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   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Writing Functions\n",
      "=================\n",
      "\n",
      "We've already been using functions, but how do you write them on your own?\n",
      "\n",
      "In Python we do this using the following structure:\n",
      "\n",
      "    def function_name(inputs):\n",
      "        commands\n",
      "        return output_value\n",
      "\n",
      "* `def` tells Python that we're creating a function\n",
      "* `function_name` is how we refer to the function in our program\n",
      "* the commands control what the function does with the `inputs`\n",
      "* `return` tells Python what output to send back to the rest of the program.\n",
      "\n",
      "Then indentation is important because it tells Python what is inside the function. You end a function by stopping the indenting.\n",
      "\n",
      "For example, if we wanted to write a function that converts pounds to kilograms:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": true,
     "input": [
      "def convert_kg_to_g(weight_kg):\n",
      "    weight_g = weight_kg * 1000\n",
      "    return weight_g"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 1
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "We can then call that function exactly like we would one of Python's built in functions:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "convert_kg_to_g(2.2)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 2,
       "text": [
        "2200.0"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "heading",
     "level": 2,
     "metadata": {},
     "source": [
      "Functions can have multiple inputs"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Let's say we've fit a linear regression to some data and want to get a prediction based on the fitted parameters."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def predicted_value(a, b, x):\n",
      "    prediction = a + b * x\n",
      "    return prediction\n",
      "\n",
      "predicted_value(2, 1, 1)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 3,
       "text": [
        "3"
       ]
      }
     ],
     "prompt_number": 3
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Define Functions Create Instructions\n",
      "------------------------------------\n",
      "\n",
      "* Defining a function creates a set of instructions, but doesn't do anything with them\n",
      "* Like describing to a friend how to make a PB&J sandwhich\n",
      "* When you call the function the computer follows those instructions and creates a value\n",
      "* If you want to do something with that value you need to store it in a variable\n",
      "* Functions need to be defined before they are called."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Local Variables\n",
      "---------------\n",
      "\n",
      "Variables created inside a Python function only exist inside that function."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def convert_kg_to_g(weight_kg):\n",
      "    weight_g = weight_kg * 1000\n",
      "    return weight_g\n",
      "result = convert_kg_to_g(3.9)\n",
      "print weight_g"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "ename": "NameError",
       "evalue": "name 'weight_g' is not defined",
       "output_type": "pyerr",
       "traceback": [
        "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mNameError\u001b[0m                                 Traceback (most recent call last)",
        "\u001b[0;32m<ipython-input-4-e642eed7c18d>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      3\u001b[0m     \u001b[0;32mreturn\u001b[0m \u001b[0mweight_g\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      4\u001b[0m \u001b[0mresult\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mconvert_kg_to_g\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m3.9\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 5\u001b[0;31m \u001b[0;32mprint\u001b[0m \u001b[0mweight_g\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
        "\u001b[0;31mNameError\u001b[0m: name 'weight_g' is not defined"
       ]
      }
     ],
     "prompt_number": 4
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "You should also treat functions as if they don't know about variables outside of them, so they should only work with things that you pass them as input."
     ]
    }
   ],
   "metadata": {}
  }
 ]
}