control_flow.ipynb.txt

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 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "heading",
     "level": 1,
     "metadata": {},
     "source": [
      "Control flow"
     ]
    },
    {
     "cell_type": "heading",
     "level": 2,
     "metadata": {},
     "source": [
      "Truth"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Python has reserved words for booleans 'True' and 'False'"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"True is\", True # 'True' always evaluates to True\n",
      "print \"False is\", False # 'False' always evaluates to False"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "We can combine booleans with the reserved keyword 'and'.\n",
      "'and' checks to see whether _both_ expressions are True."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"True and True are\", True and True\n",
      "print \"True and False are\", True and False\n",
      "print \"False and True are\", False and True\n",
      "print \"False and False are\", False and False"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "We can also combine booleans with the reserved keyword 'or'.\n",
      "'or' checks to see whether _either_ expression is True."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"True or True are,\", True or True\n",
      "print \"True or False are,\" True or False\n",
      "print \"False or True are,\" False or True\n",
      "print \"False and False are,\" False and False"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "We can test whether something is true and use this to decide whether to evaluate code.  Make sure to follow the 'if' statement with a colon and indent on the next line."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "x = True\n",
      "y = False\n",
      "\n",
      "if x:\n",
      "    print \"x must have been true.\"\n",
      "\n",
      "if y:\n",
      "    print \"y must have been true.\""
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "When the 'if' statement evaluates to False, the block of code is skipped.\n",
      "The 'else' statement will be evaluated if the 'if' statement was False."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "if x:\n",
      "    print \"x must have been true.\"\n",
      "else:\n",
      "    print \"x must have been false.\""
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "You can negate an expression with the reserved keyword 'not'."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "if not x:\n",
      "    print \"x must have been false.\"\n",
      "else:\n",
      "    print \"x must have been true.\"\n",
      "    \n",
      "if not y:\n",
      "    print \"y must have been false.\"\n",
      "else:\n",
      "    print \"y must have been true.\""
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Challenge:\n",
      "1. Try these first in your head (or on paper), then test them in ipython (notebook or interpreter, your choice).\n",
      "    ```\n",
      "    False and True and False # think about what order of operations is doing here\n",
      "    not(True or not(True))\n",
      "    not(not(False) and not(not(True)))\n",
      "    ```"
     ]
    },
    {
     "cell_type": "heading",
     "level": 2,
     "metadata": {},
     "source": [
      "Conditional expressions"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Test for equality."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"1 == 1 is\", 1 == 1 # equal to\n",
      "print \"'dog' == 'dog' is\", 'dog' == 'dog'"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "This is not the same as identity!"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "x = 'I really enjoy python and I plan to use it on a regular basis in the future.'\n",
      "y = 'I really enjoy python and I plan to use it on a regular basis in the future.'\n",
      "print \"x is y is\", x is y\n",
      "print \"x == y is\", x == y"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Syntax is realy important, = is for assignment, == tests equality"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"1 = 1 is\", 1 = 1"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "We can test for inequality using != (like ~= in MATLAB) or 'not'."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"1 != 1 is\", 1 != 1 # not equal to\n",
      "print \"not (1 == 1) is\", not(1 == 1) # not equal to"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Floating point precision can sometimes cause some headaches."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "x = 1.2 - 1.0\n",
      "y = 0.2\n",
      "print \"x == y is\", x == y"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "1.2 - 1.0"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "We can also use numerical comparisons."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"2 > 1 is\", 2 > 1\n",
      "print \"2 < 1 is\", 2 < 1\n",
      "print \"2 >= 1 is\", 2 >= 1\n",
      "print \"1 <= 2 is\", 1 <= 2"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "And you can use these comparisons on strings too.\n",
      "`>, <, >=, <=` tests use ascii numerical order (for ascii strings).\n",
      "You can find out more [here](http://www.asciitable.com/), but in general for strings:\n",
      "\n",
      "0-9 < UPPERCASE < lowercase"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print \"'abc' > 'acb' is\", 'abc' > 'acb'\n",
      "print \"'Aron' > 'Billy' is\", 'Aron' > 'Billy'\n",
      "print \"'Josh' > 'Billy' is\", 'Josh' > 'Billy'\n",
      "\n",
      "print \"'A' < 'a' is\", 'A' < 'a'"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Challenge\n",
      "How could you test (in one line) whether a variable x is in [0, 10)?  (Hint:  There is more than one way to do this.)"
     ]
    },
    {
     "cell_type": "heading",
     "level": 2,
     "metadata": {},
     "source": [
      "Tying it all together - Fizzbuzz"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "The modulus operator (modulo) gives you the remainder after division."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print '12 mod 10 =', 12 % 10\n",
      "print '8 mod 4 =', 8 % 4"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If you want to test for more than one thing, serially, you can use the `elif` keyword (short for else if)."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def fizzbuzz(n):\n",
      "\t'''\n",
      "\tFor the numbers \n",
      "\t\n",
      "\tFor an input integer, n, print the numbers from\n",
      "\t1 to n, replacing multiples of 3 with \"Fizz\", \n",
      "\tmultiples of 5 with \"Buzz\", and multiples of\n",
      "\tboth with \"FizzBuzz\".\n",
      "\t'''\n",
      "\t\n",
      "\tfor number in range(1, n+1):\n",
      "\t\tif (number % 3 == 0) and (number % 5 == 0):\n",
      "\t\t\tprint \"FizzBuzz!\"\n",
      "\t\telif (number % 3 == 0):\n",
      "\t\t\tprint \"Fizz!\"\n",
      "\t\telif (number % 5 == 0):\n",
      "\t\t\tprint \"Buzz!\"\n",
      "\t\telse:\n",
      "\t\t\tprint number"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "heading",
     "level": 2,
     "metadata": {},
     "source": [
      "Simple 'tests' using assertions"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Sometimes we might want to test our inputs before starting some sort of time-consuming operation.  A quick way to test for this is to use an assertion."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def fahr_to_cel(fahr):\n",
      "    '''Input fahrenheit temperature, output celsious temperature.'''\n",
      "    return (fahr-32) * 5/9.0"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "This is our old friend, the Fahrenheit to Celsius conversion.  It isn't computationally expensive, but let's pretend that it is."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print '98.6 in F is ', fahr_to_cel(98.6), 'in C'"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print '\"dog\" in in F is', fahr_to_cel(\"dog\"), 'in C'"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "It fails for strings (no big surprise).  Let's re-write the function, but this time we'll test to make sure the input is numerical.  First, we can use the `assert` statement."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "assert type('hello') is str\n",
      "assert type(4) is int\n",
      "assert type(4.0) is float"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "`assert` will test whether the following statement is True.  If it is True, nothing happens (like above).  Let's see what happens if the statement is False."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "assert type(4) is str"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "So let's add an assert to our script to check for non-numerical input."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def fahr_to_cel(fahr):\n",
      "    '''Input fahrenheit temperature, output celsious temperature.'''\n",
      "    assert (type(fahr) is int) or (type(fahr) is float)\n",
      "    return (fahr-32) * 5/9.0"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "fahr_to_cel('dog')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If we ever need to use an assert to compare floating point numbers, we should use a tolerance."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "x = 1.2 - 1.0\n",
      "y = 0.2\n",
      "\n",
      "assert (x == y)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "epsilon = 1e-10\n",
      "\n",
      "assert x-y < epsilon"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [],
     "language": "python",
     "metadata": {},
     "outputs": []
    }
   ],
   "metadata": {}
  }
 ]
}