{
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   "source": [
    "# Conditionals - flow-control structures\n",
    "\n",
    "- conditional statements control the flow of execution of codes\n",
    "    - makes some block of code to skip or execute based on some conditions\n",
    "    - loosely speaking, it helps computer think and make decision\n",
    "- boolean values - True or False\n",
    "- boolean expressions - comparison operators are used to compare values that result in True or False value."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Comparison operators\n",
    "- allow us to compare data resulting in True or False outcome\n",
    "- ==, !=, >, >=, <, <="
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "False\n",
      "True\n",
      "False\n",
      "True\n",
      "True\n",
      "False\n"
     ]
    }
   ],
   "source": [
    "# Examples of various comparison operators\n",
    "# x == y => True if x is equal to y\n",
    "# x != y => True if x is not equal to y\n",
    "# x > y => True if x is greater than y\n",
    "# x < y => True if x is less than y\n",
    "# x >= y => True if x is greater than or equal to y\n",
    "# x <= y => True if x is less than or equal to y\n",
    "x = 5\n",
    "y = 10\n",
    "print(x==y)\n",
    "print(x!=y)\n",
    "print(x>y)\n",
    "print(x<y)\n",
    "print(x<=y)\n",
    "print(x>=y)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Conditional execution\n",
    "- Three types of conditional selectors\n",
    "- One-way, Two-way and Multi-way selectors"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## One-way selector\n",
    "- just one if statement by itself\n",
    "- syntax:\n",
    "```python\n",
    "if <boolean expression> == True:\n",
    "    # execute this block of code\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "hello\n"
     ]
    }
   ],
   "source": [
    "if True:\n",
    "    print('hello')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "a equals a\n"
     ]
    }
   ],
   "source": [
    "if 'a' == 'a':\n",
    "    print('a equals to a')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Two-way selector\n",
    "- if statement followed by else statement\n",
    "- syntax:\n",
    "\n",
    "```python\n",
    "if <boolean expression> == True:\n",
    "    # execute this block of code\n",
    "else:\n",
    "    # do this\n",
    "    \n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "print 10 is NO greater than or equal to 20\n"
     ]
    }
   ],
   "source": [
    "if 10 >= 20:\n",
    "    print ('print 10 is greater than or equal to 20')\n",
    "else:\n",
    "    print ('print 10 is NO greater than or equal to 20')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "True\n"
     ]
    }
   ],
   "source": [
    "if 100.5 >= 100.5:\n",
    "    print('True')\n",
    "else:\n",
    "    print('False')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Multi-way selector\n",
    "- similar to multiple-choice questions with only one valid answer\n",
    "- start from first if statement and as soon as a if expression is evaluated True all the rest elif, else are ignored\n",
    "\n",
    "```python\n",
    "if <condition1>: \n",
    "    # block1\n",
    "    # execute this...\n",
    "elif <condition2>:\n",
    "    # block2 \n",
    "    # execute this\n",
    "... \n",
    "... \n",
    "else: \n",
    "    # else block\n",
    "    # execute code in this block\n",
    "    \n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Enter a number between 1-5:10\n",
      "You entered a number larger than 5\n"
     ]
    }
   ],
   "source": [
    "# Guess the number entered by user\n",
    "ans = int(input('Enter a number between 1-5:'))\n",
    "if ans == 1:\n",
    "    print('You entered 1')\n",
    "elif ans == 2:\n",
    "    print('You entered 2')\n",
    "elif ans == 3:\n",
    "    print('You entered 3')\n",
    "elif ans == 4:\n",
    "    print('You entered 4')\n",
    "elif ans == 5:\n",
    "    print('You entered 5')\n",
    "elif ans > 5:\n",
    "    print('You entered a number larger than 5')\n",
    "else:\n",
    "    print('I do not know what you entered! Hmm....')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Visualize using pythontutor.com: https://goo.gl/ZSm9KS"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Logical Operators\n",
    "**and**, **or**, and **not** -- allow to build more complex boolean expressions <br >\n",
    "\n",
    "**Truth table for and**\n",
    "\n",
    "| a | b  | a **and** b |\n",
    "| --- | --- | --- |\n",
    "| T | T |  T |\n",
    "|T | F |  F |\n",
    "|F | T |  F |\n",
    "|F | F |  F |\n",
    "\n",
    "\n",
    "**Truth table for or**\n",
    "\n",
    "| a | b  | a **or** b |\n",
    "| --- | --- | --- |\n",
    "| T | T |  T |\n",
    "|T | F |  T |\n",
    "|F | T |  T |\n",
    "|F | F |  F |\n",
    "\n",
    "**Truth table for not**\n",
    "\n",
    "|a  |  not a |\n",
    "|---|---|\n",
    "|T  |   F |\n",
    "|F  |   T |\n",
    "\n",
    "### Order of Evaluations: Highest to Lowest: <br>\n",
    "http://www.informit.com/articles/article.aspx?p=459269&seqNum=11"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10 is positive and even\n"
     ]
    }
   ],
   "source": [
    "num = 10\n",
    "if (num %2 ==0 and num > 0):\n",
    "    print(num, \"is positive and even\")\n",
    "    "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [],
   "source": [
    "num = -14\n",
    "if (num %2 ==0 and num < 0):\n",
    "    print(num, \"is negative and even\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "True\n"
     ]
    }
   ],
   "source": [
    "if not False:\n",
    "    print('True')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Exercises\n",
    "Exercise 1: Write a program to test whether a given whole number is even, odd or zero"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "x = input('Enter a whole number:' )\n",
    "x = int(x)\n",
    "if x == 0:\n",
    "    print(x, 'is zero')\n",
    "elif x%2 == 0:\n",
    "    print(x, 'is even')\n",
    "else:\n",
    "    print(x, 'is odd')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "Exercise 2. Improve the above program to tell whether the given whole number is positive or negative."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exercise 3. Write a program that helps someone decide where to go eat lunch depending on amount of money one has in their pocket."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exercise 4. Given a day of week as integer (say 1 for Sunday) write a program that tells whether that day is weekend, or weekday and the actual name of the day."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Nested conditionals\n",
    "```python\n",
    "if condition:\n",
    "    if condition1:\n",
    "        # do something\n",
    "    else:\n",
    "        # do something else\n",
    "        if condition2:\n",
    "            # do something...\n",
    "else:\n",
    "    # do this...\n",
    "\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exercise 5: Write a program that determines whether someone is eligible to vote in the US federal election."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exercise 6:\n",
    "<pre>\n",
    "Write a function day_name that converts an integer number 0 to 6 into the name of a day. Assume day 0 is “Sunday”. Once again, return None if the arguments to the function are not valid. Here are some tests that should pass:\n",
    "\n",
    "\n",
    "assert day_name(3) == \"Wednesday\"\n",
    "assert day_name(6) == \"Saturday\"\n",
    "assert day_name(42) == None\n",
    "</pre>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exercise 7:\n",
    "<pre>\n",
    "Write a function that helps answer questions like ‘“Today is Wednesday. I leave on holiday in 19 days time. What day will that be?”’ So the function must take a day name and a delta argument — the number of days to add — and should return the resulting day name.\n",
    "\n",
    "Here are some tests that should pass:\n",
    "assert day_add(\"Monday\", 4) ==  \"Friday\"\n",
    "assert day_add(\"Tuesday\", 0) == \"Tuesday\"\n",
    "assert day_add(\"Tuesday\", 14) == \"Tuesday\"\n",
    "assert day_add(\"Sunday\", 100) == \"Tuesday\"\n",
    "assert day_add(\"Sunday\", -1) == \"Saturday\"\n",
    "assert day_add(\"Sunday\", -7) == \"Sunday\"\n",
    "assert day_add(\"Tuesday\", -100) == \"Sunday\"\n",
    "</pre>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
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