machinelearning_notebook/0_python/05 Control Flow.ipynb

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   "source": [
    "All the IPython Notebooks in this lecture series are available at https://github.com/rajathkumarmp/Python-Lectures"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Control Flow Statements"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## If"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "if some_condition:\n",
    "    \n",
    "    algorithm"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Hello\n"
     ]
    }
   ],
   "source": [
    "x = 12\n",
    "if x >10:\n",
    "    print(\"Hello\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## If-else"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "if some_condition:\n",
    "    \n",
    "    algorithm\n",
    "    \n",
    "else:\n",
    "    \n",
    "    algorithm"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "hello\n"
     ]
    }
   ],
   "source": [
    "x = 12\n",
    "if x > 10:\n",
    "    print(\"hello\")\n",
    "else:\n",
    "    print(\"world\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## if-elif"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "if some_condition:\n",
    "    \n",
    "    algorithm\n",
    "\n",
    "elif some_condition:\n",
    "    \n",
    "    algorithm\n",
    "\n",
    "else:\n",
    "    \n",
    "    algorithm"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "x<y\n"
     ]
    }
   ],
   "source": [
    "x = 10\n",
    "y = 12\n",
    "if x > y:\n",
    "    print(\"x>y\")\n",
    "elif x < y:\n",
    "    print(\"x<y\")\n",
    "else:\n",
    "    print(\"x=y\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "if statement inside a if statement or if-elif or if-else are called as nested if statements."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "x<y\n",
      "x=10\n"
     ]
    }
   ],
   "source": [
    "x = 10\n",
    "y = 12\n",
    "if x > y:\n",
    "    print(\"x>y\")\n",
    "elif x < y:\n",
    "    print(\"x<y\")\n",
    "    if x==10:\n",
    "        print(\"x=10\")\n",
    "    else:\n",
    "        print(\"invalid\")\n",
    "else:\n",
    "    print(\"x=y\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Loops"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### For"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "for variable in something:\n",
    "    \n",
    "    algorithm"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0\n",
      "1\n",
      "2\n",
      "3\n",
      "4\n"
     ]
    }
   ],
   "source": [
    "for i in range(5):\n",
    "    print(i)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "In the above example, i iterates over the 0,1,2,3,4. Every time it takes each value and executes the algorithm inside the loop. It is also possible to iterate over a nested list illustrated below."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 2, 3]\n",
      "[4, 5, 6]\n",
      "[7, 8, 9]\n"
     ]
    }
   ],
   "source": [
    "list_of_lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]\n",
    "for list1 in list_of_lists:\n",
    "    print(list1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "A use case of a nested for loop in this case would be,"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "2\n",
      "3\n",
      "4\n",
      "5\n",
      "6\n",
      "7\n",
      "8\n",
      "9\n"
     ]
    }
   ],
   "source": [
    "list_of_lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]\n",
    "for list1 in list_of_lists:\n",
    "    for x in list1:\n",
    "        print(x)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### While"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "while some_condition:\n",
    "    \n",
    "    algorithm"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "4\n",
      "Bye\n"
     ]
    }
   ],
   "source": [
    "i = 1\n",
    "while i < 3:\n",
    "    print(i ** 2)\n",
    "    i = i+1\n",
    "print('Bye')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Break"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "As the name says. It is used to break out of a loop when a condition becomes true when executing the loop."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0\n",
      "1\n",
      "2\n",
      "3\n",
      "4\n",
      "5\n",
      "6\n",
      "7\n"
     ]
    }
   ],
   "source": [
    "for i in range(100):\n",
    "    print i\n",
    "    if i>=7:\n",
    "        break"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Continue"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This continues the rest of the loop. Sometimes when a condition is satisfied there are chances of the loop getting terminated. This can be avoided using continue statement. "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0\n",
      "1\n",
      "2\n",
      "3\n",
      "4\n",
      "The end.\n",
      "The end.\n",
      "The end.\n",
      "The end.\n",
      "The end.\n"
     ]
    }
   ],
   "source": [
    "for i in range(10):\n",
    "    if i>4:\n",
    "        print(\"The end.\")\n",
    "        continue\n",
    "    elif i<7:\n",
    "        print(i)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## List Comprehensions"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Python makes it simple to generate a required list with a single line of code using list comprehensions. For example If i need to generate multiples of say 27 I write the code using for loop as,"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[27, 54, 81, 108, 135, 162, 189, 216, 243, 270]\n"
     ]
    }
   ],
   "source": [
    "res = []\n",
    "for i in range(1,11):\n",
    "    x = 27*i\n",
    "    res.append(x)\n",
    "print res"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Since you are generating another list altogether and that is what is required, List comprehensions is a more efficient way to solve this problem."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[27, 54, 81, 108, 135, 162, 189, 216, 243, 270]"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "[27*x for x in range(1,11)]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "That's it!. Only remember to enclose it in square brackets"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Understanding the code, The first bit of the code is always the algorithm and then leave a space and then write the necessary loop. But you might be wondering can nested loops be extended to list comprehensions? Yes you can."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[27, 54, 81, 108, 135, 162, 189, 216, 243, 270]"
      ]
     },
     "execution_count": 13,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "[27*x for x in range(1,20) if x<=10]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Let me add one more loop to make you understand better, "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[27, 54, 81, 108, 135, 162, 189, 216, 243, 270]"
      ]
     },
     "execution_count": 14,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "[27*z for i in range(50) if i==27 for z in range(1,11)]"
   ]
  }
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