生成器.ipynb

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   "cell_type": "markdown",
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   "source": [
    "### 生成器\n",
    "通过列表生成式，我们可以直接创建一个列表。但是，受到内存限制，列表容量肯定是有限的。而且，创建一个包含100万个元素的列表，不仅占用很大的存储空间，如果我们仅仅需要访问前面几个元素，那后面绝大多数元素占用的空间都白白浪费了。\n",
    "\n",
    "所以，如果列表元素可以按照某种算法推算出来，那我们是否可以在循环的过程中不断推算出后续的元素呢？这样就不必创建完整的list，从而节省大量的空间。在Python中，这种一边循环一边计算的机制，称为生成器：generator。\n",
    "\n",
    "要创建一个generator，有很多种方法。**第一种方法很简单，只要把一个列表生成式的[]改成()，就创建了一个generator**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "<generator object <genexpr> at 0x000002C87A5729E8>"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "L = [x * x for x in range(10)]\n",
    "g = (x * x for x in range(10))\n",
    "g"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]\n",
      "<generator object <genexpr> at 0x0000000004C20A40>\n"
     ]
    }
   ],
   "source": [
    "L = [ x*x for x in range(10)]\n",
    "print(L)\n",
    "g = ( x*x for x in range(10))\n",
    "print(g)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
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       "0"
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     "execution_count": 3,
     "metadata": {},
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   ],
   "source": [
    "next(g)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1"
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     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
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   ],
   "source": [
    "next(g)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4\n",
      "9\n",
      "16\n",
      "25\n",
      "36\n",
      "49\n",
      "64\n",
      "81\n"
     ]
    }
   ],
   "source": [
    "# for i in g:\n",
    "#     print(i)\n",
    "\n",
    "list(range(0,10000000)) #在生成列表的过程中消耗内存大，占用空间大，调用的时候速度块，因为已经什么都已经生成好，放在内存里，什么想要就能够立马用到\n",
    "g = ( x*x for x in range(10))#迭代器，不怎么消耗内存，就放了一个公式程序在里面，但是在调用的时候消耗性能，因为每一次循环调用的时候，它都要进过计算才能得出具体的值"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0\n",
      "1\n",
      "4\n",
      "9\n",
      "16\n",
      "25\n",
      "36\n",
      "49\n",
      "64\n",
      "81\n"
     ]
    }
   ],
   "source": [
    "g = (x * x for x in range(10))\n",
    "for n in g:\n",
    "    print(n)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 案例\n",
    "著名的斐波拉契数列（Fibonacci），除第一个和第二个数外，任意一个数都可由前两个数相加得到：\n",
    "\n",
    "1, 1, 2, 3, 5, 8, 13, 21, 34, ...\n",
    "\n",
    "斐波拉契数列用列表生成式写不出来，但是，用函数把它打印出来却很容易"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n",
      "8\n",
      "13\n",
      "21\n",
      "34\n",
      "55\n",
      "89\n",
      "144\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144]"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def fib(max):\n",
    "    n, a, b = 0, 0, 1\n",
    "    ll = []\n",
    "    while n < max:\n",
    "        print(b)\n",
    "        ll.append(b)\n",
    "        a, b = b, a + b\n",
    "        n = n + 1\n",
    "    return ll\n",
    "fib(12)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n",
      "8\n",
      "13\n",
      "21\n",
      "34\n",
      "55\n",
      "89\n",
      "144\n",
      "233\n",
      "377\n",
      "610\n",
      "987\n"
     ]
    }
   ],
   "source": [
    "def fib(max):\n",
    "    n, a, b = 0, 0, 1\n",
    "    ll = []\n",
    "    while n < max:\n",
    "#         print(b)\n",
    "        yield b \n",
    "        ll.append(b)\n",
    "        a, b = b, a + b\n",
    "        n = n + 1\n",
    "    return ll\n",
    "a = fib(12) #这个函数没有执行，生成1个生成器对象\n",
    "# print(next(a)) \n",
    "#调用next（）函数才执行fib函数的内容，并且是之前中断的地方继续执行，\n",
    "#然后如果再次碰到关键词yield，就会中断执行，并且将yield后面的内容返回出去。\n",
    "# print(next(a))\n",
    "# print(next(a))\n",
    "# print(next(a))\n",
    "\n",
    "\n",
    "\n",
    "for i  in fib(10000):\n",
    "    if i>1000:\n",
    "        break\n",
    "    print(i)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4\n",
      "3\n"
     ]
    }
   ],
   "source": [
    "a = 3\n",
    "b = 4\n",
    "a,b = b,a\n",
    "print(a)\n",
    "print(b)\n",
    "\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4\n",
      "3\n"
     ]
    }
   ],
   "source": [
    "a = 3\n",
    "b = 4\n",
    "\n",
    "temp = a\n",
    "a = b\n",
    "\n",
    "b = temp\n",
    "print(a)\n",
    "print(b)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[1, 1, 2, 3, 5, 8, 13, 21, 34, 55]"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def abc(n):\n",
    "    a=0\n",
    "    b=1\n",
    "    num = 0\n",
    "    L = []\n",
    "    while num < n:\n",
    "#       print(b)\n",
    "#         temp = a\n",
    "#         a = b\n",
    "#         b = temp + b\n",
    "        L.append(b)\n",
    "        a,b = b,a+b\n",
    "        num = num + 1\n",
    "    return L\n",
    "\n",
    "#生成列表的方式\n",
    "\n",
    "\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1"
      ]
     },
     "execution_count": 14,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def abc(n):\n",
    "    a=0\n",
    "    b=1\n",
    "    num = 0\n",
    "    while num < n:\n",
    "        yield b\n",
    "        a,b = b,a+b\n",
    "        num = num + 1\n",
    "\n",
    "a = abc(10)\n",
    "next(a)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1"
      ]
     },
     "execution_count": 15,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "next(a)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "2"
      ]
     },
     "execution_count": 16,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "next(a)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 生成器对象，被next调用，会执行生成器函数，直到碰到yield关键词，将yield关键词后面值返回出去，并且停止运行后面的程序。直到下次在被next调用的时候，生成器函数会继续在上一次中断的后面继续执行，直到再次碰到yield关键。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "3\n",
      "5\n",
      "8\n",
      "13\n",
      "21\n",
      "34\n",
      "55\n"
     ]
    }
   ],
   "source": [
    "for item in a:\n",
    "    print(item)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n",
      "8\n",
      "13\n",
      "21\n",
      "34\n",
      "55\n",
      "89\n",
      "144\n",
      "233\n",
      "377\n",
      "610\n"
     ]
    }
   ],
   "source": [
    "for item in abc(15):\n",
    "    print(item)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n",
      "8\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "'done'"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "fib(6)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "<generator object fib at 0x000002C87A572E60>"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def fib(max):\n",
    "    n, a, b = 0, 0, 1\n",
    "    while n < max:\n",
    "        yield b\n",
    "        a, b = b, a + b\n",
    "        n = n + 1\n",
    "    return 'done'\n",
    "fib(6)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n",
      "8\n"
     ]
    }
   ],
   "source": [
    "for n in fib(6):\n",
    "    print(n)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "### 写一个生成9*9乘法表结果列表的函数，然后将这个函数变成生成器。\n",
    "* [1,2,4,3,6,9,4,8,12,16.....9,18,27,36,45,54,63,72,81]\n",
    "* 迭代器"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
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