notebooks/Introduction_to_Python_3_Control_Flow_chapter.ipynb

{"nbformat": 4, "nbformat_minor": 0, "metadata": {"colab": {"name": "Introduction_to_Python_3_Control_Flow_chapter.ipynb", "provenance": [], "collapsed_sections": []}, "kernelspec": {"name": "python3", "display_name": "Python 3"}}, "cells": [{"cell_type": "markdown", "metadata": {"id": "1Ycy34VcX4lu"}, "source": ["# Introduction to Python 3: Control Flow\n", "## Luca de Alfaro\n", "Copyright Luca de Alfaro, 2018-19.  CC-BY-NC License.\n"]}, {"cell_type": "markdown", "metadata": {"id": "2a8149c1fafdfeac", "colab_type": "text"}, "source": "\nPrepared on: Tue Aug  3 11:56:54 2021\n\nThis is a book chapter; it is not a homework assignment.  \nDo not submit it as a solution to a homework assignment; you would receive no credit.\n"}, {"cell_type": "markdown", "metadata": {"id": "PNT4q5Ks7v79"}, "source": ["## Conditionals"]}, {"cell_type": "markdown", "metadata": {"id": "bEKIjoVGqy5H"}, "source": ["You can build boolean expressions with the usual relational operators\n", "`<`, `<=`, `>`, `>=`, `==`, and `!=`. "]}, {"cell_type": "code", "metadata": {"id": "SVoA-VsT7ys_"}, "source": ["3 < 4\n"], "execution_count": 1, "outputs": [{"output_type": "execute_result", "data": {"text/plain": ["True"]}, "metadata": {"tags": []}, "execution_count": 1}]}, {"cell_type": "markdown", "metadata": {"id": "Pdvo6tfrq6JA"}, "source": ["There are also other operators.  One is `in`, to test membership \n", "in lists or dictionaries or sets or strings: \n"]}, {"cell_type": "code", "metadata": {"id": "9Fvh8i1178HH"}, "source": ["print('a' in ['a', 'b', 'c'])\n", "print('a' in 'hello my dear')\n"], "execution_count": 2, "outputs": [{"output_type": "stream", "text": ["True\n", "True\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "44QFSlKxq-G9"}, "source": ["Another one is `is` and `is not`, to check whether two things are identical, \n", "most often used for None.  Here's one way of removing None elements from a\n", "list, preserving order:\n"]}, {"cell_type": "code", "metadata": {"id": "RfhvJmk38PdR"}, "source": ["[a for a in [1, 2, 3, None, 4] if a is not None]\n"], "execution_count": 3, "outputs": [{"output_type": "execute_result", "data": {"text/plain": ["[1, 2, 3, 4]"]}, "metadata": {"tags": []}, "execution_count": 3}]}, {"cell_type": "markdown", "metadata": {"id": "GDDgcD8hrIS8"}, "source": ["The if is expressed via `if` / `elif` / `else`:\n"]}, {"cell_type": "code", "metadata": {"id": "RDIMwV2K8flt"}, "source": ["for x in range(10): # 0, 1, 2, ..., 9\n", "    if x % 2 == 0: # The % is the modulus operator.\n", "        print(x, \"is even\")\n", "    else:\n", "        print(x, \"is odd\")\n", "    if x % 3 == 0:\n", "        print(x, \"is multiple of 3\")\n", "    elif x % 3 == 1:\n", "        print(x, \"is 1 above a multiple of 3\")\n", "    else:\n", "        print(x, \"is 1 below a multiple of 3\")\n"], "execution_count": 4, "outputs": [{"output_type": "stream", "text": ["0 is even\n", "0 is multiple of 3\n", "1 is odd\n", "1 is 1 above a multiple of 3\n", "2 is even\n", "2 is 1 below a multiple of 3\n", "3 is odd\n", "3 is multiple of 3\n", "4 is even\n", "4 is 1 above a multiple of 3\n", "5 is odd\n", "5 is 1 below a multiple of 3\n", "6 is even\n", "6 is multiple of 3\n", "7 is odd\n", "7 is 1 above a multiple of 3\n", "8 is even\n", "8 is 1 below a multiple of 3\n", "9 is odd\n", "9 is multiple of 3\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "VUGfTn8lrPYh"}, "source": ["`if` / `then` / `else` can be used also as an expression, using the following syntax:\n"]}, {"cell_type": "code", "metadata": {"id": "oI5OLlZT9L9d"}, "source": ["x = -3\n", "y = x + 1 if x > 0 else -x + 1\n", "print(y)\n"], "execution_count": 5, "outputs": [{"output_type": "stream", "text": ["4\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "9CtGvd_1rXaY"}, "source": ["This can be very handy in list comprehensions.\n"]}, {"cell_type": "code", "metadata": {"id": "y-Hx6dEd9dzT"}, "source": ["[x if x % 2 == 0 else - x for x in range(10)]\n"], "execution_count": 6, "outputs": [{"output_type": "execute_result", "data": {"text/plain": ["[0, -1, 2, -3, 4, -5, 6, -7, 8, -9]"]}, "metadata": {"tags": []}, "execution_count": 6}]}, {"cell_type": "markdown", "metadata": {"id": "xNpgmyOd0gHR"}, "source": ["## Iteration"]}, {"cell_type": "markdown", "metadata": {"id": "u6k59l9NtI6p"}, "source": ["In old poor languages like Fortran and C, when you iterate, you have to \n", "have a counter, increment it, and all that stuff.  Yeech. \n", "Not so in Python.  You iterate over something that is iterable, that is, \n", "that has (or can produce) a sequence of elements.  Like... a list! \n"]}, {"cell_type": "code", "metadata": {"id": "mva7sbs-y7lR"}, "source": ["my_words = \"I like to eat pizza with anchovies, I actually do!\".split()\n", "for w in my_words:\n", "    print(\"My word is:\", w)\n"], "execution_count": 7, "outputs": [{"output_type": "stream", "text": ["My word is: I\n", "My word is: like\n", "My word is: to\n", "My word is: eat\n", "My word is: pizza\n", "My word is: with\n", "My word is: anchovies,\n", "My word is: I\n", "My word is: actually\n", "My word is: do!\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "S0pmA6pMtNgx"}, "source": ["You can also iterate over pairs, consisting of the element index and the list element:\n"]}, {"cell_type": "code", "metadata": {"id": "TVSICPjF1MHt"}, "source": ["for i, w in enumerate(my_words):\n", "    print(\"The word number\", i, \"is:\", w)\n"], "execution_count": 8, "outputs": [{"output_type": "stream", "text": ["The word number 0 is: I\n", "The word number 1 is: like\n", "The word number 2 is: to\n", "The word number 3 is: eat\n", "The word number 4 is: pizza\n", "The word number 5 is: with\n", "The word number 6 is: anchovies,\n", "The word number 7 is: I\n", "The word number 8 is: actually\n", "The word number 9 is: do!\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "zDvlzIlktRAC"}, "source": ["If you get tired of iteration, you can break out of it: \n"]}, {"cell_type": "code", "metadata": {"id": "pRbVD4DT1aEI"}, "source": ["for w in my_words:\n", "    print(w)\n", "    if w.startswith('anchovies'):\n", "        print(\"   Indeed, they are delicious, no need to say more!\")\n", "        break\n"], "execution_count": 9, "outputs": [{"output_type": "stream", "text": ["I\n", "like\n", "to\n", "eat\n", "pizza\n", "with\n", "anchovies,\n", "   Indeed, they are delicious, no need to say more!\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "dOfcNhaWtUQ2"}, "source": ["And if you need to iterate over indices, like you used to do in C? \n", "Well, you just create... a list of indices! \n"]}, {"cell_type": "code", "metadata": {"id": "9wtkml541jVB"}, "source": ["for i in range(10):\n", "    print(\"My integer is:\", i)\n"], "execution_count": 10, "outputs": [{"output_type": "stream", "text": ["My integer is: 0\n", "My integer is: 1\n", "My integer is: 2\n", "My integer is: 3\n", "My integer is: 4\n", "My integer is: 5\n", "My integer is: 6\n", "My integer is: 7\n", "My integer is: 8\n", "My integer is: 9\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "94Zkin27tYaT"}, "source": ["Well, `range(...)` gives a list in Python 2.  In Python 3, it gives \n", "an iterator, that will let you iterate on the numbers in the given \n", "range.  The point is that if you want to do a billion iterations, \n", "you may not want to build a list of a billion elements (and possibly\n", "run out of memory!). \n"]}, {"cell_type": "code", "metadata": {"id": "7qnNcRmX7MFY"}, "source": ["print(range(1, 10000000000))\n"], "execution_count": 11, "outputs": [{"output_type": "stream", "text": ["range(1, 10000000000)\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "DhwBnu1BtfHA"}, "source": ["Note that you can also iterate on list slices:\n"]}, {"cell_type": "code", "metadata": {"id": "LhzuMNq62FXn"}, "source": ["for w in my_words[:5]:\n", "    print(w)\n"], "execution_count": 12, "outputs": [{"output_type": "stream", "text": ["I\n", "like\n", "to\n", "eat\n", "pizza\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "rLJGUjp22xL5"}, "source": ["Oh btw, did you notice that we are using indentation rather than those \n", "pesky { } ?  Some people think it's silly, a throwback to Fortran and \n", "punched cards.  I think it's brilliant.  See, in C or Java you have \n", "two things: the real structure of the code (indicated by braces) and the illustrated structure (indicated by indentation).  The problem with this is that sometimes indentation and braces they differ, and when they do, the visual indication is fallacious.  In Python, the visual indication is also the structural one, and is always truthful. \n", "\n", "I am sure you prefer this to a language where there is only structure and no visuals! "]}, {"cell_type": "markdown", "metadata": {"id": "rCcgxNhPtikm"}, "source": ["If you have a dictionary, you can iterate on it like this. \n", "On keys only (because .keys() returns a view over the keys):\n"]}, {"cell_type": "code", "metadata": {"id": "05zZ65Yc2Vej"}, "source": ["n_of_paws = {'cat': 4, 'fish': 0, 'bird': 2, 'snake': 0}\n", "\n", "for k in n_of_paws.keys():\n", "    print (\"I have a\", k)\n"], "execution_count": 13, "outputs": [{"output_type": "stream", "text": ["I have a cat\n", "I have a fish\n", "I have a bird\n", "I have a snake\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "0hpVr9M-tmfx"}, "source": ["... and on key-value pairs, via .items() : \n"]}, {"cell_type": "code", "metadata": {"id": "Hee0NxlZ3ahm"}, "source": ["for k, v in n_of_paws.items():\n", "    print(\"A\", k, \"has\", v, \"paws\")\n"], "execution_count": 14, "outputs": [{"output_type": "stream", "text": ["A cat has 4 paws\n", "A fish has 0 paws\n", "A bird has 2 paws\n", "A snake has 0 paws\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "uaeqs_J1tqkl"}, "source": ["There is also a while statement, which works as usual... \n"]}, {"cell_type": "code", "metadata": {"id": "-OKmn62J9uco"}, "source": ["x = 3.\n", "while x > 1.1:\n", "    print(x)\n", "    x = x / 1.6\n", "print(\"The final result is:\", x)\n"], "execution_count": 15, "outputs": [{"output_type": "stream", "text": ["3.0\n", "1.875\n", "1.171875\n", "The final result is: 0.732421875\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "dRtVOqsBC4AJ"}, "source": ["## Exceptions"]}, {"cell_type": "markdown", "metadata": {"id": "REqC53Ccujyp"}, "source": ["When things go wrong, an exception is raised.  You can \n", "catch it and handle it like so:\n"]}, {"cell_type": "code", "metadata": {"id": "nzl7G2J6C7IR"}, "source": ["try:\n", "    x = 34 / 'a'\n", "except:\n", "    x = None\n", "print(x)\n"], "execution_count": 16, "outputs": [{"output_type": "stream", "text": ["None\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "Ht4T5Vwfunw3"}, "source": ["The module 'traceback' is very useful to figure out where \n", "exceptions happen, and what happened:\n"]}, {"cell_type": "code", "metadata": {"id": "LlWMZjRaC8DJ"}, "source": ["import traceback\n", "try:\n", "    x = 34 / 'a'\n", "except:\n", "    print(traceback.format_exc())\n"], "execution_count": 17, "outputs": [{"output_type": "stream", "text": ["Traceback (most recent call last):\n", "  File \"<ipython-input-17-5698555f1f48>\", line 3, in <module>\n", "    x = 34 / 'a'\n", "TypeError: unsupported operand type(s) for /: 'int' and 'str'\n", "\n"], "name": "stdout"}]}, {"cell_type": "markdown", "metadata": {"id": "VwtDldcVur32"}, "source": ["You can also define and raise your own exceptions, \n", "which are defined similarly to classes:\n"]}, {"cell_type": "code", "metadata": {"id": "Dw70PaQVDdV9"}, "source": ["class Indigestion(Exception):\n", "    pass\n", "\n", "def eat(m, l):\n", "    if len(l) > 2:\n", "        raise Indigestion()\n", "    return m + l\n", "\n", "l = ['eggs', 'bacon', 'peanuts']\n", "\n", "try:\n", "    eat(['bananas'], l)\n", "except Indigestion:\n", "    print(\"Hey, that's too much.\")\n"], "execution_count": 18, "outputs": [{"output_type": "stream", "text": ["Hey, that's too much.\n"], "name": "stdout"}]}]}