02-python-essentials/02.14-if-statement.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 判断语句"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 基本用法"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "判断，基于一定的条件，决定是否要执行特定的一段代码，例如判断一个数是不是正数："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Hey!\n",
      "x is positive\n"
     ]
    }
   ],
   "source": [
    "x = 0.5\n",
    "if x > 0:\n",
    "    print \"Hey!\"\n",
    "    print \"x is positive\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在这里，如果 `x > 0` 为 `False` ，那么程序将不会执行两条 `print` 语句。\n",
    "\n",
    "虽然都是用 `if` 关键词定义判断，但与**C，Java**等语言不同，**Python**不使用 `{}` 将 `if` 语句控制的区域包含起来。**Python**使用的是缩进方法。同时，也不需要用 `()` 将判断条件括起来。\n",
    "\n",
    "上面例子中的这两条语句：\n",
    "```python\n",
    "    print \"Hey!\"\n",
    "    print \"x is positive\"\n",
    "```\n",
    "就叫做一个代码块，同一个代码块使用同样的缩进值，它们组成了这条 `if` 语句的主体。\n",
    "\n",
    "不同的缩进值表示不同的代码块，例如：\n",
    "\n",
    "`x > 0` 时："
   ]
  },
  {
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Hey!\n",
      "x is positive\n",
      "This is still part of the block\n",
      "This isn't part of the block, and will always print.\n"
     ]
    }
   ],
   "source": [
    "x = 0.5\n",
    "if x > 0:\n",
    "    print \"Hey!\"\n",
    "    print \"x is positive\"\n",
    "    print \"This is still part of the block\"\n",
    "print \"This isn't part of the block, and will always print.\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`x < 0` 时："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
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   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "This isn't part of the block, and will always print.\n"
     ]
    }
   ],
   "source": [
    "x = -0.5\n",
    "if x > 0:\n",
    "    print \"Hey!\"\n",
    "    print \"x is positive\"\n",
    "    print \"This is still part of the block\"\n",
    "print \"This isn't part of the block, and will always print.\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "在这两个例子中，最后一句并不是`if`语句中的内容，所以不管条件满不满足，它都会被执行。\n",
    "\n",
    "一个完整的 `if` 结构通常如下所示（注意：条件后的 `:` 是必须要的，缩进值需要一样）：\n",
    "    \n",
    "    if <condition 1>:\n",
    "        <statement 1>\n",
    "        <statement 2>\n",
    "    elif <condition 2>: \n",
    "        <statements>\n",
    "    else:\n",
    "        <statements>\n",
    "\n",
    "当条件1被满足时，执行 `if` 下面的语句，当条件1不满足的时候，转到 `elif` ，看它的条件2满不满足，满足执行 `elif` 下面的语句，不满足则执行 `else` 下面的语句。\n",
    "\n",
    "对于上面的例子进行扩展："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "x is zero\n"
     ]
    }
   ],
   "source": [
    "x = 0\n",
    "if x > 0:\n",
    "    print \"x is positive\"\n",
    "elif x == 0:\n",
    "    print \"x is zero\"\n",
    "else:\n",
    "    print \"x is negative\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`elif` 的个数没有限制，可以是1个或者多个，也可以没有。\n",
    "\n",
    "`else` 最多只有1个，也可以没有。\n",
    "\n",
    "可以使用 `and` ， `or` , `not` 等关键词结合多个判断条件："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x = 10\n",
    "y = -5\n",
    "x > 0 and y < 0"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "False"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "not x > 0"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x < 0 or y < 0"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "这里使用这个简单的例子，假如想判断一个年份是不是闰年，按照闰年的定义，这里只需要判断这个年份是不是能被4整除，但是不能被100整除，或者正好被400整除："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "This is not a leap year.\n"
     ]
    }
   ],
   "source": [
    "year = 1900\n",
    "if year % 400 == 0:\n",
    "    print \"This is a leap year!\"\n",
    "# 两个条件都满足才执行\n",
    "elif year % 4 == 0 and year % 100 != 0:\n",
    "    print \"This is a leap year!\"\n",
    "else:\n",
    "    print \"This is not a leap year.\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 值的测试"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Python**不仅仅可以使用布尔型变量作为条件，它可以直接在`if`中使用任何表达式作为条件：\n",
    "\n",
    "大部分表达式的值都会被当作`True`，但以下表达式值会被当作`False`：\n",
    "\n",
    "- False\n",
    "- None\n",
    "- 0\n",
    "- 空字符串，空列表，空字典，空集合"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The first element is: 3\n"
     ]
    }
   ],
   "source": [
    "mylist = [3, 1, 4, 1, 5, 9]\n",
    "if mylist:\n",
    "    print \"The first element is:\", mylist[0]\n",
    "else:\n",
    "    print \"There is no first element.\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "修改为空列表："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "There is no first element.\n"
     ]
    }
   ],
   "source": [
    "mylist = []\n",
    "if mylist:\n",
    "    print \"The first element is:\", mylist[0]\n",
    "else:\n",
    "    print \"There is no first element.\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "当然这种用法并不推荐，推荐使用 `if len(mylist) > 0:` 来判断一个列表是否为空。"
   ]
  }
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