lesson_02_SRK.ipynb

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   "source": [
    "# 变量"
   ]
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   "source": [
    "python最简单的功能相当于一个计算器.\n",
    "\n",
    "比如, 眼科最简单的公式SRK公式:\n",
    "\n",
    "P = A - 0.9*K - 2.5*AL\n",
    "\n",
    "直接输入数就可以了, 要进行运算时使用shift+return运行. "
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     "text": [
      "24.900000000000006\n"
     ]
    }
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   "source": [
    "AL=23\n",
    "A=118.4\n",
    "K=40\n",
    "P = A - 0.9*K - 2.5*AL\n",
    "print(P) "
   ]
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   "source": [
    "其中AL,A,K,P这些都叫做变量, =符号并不是\"等于\", 而是被称做赋值的操作. 可以想象变量是一个盒子, 每次可以往里面装不同的东西. \n",
    "\n",
    "例如, 经常出现这样的问题, 一个病人本来用A常数为118.4的人工晶体, 现在因为某种原因, 突然需要换成A常数为119.0的人工晶体. \n",
    "\n",
    "变量A可以这样写: "
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     "text": [
      "119.0\n"
     ]
    }
   ],
   "source": [
    "A=A+0.6\n",
    "print(A)"
   ]
  },
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   "source": [
    "如果=符号是\"等于\", 那么A=A+0.6, 只能推导出A=0, 显然是不对的. \n",
    "当改变了A常数以后, 如果需要计算新的人工晶体度数"
   ]
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  {
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     "text": [
      "25.5\n"
     ]
    }
   ],
   "source": [
    "P = A - 0.9*K - 2.5*AL\n",
    "print(P)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## 练习\n",
    "\n",
    "好了, 该你动手试一试了: \n",
    "* 患者眼轴长24.5mm\n",
    "* 角膜曲率: K_1=40, K_2=42\n",
    "* 选择使用A常数=119.0 的IOL\n",
    "\n",
    "求目标度数"
   ]
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      "None\n"
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   "source": [
    "# 在此行下面修改代码\n",
    "AL=None # 将None改成需要的数值\n",
    "A=None\n",
    "K_1=None\n",
    "K_2=None\n",
    "K=None\n",
    "P=None # 将None改成需要的计算公式\n",
    "# 在此行上面修改代码\n",
    "# 不要改变此行之后的代码\n",
    "print(P)"
   ]
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  {
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   "source": [
    "参考答案: \n",
    "    20.849999999999994"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 函数"
   ]
  },
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   "cell_type": "markdown",
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   "source": [
    "如果每次计算都要把P=A - 0.9*K - 2.5*AL写一遍, 是很麻烦的事情, 于是引出一个叫做函数的东西: \n",
    "函数相当于把常用的计算过程写在一起, 方便反复使用. "
   ]
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   "source": [
    "def SRK(A,K,AL):\n",
    "    P= A - 0.9*K - 2.5*AL\n",
    "    return P"
   ]
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     "text": [
      "25.5\n"
     ]
    }
   ],
   "source": [
    "print(SRK(119.0,40,23))"
   ]
  },
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   "source": [
    "Python中定义函数的方法是: \n",
    "```python\n",
    "def 函数名(参数1, 参数2, 参数3, 参数4):\n",
    "    空4格写函数体的内容\n",
    "    return 函数值\n",
    "```\n",
    "\n",
    "比如: def IOL_power(A,K,AL):\n",
    "\n",
    "IOL_power就是函数名, A, K, AL就是参数.\n",
    "\n",
    "在召唤函数的时候, 要把赋予参数数值\n",
    "\n",
    "IOL_power(119.0,40,23)\n",
    "\n",
    "python会按照数字的顺序依次把119.0赋给A, 40赋给K, 23赋给AL"
   ]
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   "source": [
    "## 练习: \n",
    "重新定义SRK函数, 要使之能够计算两个角膜曲率K_1,K_2的情况. 其中: \n",
    "$$ K=\\frac{ K_1+K_2}{2} ,  \n",
    "\\\\\n",
    "P=A - 0.9 \\times K + 2.5 \\times AL\n",
    "$$"
   ]
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      "None\n"
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   "source": [
    "def SRK(A,K_1,K_2,AL):\n",
    "    # 在此行下面修改代码, 不要改变此行之前的代码\n",
    "    K=None\n",
    "    P=None \n",
    "    # 在此行上面修改代码, 不要改变此行之后的代码\n",
    "    return P\n",
    "print( SRK (119.0, 40, 42, 24.5) )"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "参考答案:  20.849999999999994"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 其他语法注意事项\n",
    "\n",
    "## 变量命名\n",
    "python中的变量命名有一些规矩: \n",
    "* 应当是以字母或者下划线开头的: 比如a, ABC, _A\n",
    "* 不可以是数字开头, 如果有 1=2 就很尴尬了\n",
    "\n",
    "## 格式\n",
    "python的代码格式: \n",
    "* 如果使用 # 表示注释\n",
    "* 使用缩进来表示代码块, 其他语言例如C语言, 是使用{}, python一定是要使用缩进, 不同的缩进表示代码的层级不同\n",
    "下面这段代码中: \n",
    "```python\n",
    "def SRK(A,K,AL):\n",
    "    P= A - 0.9*K - 2.5*AL\n",
    "    return P\n",
    "print(SRK(119.0,40,23))\n",
    "```\n",
    "return和print是两个不同的代码块. 对于有些强迫症患者, 这样写可能会更清晰\n",
    "```python\n",
    "def SRK(A,K,AL): #{\n",
    "    P= A - 0.9*K - 2.5*AL\n",
    "    return P\n",
    "#}\n",
    "print(SRK(119.0,40,23))\n",
    "```\n",
    "\n",
    "## python2与python3\n",
    "\n",
    "本教程是基于python 3的, python目前有两个主流版本python 2.7和python 3.6, 这两个在语法上有细微的差别, 比如\n",
    "* 在python 2中, print \"Hello World!\"\n",
    "* 在python 2中, 100/7=14, \n",
    "  * 因为整数型数据除以整数型数据还是等于整数型数据\n",
    "  * 要想得到小数, 应当写成100.0/7\n",
    "\n",
    "* 在python 3中, print(\"Hello World!\")\n",
    "* 在python 3中, 100/7=14.285714285714286\n",
    "\n",
    "我个人认为python 3用起来舒服一些, 至少100/7=14.285714285714286, 但还有可能有许多遗留的工具包是基于python 2的, 在安装和使用前一定要仔细看清楚. "
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   "source": [
    "# 目录\n",
    "* [第 0 课, 动机](./lesson_00_motivation.html)\n",
    "* [第 1 课, CoCalc](./lesson_01_jupyter.html)\n",
    "* [第 2 课, SRK公式](./lesson_02_SRK.html)\n",
    "  * [第 2 课, jupyter笔记本下载](./lesson_02_SRK.ipynb)\n",
    "* [第 3 课, SRK-II公式](./lesson_03_SRKII.html)\n",
    "  * [第 3 课, jupyter笔记本下载](./lesson_03_SRKII.ipynb)\n",
    "* [第 4 课, 一千零一个病人](./lesson_04_1001patients.html)\n",
    "  * [第 4 课, jupyter笔记本下载](./lesson_04_1001patients.ipynb)\n",
    "* [第 5 课, save 和 load](./lesson_05_pandas.html)\n",
    "  * [第 5 课, jupyter笔记本下载](./lesson_05_pandas.ipynb)\n"
   ]
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