学到的函数.ipynb

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   "source": [
    "# enumerate \n",
    "\n",
    "enumerate() 函数用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标，一般用在 for 循环当中。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "enumerate(sequence, start=0)\n",
    "\n",
    "'''\n",
    "sequence -- 一个序列、迭代器或其他支持迭代对象。\n",
    "start -- 下标起始位置。\n",
    "返回 enumerate(枚举) 对象\n",
    "'''"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "#示例\n",
    "seasons = ['Spring', 'Summer', 'Fall', 'Winter']\n",
    "list(enumerate(seasons))\n",
    "list(enumerate(seasons, start=1))       # 下标从 1 开始\n",
    "tuple(enumerate(seasons, start=1))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "#普通的for循环\n",
    "i = 0\n",
    "seq = ['one', 'two', 'three']\n",
    "for element in seq:\n",
    "    print(i, seq[i])\n",
    "    i +=1\n",
    "#for循环使用enumerate示例1\n",
    "seq = ['one', 'two', 'three']\n",
    "for temp in enumerate(seq):\n",
    "    print(temp)   \n",
    "#for循环使用enumerate示例2\n",
    "seq = ['one', 'two', 'three']\n",
    "for i, element in enumerate(seq):\n",
    "    print (i, element)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Collection"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### collection.defaultdict()定义字典（可以含有不存在的键）"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "defaultdict(None, {'yellow': 3, 'blue': 4, 'red': 1})\n",
      "defaultdict(<class 'list'>, {'yellow': 3, 'blue': 4, 'red': 1})\n"
     ]
    }
   ],
   "source": [
    "import collections \n",
    "s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]  \n",
    "## 元组列表  ## 元组是里面的元素，列表是最终的数据类型 type \n",
    "\n",
    "d = collections.defaultdict()   ## 注意这里并没有指明数据类型 \n",
    "for k, v in s:\n",
    "    d[k] = v  \n",
    "    \n",
    "print(d)\n",
    "#### defaultdict(<class 'list'>, {'yellow': 3, 'blue': 4, 'red': 1})\n",
    "\n",
    "\n",
    "##############  注意查看 defaultdict() 默认是 list 类型  ###############  \n",
    "import collections \n",
    "s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]\n",
    "d = collections.defaultdict(list)\n",
    "for k, v in s:\n",
    "    d[k] = v\n",
    "print(d)\n",
    "# defaultdict(<class 'list'>, {'yellow': 3, 'blue': 4, 'red': 1})\n",
    "\n",
    "#注意看，这里的是赋值，所以只有最后一个blue\n",
    "\n",
    "#对比结果\n",
    "import collections \n",
    "s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]\n",
    "\n",
    "d = collections.defaultdict(list)  ## 指明了数据类型 list \n",
    "for k, v in s:\n",
    "    d[k].append(v)   ## 注意结果是将所有的值做成了 list \n",
    "    \n",
    "print(d)\n",
    "#### defaultdict(<class 'list'>, {'yellow': [1, 3], 'blue': [2, 4], 'red': [1]})\n",
    "\n",
    "#既然是字典类型就会存在 .keys() , .values(), .items()\n",
    "d.keys()\n",
    "d.values()\n",
    "d.items()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "dict_items([('red', {1, 3}), ('blue', {2, 4})])\n",
      "dict_items([('m', 1), ('i', 4), ('s', 4), ('p', 2)])\n",
      "abc\n",
      "N/A\n"
     ]
    }
   ],
   "source": [
    "#通过 集合 set() 构建 defaultdict\n",
    "from collections import defaultdict\n",
    "s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)]\n",
    "d = defaultdict(set)\n",
    "for k, v in s:\n",
    "    d[k].add(v)   ## 元组应该是 add() 操作 \n",
    "\n",
    "print(d.items())\n",
    "#### dict_items([('red', {1, 3}), ('blue', {2, 4})])\n",
    "\n",
    "#通过 defaultdict 构建 计数器\n",
    "from collections import defaultdict\n",
    "s = 'mississippi'\n",
    "d = defaultdict(int)  ## 注意这里是指明了 int 类型的 \n",
    "for k in s:\n",
    "    d[k] += 1  ## 完成计数+1操作\n",
    "\n",
    "print(d.items()) \n",
    "#### dict_items([('m', 1), ('i', 4), ('s', 4), ('p', 2)])\n",
    "\n",
    "#特殊之处：通过设置可以使得不存在的键会自动用 空值 填充\n",
    "from collections import defaultdict\n",
    "dd = defaultdict(lambda : 'N/A')#这里如果不设置lambda:'N/A'的话，会报错说没有key2\n",
    "dd['key1'] = 'abc'\n",
    "print(dd['key1'])\n",
    "print(dd['key2'])\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "<class 'zip'>\n",
      "('yellow', 1)\n",
      "yellow 1\n",
      "blue 2\n",
      "yellow 3\n",
      "blue 4\n",
      "red 1\n"
     ]
    }
   ],
   "source": [
    "#如何通过 两个列表 得到 存放元组的容器 呢？ 可以通过 zip() 函数实现\n",
    "color = ['yellow', 'blue', 'yellow', 'blue', 'red']\n",
    "number = [1,2,3,4,1]\n",
    "\n",
    "s = zip(color, number)\n",
    "\n",
    "print(type(s)) \n",
    "print(next(iter(s))) \n",
    "\n",
    "for k, v in zip(color, number):\n",
    "    print(k, v)\n",
    "\n",
    "# yellow 1\n",
    "# blue 2\n",
    "# yellow 3\n",
    "# blue 4\n",
    "# red 1\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# join"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 连接任意数量的字符串（包括要连接的元素字符串、元组、列表、字典），用新的目标分隔符连接，返回新的字符串。\n",
    "### 'sep'.join(sep_object) \n",
    "#### sep：分割符，可为“，、；”等。\n",
    "#### sep_object：分割对象，可为字符串、以及储存字符串的元组、列表、字典。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'hello peace world'"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "#join函数的实例\n",
    "#（1）对象为字符串\n",
    "';'.join('abc') #输出结果为：'a;b;c'  \n",
    " \n",
    "string1 = 'good idea'#定义字符串\n",
    "' '.join(string1)  \n",
    "#输出结果：'g o o d   i d e a' \n",
    "#说明：由于字符串里没指明按字符串之间是怎么连接的，默认每个字符之间插入目标字符\n",
    "\n",
    "#（2）对象为元组\n",
    "tuple1 = ('a','b','c')  #定义元组tuple1\n",
    "'、'.join(tuple1)\n",
    " \n",
    "tuple2 = ('hello','peace','world')  #定义元组tuple2\n",
    "' '.join(tuple2)\n",
    "\n",
    "#（3）对象为列表\n",
    "b = ['a','b','c'] #定义列表\n",
    "'、'.join(b)\n",
    " \n",
    "list1 = ['hello','peace','world'] #定义列表\n",
    "' '.join(list1)\n",
    "\n",
    "#（4）对象为字典\n",
    "c={'hello':1,'world':2}\n",
    "';'.join(c)\n",
    "d = {'hello':'hi','world':'2'}\n",
    "' '.join(d)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "#### 易错点及解决办法\n",
    "问题：储存非字符串的元组、列表、字典等报错，比如元组储存数字进行连接"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'1、2、3'"
      ]
     },
     "execution_count": 13,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "a = (1,2,3) #定义元组a\n",
    "';'.join(a)\n",
    "#报错:TypeError: sequence item 0: expected str instance, int found\n",
    "\n",
    "#解决办法：要将数字连接起来成为一个字符串，则结合for循环语句并将数字转为字符串再连接起来\n",
    "a = (1,2,3) #定义元组a\n",
    "';'.join(str(i) for i in a)\n",
    " \n",
    "b = (186234,1385475,1235462)\n",
    "';'.join(str(i) for i in b)\n",
    " \n",
    "#调用set函数去重处理\n",
    "e = (1,2,3,2) #定义元组a\n",
    "'、'.join(str(i) for i in set(e))\n",
    "# 分割对象储存的对象要为字符串，否则会报错"
   ]
  }
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