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   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Writing Functions\n",
      "=================\n",
      "You're probably already familiar with [usingfunctions](http://nbviewer.ipython.org/urls/github.com/ethanwhite/progbio/raw/master/ipynbs/functions-using.ipynb), but how do you write you 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",
      "The `def` tells Python that we're creating a function, the `function_name` is how we refer to the function in our program, the commands control what the function does with the `inputs`, and the `return` statement tells Python what output to send back to the rest of the program. 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_lbs_to_kg(weight_lbs):\n",
      "    weight_kg = weight_lbs / 2.20462\n",
      "    return weight_kg"
     ],
     "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_lbs_to_kg(22)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 2,
       "text": [
        "9.979044007584074"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Define Functions Before Calling Them\n",
      "------------------------------------\n",
      "In the same way that we have to define a variable before we call it, we also have to define a function before we call it. So, the def statement must appear in the program before the function is used."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Local Variables\n",
      "---------------\n",
      "\n",
      "Variables created inside a Python function only exist inside that function. For example, if we define our weight conversion function, and then call it, the variable weight_kg does not exist after the function has been called."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def convert_lbs_to_kg(weight_lbs):\n",
      "    weight_kg = weight_lbs / 2.20462\n",
      "    return weight_kg\n",
      "result = convert_lbs_to_kg(170)\n",
      "print weight_kg"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "ename": "NameError",
       "evalue": "name 'weight_kg' 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/home/ethan/Dropbox/Teaching/ProgBiol/repo/ipynbs/<ipython-input-1-e6d100b8310b>\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_kg\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_lbs_to_kg\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m170\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_kg\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
        "\u001b[0;31mNameError\u001b[0m: name 'weight_kg' is not defined"
       ]
      }
     ],
     "prompt_number": 1
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Software Carpentry video on functions\n",
      "<iframe width=\"420\" height=\"315\" src=\"http://www.youtube.com/embed/gkTga4kP6uo\" frameborder=\"0\" allowfullscreen></iframe>"
     ]
    }
   ],
   "metadata": {}
  }
 ]
}