2. Control flow.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Control Flow"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<h3>Control flow statements allow you to specify the order in which statements are being executed, for instance by using loops or choices based on some condition(s).</h3>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# If statement"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```\n",
    "if <condition1>:\n",
    "    <statements1>\n",
    "elif <condition2>:   # This is optional. Also, multiple\n",
    "    <statements2>    # elif clauses are possible.\n",
    "else <condition3>:   # The else clause is optional as well.\n",
    "    <statements3>\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "x is smaller than 10.\n"
     ]
    }
   ],
   "source": [
    "x = 5\n",
    "if x < 10:\n",
    "    print('x is smaller than 10.')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "x modulo two is not zero.\n",
      "Therefore, x is odd!\n"
     ]
    }
   ],
   "source": [
    "if x % 2 == 0:\n",
    "    print('x modulo two is zero.')\n",
    "    print('Therefore, x is even!')\n",
    "else:\n",
    "    print('x modulo two is not zero.')\n",
    "    print('Therefore, x is odd!')\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Goedemiddag!\n"
     ]
    }
   ],
   "source": [
    "language = 'Dutch'\n",
    "if language == 'English':\n",
    "    print('Good afternoon!')\n",
    "elif language == 'Dutch':\n",
    "    print('Goedemiddag!')\n",
    "elif language == 'French':\n",
    "    print('Bonjour!')\n",
    "else:\n",
    "    print('Sorry, I do not speak your language...')\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Loops"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "## for loop: iterate over a sequence"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```\n",
    "for <variable_name> in <sequence_name>:\n",
    "    <statements>\n",
    "else:                                    # Optional\n",
    "    <statements>\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "2\n",
      "3\n"
     ]
    }
   ],
   "source": [
    "for i in range(1,4):\n",
    "    print(i)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "string\n",
      "True\n",
      "3.1415\n"
     ]
    }
   ],
   "source": [
    "for item in [1, 'string', True, 3.1415]:\n",
    "    print(item)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### Looping over dictionaries"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "three\n",
      "two\n",
      "one\n"
     ]
    }
   ],
   "source": [
    "mydict = {\"one\":\"één\", \"two\":\"twee\", \"three\":\"drie\"}\n",
    "\n",
    "for k in mydict:\n",
    "    print(k)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "three is drie in Dutch.\n",
      "two is twee in Dutch.\n",
      "one is één in Dutch.\n"
     ]
    }
   ],
   "source": [
    "for key, value in mydict.items():\n",
    "    print(\"{} is {} in Dutch.\".format(key, value))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### Using the index of elements in a loop using enumerate"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Character 0 of Python is P.\n",
      "Character 1 of Python is y.\n",
      "Character 2 of Python is t.\n",
      "Character 3 of Python is h.\n",
      "Character 4 of Python is o.\n",
      "Character 5 of Python is n.\n"
     ]
    }
   ],
   "source": [
    "myword = \"Python\"\n",
    "\n",
    "for index, char in enumerate(myword):\n",
    "    print(\"Character {} of {} is {}.\".format(index, myword, char))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "(0, 0) : 0\n",
      "(0, 1) : 1\n",
      "(0, 2) : 2\n",
      "(1, 0) : 3\n",
      "(1, 1) : 4\n",
      "(1, 2) : 5\n",
      "(2, 0) : 6\n",
      "(2, 1) : 7\n",
      "(2, 2) : 8\n"
     ]
    }
   ],
   "source": [
    "r = range(9)\n",
    "\n",
    "# Assume we want to convert r into a 3x3 matrix.\n",
    "for index, value in enumerate(r):\n",
    "    row = index // 3\n",
    "    col = index % 3\n",
    "    print(\"({}, {}) : {}\".format(row, col, value))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### Looping over multiple sequences at the same time"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The sum of [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] and [10, 11, 12, 13, 14, 15, 16, 17, 18, 19] is [10, 12, 14, 16, 18, 20, 22, 24, 26, 28].\n"
     ]
    }
   ],
   "source": [
    "vector1 = range(10)\n",
    "vector2 = range(10,20)\n",
    "result  = []\n",
    "\n",
    "# Calculate the element-wise sum of the vectors.\n",
    "for v1, v2 in zip(vector1, vector2):\n",
    "    result.append(v1 + v2)\n",
    "    \n",
    "print(\"The sum of {} and {} is {}.\".format(list(vector1), list(vector2), result))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### Looping over a sorted or reversed version of a sequence"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4\n",
      "3\n",
      "2\n",
      "1\n",
      "0\n"
     ]
    }
   ],
   "source": [
    "for value in reversed(range(5)):\n",
    "    print(value)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "2\n",
      "3\n",
      "6\n",
      "8\n"
     ]
    }
   ],
   "source": [
    "for value in sorted([ 6, 1, 3, 8 ,2]):\n",
    "    print(value)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "## while loop: iterate until some condition is no longer valid"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```\n",
    "while <condition>:\n",
    "    <statements>\n",
    "else:                # Optional\n",
    "    <statements>\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "2\n",
      "3\n",
      "4\n"
     ]
    }
   ],
   "source": [
    "x = 1\n",
    "while x < 5:\n",
    "    print(x)\n",
    "    x += 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "# This is dangerous, do not try to execute this in a IPython Notebook!\n",
    "#while True:\n",
    "#    print(\"Stuck in loop!\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Loop control"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### break: completely break out of the loop"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "2\n"
     ]
    }
   ],
   "source": [
    "for n in range(1,5):\n",
    "    if n == 3:\n",
    "        break\n",
    "    print(n)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### continue: break out of this iteration of the loop and continue with the next one"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "2\n",
      "4\n"
     ]
    }
   ],
   "source": [
    "for n in range(1,5):\n",
    "    if n == 3:\n",
    "        continue\n",
    "    print(n)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 22,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Found an even number 2\n",
      "Found a number 3\n",
      "Found an even number 4\n",
      "Found a number 5\n",
      "Found an even number 6\n",
      "Found a number 7\n",
      "Found an even number 8\n",
      "Found a number 9\n"
     ]
    }
   ],
   "source": [
    "for num in range(2, 10):\n",
    "    if num % 2 == 0:\n",
    "        print(\"Found an even number\", num)\n",
    "        continue\n",
    "    print(\"Found a number\", num)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### break and else: check whether we \"completed\" the loop or used \"break\" to kill the loop"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Checking item: 1\n",
      "Checking item: 2\n",
      "Checking item: 3\n",
      "Checking item: TEXT\n",
      "Found your item in the list!\n"
     ]
    }
   ],
   "source": [
    "mylist   = [1, 2, 3, 'TEXT', 4, 5, 6]\n",
    "look_for = 'TEXT'\n",
    "for item in mylist:\n",
    "    print('Checking item:', item)\n",
    "    if item == look_for:\n",
    "        print('Found your item in the list!')\n",
    "        break\n",
    "else:\n",
    "    print('Processed all items in the list, but could not find the item you are looking for!')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "source": [
    "### Advanced example using a break and else"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2 is a prime number\n",
      "3 is a prime number\n",
      "4 equals 2 * 2.0\n",
      "5 is a prime number\n",
      "6 equals 2 * 3.0\n",
      "7 is a prime number\n",
      "8 equals 2 * 4.0\n",
      "9 equals 3 * 3.0\n"
     ]
    }
   ],
   "source": [
    "for n in range(2, 10):\n",
    "    for x in range(2, n):\n",
    "        if n % x == 0:\n",
    "            print(n, 'equals', x, '*', n/x)\n",
    "            break\n",
    "    else:\n",
    "        # loop fell through without finding a factor\n",
    "        print(n, 'is a prime number')"
   ]
  }
 ],
 "metadata": {
  "celltoolbar": "Slideshow",
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.4.0"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 0
}