{
 "metadata": {
  "name": "common-confusions"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Common points of confusion\n",
      "==========================\n",
      "Learning to program is hard and there are lots of points of confusion among beginners.\n",
      "Here are some reminders and demonstrations to help overcome some of the most common."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Functions\n",
      "---------\n",
      "\n",
      "* You have to call/run/execute a function for it to do anything.\n",
      "* You must store the output of the function to work with it (or use it immediately)\n",
      "* You must ``return`` something from the function to us it.\n",
      "Variables inside the function do not exist after the function has finished executing.\n",
      "This ``return`` is the last thing done by the function.\n",
      "\n",
      "\n",
      "### Python Tutor demo\n",
      "On it's own, the following function does nothing"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def concatenate(string1, string2):\n",
      "    combined_string = string1 + string2\n",
      "    return combined_string"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 1
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If we call it but do not do something with the output, it is lost."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "concatenate('ethan', 'white')\n",
      "print combined_string"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "ename": "NameError",
       "evalue": "name 'combined_string' is not defined",
       "output_type": "pyerr",
       "traceback": [
        "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[1;31mNameError\u001b[0m                                 Traceback (most recent call last)",
        "\u001b[1;32m<ipython-input-2-882da32f77aa>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m      1\u001b[0m \u001b[0mconcatenate\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'ethan'\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;34m'white'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m \u001b[1;32mprint\u001b[0m \u001b[0mcombined_string\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
        "\u001b[1;31mNameError\u001b[0m: name 'combined_string' is not defined"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If we store the output then we can use it, or we can use it directly."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "result = concatenate('ethan', 'white')\n",
      "print result"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "ethanwhite\n"
       ]
      }
     ],
     "prompt_number": 3
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"The name of our professor is %s\" % concatenate('ethan ', 'white')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "The name of our professor is ethan white\n"
       ]
      }
     ],
     "prompt_number": 4
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "But, this only works if we have returned the value explicitly."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def concatenate(string1, string2):\n",
      "    combined_string = string1 + string2\n",
      "\n",
      "result = concatenate('ethan', 'white')\n",
      "print result"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "None\n"
       ]
      }
     ],
     "prompt_number": 5
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If/elif/else\n",
      "------------\n",
      "Only one set of commands in an if/elif/else block will execute. If multiple sets\n",
      "of conditions are met, then the first set of conditions will be used.\n"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "a = 10\n",
      "b = 20\n",
      "if a > 15:\n",
      "    print 1\n",
      "elif a == 10 and b < 100:\n",
      "    print 2\n",
      "elif b == 20:\n",
      "    print 3\n",
      "else:\n",
      "    print 4"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "2\n"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If we wanted to actually check all of the conditions, regardless of what had\n",
      "already happened, then we would just want a series of if statements."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "a = 10\n",
      "b = 20\n",
      "if a > 15:\n",
      "    print 1\n",
      "if a == 10 and b < 100:\n",
      "    print 2\n",
      "if b == 20:\n",
      "    print 3\n",
      "else:\n",
      "    print 4"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "2\n",
        "3\n"
       ]
      }
     ],
     "prompt_number": 7
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "However, in this case the ``else`` will only apply to the last if statement."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Methods\n",
      "-------\n",
      "Methods are functions that are attached to particular objects and they operate on those objects.\n",
      "As a result, sometimes they do not require arguments (but sometimes they do)."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mystring = \"hello world\"\n",
      "print mystring.upper()\n",
      "print mystring.count('l')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "HELLO WORLD\n",
        "3\n"
       ]
      }
     ],
     "prompt_number": 8
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### List methods often return ``None``\n",
      "\n",
      "In contrast to String methods, methods that work on lists often change the list\n",
      "and return a special keyword ``None``."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mylist = [1, 3, 4, 2, 7]\n",
      "print mylist.sort()\n",
      "print mylist"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "None\n",
        "[1, 2, 3, 4, 7]\n"
       ]
      }
     ],
     "prompt_number": 9
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "This means that an easy mistake to make is to treat lists methods like we do most things\n",
      "and end up replacing our list with ``None``."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mylist = [1, 3, 4, 2, 7]\n",
      "mylist = mylist.sort()\n",
      "print mylist"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "None\n"
       ]
      }
     ],
     "prompt_number": 10
    }
   ],
   "metadata": {}
  }
 ]
}