0处，有一个列表`['apple','orange']` 而在索引为1处有另外一个列表`['carrot','potato']` 。\n",
    "* 因此z[0] 应该给我们第一个包含'apple'的列表。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['apple', 'orange', 'peach']\n"
     ]
    }
   ],
   "source": [
    "z1 = z[0]\n",
    "print(z1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "现在观察z1并不是一个嵌套列表，因此为了获得'apple'，z1的索引应该为0。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'apple'"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "z1[0]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在python中，你可以通过每次并排写索引值来访问“apple”，而不是像上面那样做。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'apple'"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "z[0][0]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "如果列表中有一个列表，那么您可以通过执行 z[ ][ ][ ] 来访问最里面的值。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 1.2 切片"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "索引只限于访问单个元素，而切片则是访问列表内的一系列数据。换句话说，`切片`返回的是一个列表。\n",
    "\n",
    "切片是通过定义切片列表中需要的父列表中的第一个元素和最后一个元素的索引值来完成的。它被写成`parentlist[a: b]`，其中`a`,`b`是父列表的索引值。如果`a`或`b`未定义，则认为该索引值是`a`未定义时的第一个值，以及`b`未定义时的最后一个值。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[3, 2, 3]\n",
      "[2, 3, 2, 3, 4, 5, 6, 7, 8, 9]\n",
      "[2, 3, 2, 3, 4, 5, 6, 7, 8, 9]\n",
      "[2, 3, 2, 3, 4, 5, 6, 7, 8, 9]\n"
     ]
    }
   ],
   "source": [
    "num = [2,3,2,3,4,5,6,7,8,9]\n",
    "print(num[1:4])\n",
    "print(num[0:])\n",
    "print(num[:])\n",
    "print(num)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[2, 3, 2, 3]\n",
      "[4, 5, 6, 7, 8, 9]\n"
     ]
    }
   ],
   "source": [
    "print(num[0:4])\n",
    "print(num[4:])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "您还可以使用固定长度或步长对父列表进行切片。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[2, 3, 6]"
      ]
     },
     "execution_count": 14,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "num[:9:3]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 1.3 列表的内置函数"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "为了找到列表的长度或者列表中元素的数量，我们可以使用**len( )**。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "10"
      ]
     },
     "execution_count": 15,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "len(num)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "如果列表包含所有的整数元素，那么 **min( )** 和 **max( )** 给出列表中的最大值和最小值。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[2, 3, 2, 3, 4, 5, 6, 7, 8, 9]\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "2"
      ]
     },
     "execution_count": 16,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "print(num)\n",
    "min(num)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "9"
      ]
     },
     "execution_count": 17,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "max(num)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "列表可以通过添加\"`+`\"来连接。生成的列表将包含添加的列表的所有元素。结果列表将不是嵌套列表。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[1, 2, 3, 5, 4, 7]"
      ]
     },
     "execution_count": 18,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "[1,2,3] + [5,4,7]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可能会出现这样的需求，您可能需要检查预定义列表中是否存在特定的元素。考虑下面的列表。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {},
   "outputs": [],
   "source": [
    "names = ['Earth','Air','Fire','Water']"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "检查“Fire”和“Rajath”是否出现在列表名称中。传统的方法是使用for循环遍历列表并使用if条件。但在python中，你可以使用\" a在b中\"的概念，如果a在b中出现，它会返回\"True\"如果不是，它会返回\"False\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 20,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "False"
      ]
     },
     "execution_count": 20,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "'Fir' in names"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 21,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 21,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "'Fire' in names"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 22,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "False"
      ]
     },
     "execution_count": 22,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "'fire' in names"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在一个有字符串作为元素的列表中，**max( )** 和 **min( )** 可以使用。**max( )**  会返回一个ASCII码最大的元素而最小的元素会在使用**min( )** 返回。注意，每次只考虑每个元素的第一个索引，如果它们的值相同，则考虑第二个索引，依此类推。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 23,
   "metadata": {},
   "outputs": [],
   "source": [
    "mlist = ['bzaa','ds','nc','az','z','klm']"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "z\n",
      "az\n"
     ]
    }
   ],
   "source": [
    "print(max(mlist))\n",
    "print(min(mlist))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "这里考虑每个元素的第一个索引，因此z有最高的ASCII值，因此它被返回，最小的ASCII值是a。但是如果数字被声明为字符串呢?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 25,
   "metadata": {},
   "outputs": [],
   "source": [
    "nlist = ['1','94','93','1000']"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 26,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "94\n",
      "1\n"
     ]
    }
   ],
   "source": [
    "print(max(nlist))\n",
    "print(min(nlist))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "即使数字是在字符串中声明的，也会考虑每个元素的第一个索引，并相应地返回最大值和最小值。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "但是如果你想找到给予字符串长度的 **max( )** 字符串元素，那么我们要在 **max( )** 和 **min( )** 中声明参数'key=len'。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 27,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Earth\n",
      "Jet\n"
     ]
    }
   ],
   "source": [
    "names = ['Earth','Jet', 'Air','Fire','Water']\n",
    "print(max(names, key=len))\n",
    "print(min(names, key=len))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "但是即使'Water'的长度为5。**max()** 或 **min()** 函数返回第一个元素当两个或者多个元素具有相同的长度。\n",
    "\n",
    "可以使用任何其他内建函数或lambda函数(后面将讨论)来代替len。\n",
    "\n",
    "通过使用**list()** 函数，一个字符串可以被转化成列表。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 28,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['h', 'e', 'l', 'l', 'o']"
      ]
     },
     "execution_count": 28,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "list('hello')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**append( )** 被用来在列表的最后添加一个元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 29,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst = [1,1,4,8,7]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 30,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 1, 4, 8, 7, 1]\n"
     ]
    }
   ],
   "source": [
    "lst.append(1)\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**count( )** 用于计算列表中出现的特定元素的数量。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 31,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "3"
      ]
     },
     "execution_count": 31,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "lst.count(1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**append( )** 函数也可以被用来在末尾添加一整个列表。观察可以发现最终得到的列表是嵌套列表。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 32,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst1 = [5,4,2,8]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 33,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 1, 4, 8, 7, 1, [5, 4, 2, 8]]\n"
     ]
    }
   ],
   "source": [
    "lst.append(lst1)\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "但是如果嵌套列表不是需要的，那么可以使用**extend()** 函数。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 34,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 1, 4, 8, 7, 1, [5, 4, 2, 8], 5, 4, 2, 8]\n"
     ]
    }
   ],
   "source": [
    "lst.extend(lst1)\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**index( )** 被用来找到一个特殊元素的索引值。注意如果有许多个元素具有相同的值那么元素第一个索引值会被返回。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 35,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "0"
      ]
     },
     "execution_count": 35,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "lst.index(1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 36,
   "metadata": {},
   "outputs": [
    {
     "ename": "ValueError",
     "evalue": "999 is not in list",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-36-344da5bcf759>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mlst\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mindex\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m999\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mValueError\u001b[0m: 999 is not in list"
     ]
    }
   ],
   "source": [
    "lst.index(999)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**insert(x,y)** 用于在指定的索引值x处插入元素y。**append( )** 函数使得它只能插在最后。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst.insert(5, 'name')\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**insert(x,y)** 插入但不替换元素。如果希望用另一个元素替换该元素，只需将值赋给该特定索引。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst[5] = 'Python'\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**pop( )** 函数返回列表中的最后一个元素。这类似于堆栈的操作。因此，说列表可以作为堆栈使用是正确的。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst.pop()\n",
    "lst"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以指定索引值来弹出与该索引值对应的元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst.pop(2)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(lst)\n",
    "lst.pop(-2)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**pop( )** 用于根据可分配给变量的元素的索引值来删除元素。还可以通过使用**remove()** 函数指定元素本身来删除元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst.remove('Python')\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以替代 **remove** 但是使用索引值的函数是 **del**。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(lst)\n",
    "del lst[1]\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "可以使用**reverse()** 函数反转列表中出现的所有元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst.reverse()\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "注意嵌套列表 [5,4,2,8] 被视为父列表lst的单个元素。因此在嵌套列表里的元素是不可以被翻转的。\n",
    "\n",
    "Python提供了内置函数 **sort( )** 去按升序排列元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst = [1, 4, 8, 8, 10]\n",
    "lst.sort()\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "对于降序，因为默认情况下反向条件为False。因此，将其更改为True将按降序排列元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lst.sort(reverse=True)\n",
    "print(lst)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "相似地对于包含字符串元素的列表， **sort( )** 会根据他们的ASCII值以升序的方式排列而通过确定reverse=True可以让他们以降序的方式排列。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "names = ['apple', 'orange', 'peach']\n",
    "names.sort()\n",
    "print(names)\n",
    "names.sort(reverse=True)\n",
    "print(names)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "如果要根据长度排序我们应该像图示的一样确定key=len。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "names.sort(key=len)\n",
    "print(names)\n",
    "names.sort(key=len,reverse=True)\n",
    "print(names)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 1.4 复制一个列表"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "大多数新的python程序员都会犯这个错误，即**对象的赋值和拷贝的差异**。考虑以下的例子："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lista= [2,1,4,3]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "listb = lista # 对象赋值\n",
    "print(listb)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "这里，我们声明了一个列表，lista = [2,1,4,3]。通过赋值将该列表复制到listb，并复制该列表。现在我们对lista执行一些随机操作。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lista.pop()\n",
    "print(lista)\n",
    "lista.append(9)\n",
    "print(lista)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(listb)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "虽然没有对listb执行任何操作，但它也发生了变化。这是因为您将lista、listb指向相同的内存空间。那么如何解决这个问题呢?\n",
    "\n",
    "在切片中我们已经看到parentlist[a:b]从父列表返回一个起始索引a和结束索引b的列表，如果a和b没有被提及，那么默认情况下它会包含第一个到最后一个元素。我们在这里使用相同的概念。通过这样做，我们将lista的数据作为变量分配给listb。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lista = [2,1,4,3]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "listb = lista[:]\n",
    "print(listb)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "lista.pop()\n",
    "print(lista)\n",
    "lista.append(9)\n",
    "print(lista)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print(listb)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "还有其他什么方法能够拷贝一个对象到一个新的变量名字？"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 2. 元组"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "元组与列表相似，但唯一大的区别是列表中的元素可以更改，而**元组中的元素不能更改**。为了更好地理解，请回忆**divmod()** 函数。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "xyz = divmod(10,3)\n",
    "print(xyz)\n",
    "print(type(xyz))\n",
    "xyz[0]=10"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "这里的商必须是3余数必须是1。当10除以3时，这些值不能改变。因此，divmod以元组的形式返回这些值。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "要定义元组，将一个变量分配给paranthesis()或tuple()。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 37,
   "metadata": {},
   "outputs": [],
   "source": [
    "tup = ()\n",
    "tup2 = tuple()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "如果想直接声明元组，可以在数据的末尾使用逗号。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 38,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "(27,)"
      ]
     },
     "execution_count": 38,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "27,"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "27乘以2得到54，但是乘以一个元组，数据重复两次。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 39,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "(27, 27)"
      ]
     },
     "execution_count": 39,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "2*(27,)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在声明元组时可以分配值。它接受一个列表作为输入并将其转换为元组，或者接受一个字符串并将其转换为元组。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 40,
   "metadata": {
    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "(1, 2, 3)\n",
      "('H', 'e', 'l', 'l', 'o')\n"
     ]
    }
   ],
   "source": [
    "tup3 = tuple([1,2,3])\n",
    "print(tup3)\n",
    "tup4 = tuple('Hello')\n",
    "print(tup4)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "它遵循与列表相同的索引和切片。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 41,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2\n",
      "('H', 'e', 'l')\n"
     ]
    }
   ],
   "source": [
    "print(tup3[1])\n",
    "tup5 = tup4[:3]\n",
    "print(tup5)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 2.1 将一个元组映射到另一个元组"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 42,
   "metadata": {},
   "outputs": [],
   "source": [
    "(a,b,c)= ('alpha','beta','gamma')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 43,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "alpha beta gamma\n"
     ]
    }
   ],
   "source": [
    "print(a,b,c)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 44,
   "metadata": {},
   "outputs": [],
   "source": [
    "(c, b, a) = (a, b, c)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 45,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "('R', 'a', 'j', 'a', 't', 'h', 'K', 'u', 'm', 'a', 'r', 'M', 'P')\n"
     ]
    }
   ],
   "source": [
    "d = tuple('RajathKumarMP')\n",
    "print(d)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 2.2 元组内置函数"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**count()** 函数计算元组中存在的指定元素的数量。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 46,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "3"
      ]
     },
     "execution_count": 46,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "d.count('a')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**index()** 函数返回指定元素的索引。如果元素大于1，则返回该指定元素的第一个元素的索引"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 47,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1"
      ]
     },
     "execution_count": 47,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "d.index('a')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 3. 集合"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "集合主要用于消除序列/列表中的重复数字。它还用于执行一些标准的集合操作。\n",
    "\n",
    "set被声明为set()，它将初始化一个空集。set([sequence])也可以被执行来声明一个包含元素的集"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 48,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "<class 'set'>\n"
     ]
    }
   ],
   "source": [
    "set1 = set()\n",
    "print(type(set1))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 49,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{1, 2, 3, 4}\n"
     ]
    }
   ],
   "source": [
    "set0 = set([1,2,2,3,3,4])\n",
    "print(set0)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 50,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{1, 2, 3, 4}\n"
     ]
    }
   ],
   "source": [
    "set1 = set((1,2,2,3,3,4))\n",
    "print(set1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "重复两次的元素2,3只会出现一次。因此在一个集合中，每个元素都是不同的。"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 3.1 内置函数"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 51,
   "metadata": {},
   "outputs": [],
   "source": [
    "set1 = set([1,2,3])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 52,
   "metadata": {},
   "outputs": [],
   "source": [
    "set2 = set([2,3,4,5])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**union( )** 函数返回一个并集合，该集合包含两个集合的所有元素，但是没有重复。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 53,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{1, 2, 3, 4, 5}"
      ]
     },
     "execution_count": 53,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "set1.union(set2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**add()** 将向集合中添加一个特定的元素。注意，新添加的元素的索引是任意的，可以放在末尾不需要的任何位置。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 54,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{1, 2, 3}\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "{0, 1, 2, 3}"
      ]
     },
     "execution_count": 54,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "print(set1)\n",
    "set1.add(0)\n",
    "set1"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**intersection( )** 函数输出一个交集合，该集合包含两个集合中的所有元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 55,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{2, 3}"
      ]
     },
     "execution_count": 55,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "set1.intersection(set2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**difference( )** 函数输出一个集合，其中包含在set1中而不在set2中的元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 56,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{0, 1, 2, 3}\n",
      "{2, 3, 4, 5}\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "{0, 1}"
      ]
     },
     "execution_count": 56,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "print(set1)\n",
    "print(set2)\n",
    "set1.difference(set2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**pop( )** 是用来移除集合中的任意元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 57,
   "metadata": {},
   "outputs": [],
   "source": [
    "set1=set([10, 9, 1, 2, 4])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 58,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{2, 4, 9, 10}\n"
     ]
    }
   ],
   "source": [
    "set1.pop()\n",
    "print(set1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**remove( )** 函数从集合中删除指定的元素。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 59,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{4, 9, 10}"
      ]
     },
     "execution_count": 59,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "set1.remove(2)\n",
    "set1"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**clear( )** 用于清除所有元素并将其设置为空集。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 60,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "set()"
      ]
     },
     "execution_count": 60,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "set1.clear()\n",
    "set1"
   ]
  }
 ],
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   "language": "python",
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