03-Variables.ipynb

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    "**Course Announcements**\n",
    "\n",
    "- **A1** is now available, due Mon April 12 (Week 3)\n",
    "- All assignments released and submitted through Datahub\n",
    "- **Q&A** notes from [lecture participation survey](https://forms.gle/y4gY4DYTq9aFSkC76)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "notes"
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   "source": [
    "**Q&A**\n",
    "\n",
    "Q: What is the difference between Datahub and Anaconda?  \n",
    "A: Datahub is where you will access & submit all assignments. Anaconda is a way for you to run Python and edit Jupyter notebooks on your computer - Any files you work with in Anaconda's Jupyter Notebooks are saved & accessed on your own computer (not visible to Datahub).  \n",
    "\n",
    "Q: Why can't I find my assignment/coding lab files on Anaconda?  \n",
    "A: You need to use Datahub for all assignments/coding labs. You can use Anaconda's Jupyter Notebooks to save and work on files, but to turn in work for the class it must be uploaded to Datahub.  \n",
    "\n",
    "Q: Some poll questions are confusing because I don't understand the content.  \n",
    "A: Sometimes the in-class polls are meant to check your understanding of what was just introduced. Other times, they will ask you to try something new! Don't worry if you're unable to answer some questions. However, if you are consistently lost/confused, you should probably start coming to office hours to work through your questions.  "
   ]
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  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "# Variables\n",
    "\n",
    "- definition\n",
    "- Namespace\n",
    "- Variable types"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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    }
   },
   "source": [
    "## Programming With Python"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Programming: a way to ask computer to store values (variables), and do things with them (operations).`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   },
   "outputs": [],
   "source": [
    "# This is a comment. You can write a comment by using a `#`\n",
    "my_variable = 12 \n",
    "\n",
    "my_other_variable = 13 # Comments can be 'inline', like this one"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Defining Variables"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "In programming, variables are things that store values. Variables are defined with <code>name = value</code>.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "my_var = 1  # `my_var` is a variable\n",
    "\n",
    "# This defines another variable\n",
    "other_var = 'variables are cool'\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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     "slide_type": "fragment"
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   },
   "outputs": [],
   "source": [
    "# once you create a variable it's stored in your namespace\n",
    "other_var\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Code Variables != Math Variables\n",
    "\n",
    "In mathematics: `=` refers to equality (as a statement of truth).\n",
    "\n",
    "In coding: `=` refers to assignment. "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "Math: What is x?\n",
    "\n",
    "$y = 10x + 2$"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Code: What is x?\n",
    "\n",
    "`x = x + 1`"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Poll Question #1\n",
    "\n",
    "After executing the following code, what will be the value of `my_var`?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "my_var = 2 \n",
    "\n",
    "my_var = my_var + 1\n",
    "\n",
    "print(my_var)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) 2\n",
    "- B) 3\n",
    "- C) \"my_var + 1\"\n",
    "- D) This code will fail"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Poll Question #2\n",
    "\n",
    "After executing the following code, what will be the value of `diff_var`?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   },
   "outputs": [],
   "source": [
    "diff_var = my_variabel - my_var\n",
    "\n",
    "print(diff_var)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) 4\n",
    "- B) 9\n",
    "- C) \"my_variable - my_var\"\n",
    "- D) This code will fail"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "slide"
    }
   },
   "source": [
    "### Assignment Notes\n",
    "\n",
    "- In programming `=` means assignment\n",
    "- There can be more than one assignment in a single line\n",
    "- Anything to the right of the `=` is evaluated before assignment\n",
    "    - This process proceeds from right to left"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Declaring Variables Cheat Sheet\n",
    "\n",
    "- Names are always on the left of the `=`, values are always on the right\n",
    "- Names are case sensitive\n",
    "- Variables must start with letters (or underscores)\n",
    "    - After that, they can include numbers\n",
    "    - They cannot include special characters (like &, *, #, etc)\n",
    "- Python doesn't care what you name your variables\n",
    "    - Humans do care. Pick names that describe the data / value that they store"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Reserved Words\n",
    "\n",
    "There are 33 words that are not allowed to be used for variable assignment in Python 3.6."
   ]
  },
  {
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   "metadata": {
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   },
   "source": [
    "<table type=\"text/css\">\n",
    "  <tr>\n",
    "      <td><code>False</code></td>\n",
    "      <td><code>None</code></td>\n",
    "      <td><code>True</code></td>\n",
    "      <td><code>and</code></td>\n",
    "      <td><code>as</code></td>\n",
    "      <td><code>assert</code></td>\n",
    "      <td><code>break</code></td>\n",
    "  </tr>\n",
    "  <tr>\n",
    "      <td><code>class</code></td>\n",
    "      <td><code>continue</code></td>\n",
    "      <td><code>def</code></td>\n",
    "      <td><code>del</code></td>\n",
    "      <td><code>elif</code></td>\n",
    "      <td><code>else</code></td>\n",
    "      <td><code>except</code></td>\n",
    "  </tr>\n",
    "  <tr>\n",
    "      <td><code>finally</code></td>\n",
    "      <td><code>for</code></td>\n",
    "      <td><code>from</code></td>\n",
    "      <td><code>global</code></td>\n",
    "      <td><code>if</code></td>\n",
    "      <td><code>import</code></td>\n",
    "      <td><code>in</code></td>\n",
    "  </tr>\n",
    "  <tr>\n",
    "      <td><code>is</code></td>\n",
    "      <td><code>lambda</code></td>\n",
    "      <td><code>nonlocal</code></td>\n",
    "      <td><code>not</code></td>\n",
    "      <td><code>or</code></td>\n",
    "      <td><code>pass</code></td>\n",
    "      <td><code>raise</code></td>\n",
    "  </tr>    \n",
    "  <tr>\n",
    "      <td><code>return</code></td>\n",
    "      <td><code>try</code></td>\n",
    "      <td><code>while</code></td>\n",
    "      <td><code>with</code></td>\n",
    "      <td><code>yield</code></td>\n",
    "  </tr>    \n",
    "</table>"
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {
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   "outputs": [],
   "source": [
    "# you will get an error if you try to assign a variable to one of these words\n",
    "try = 6"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Kernels"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "The <b>kernel</b> is the thing that executes your code. It is what connects the notebook (as you see it) with the part of your computer that runs code. \n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
    }
   },
   "source": [
    "Your kernel also stores your **namespace** - all the variables and code that you have declared (executed). \n",
    "\n",
    "It can be useful to clear and re-launch the kernel. You can do this from the 'kernel' drop down menu, at the top, optionally also clearing all ouputs. Note that this will erase any variables that are stored in memory. "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Namespace"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "The <b>namespace</b> is the 'place' where all your currently defined code is declared - all the things you have stored in active memory. \n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "whos?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "# You can list everything declared in the namespace with '%whos'\n",
    "%whos"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Variable Types"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "Every variable has a <b>type</b>, which refers to the kind of variable that it is, and how the computer stores that data.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "outputs": [],
   "source": [
    "# Declare a variable\n",
    "variable_name = 1\n",
    "\n",
    "# You can always ask Python 'what type is this variable' using:\n",
    "type(variable_name)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Int"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "<b>Integers</b> store whole numbers.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "my_integer = 1\n",
    "another_integer = 321"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "# integers can be signed\n",
    "yet_another_integer = -4\n",
    "type(yet_another_integer)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Float"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "<b>Floats</b> store signed, decimal-point numbers.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "my_float = 1.0\n",
    "another_float = -231.45"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "type(another_float)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### String"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "<b>Strings</b> store characters, as text. \n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "my_string = 'words, words, words'\n",
    "another_string = 'more words'\n",
    "\n",
    "# Note that strings can be defined with either '' or \"\"\n",
    "and_another = \"and some more\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "print(and_another)\n",
    "type(and_another)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Quotation Marks\n",
    "\n",
    "About those quotation marks..."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "my_string = 'This is a single-quoted string.'\n",
    "my_string"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "my_string = \"This is a double-quoted string.\"\n",
    "my_string"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Note that Python will put single quotes around it, even if you specify double quotes. \n",
    "\n",
    "A general principle is to pick something and be consistent. In this course, I'll do my best to only use single quotes."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Aside: What if you want to print a quotation mark?\n",
    "- use double quotes outside with apostraphe inside quotes\n",
    "- use an escape `\\` (backslash) before charater"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "# double quotes on outside; single quote inside\n",
    "my_string = \"i wan't to see a quote.\"\n",
    "my_string"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "# backslash to \"escape\" quotation mark\n",
    "string_quote = \"And she said, \\\"Please teach me Python!\\\"\"\n",
    "string_quote"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Boolean"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "<b>Booleans</b> store <code>True</code> or <code>False</code>. \n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "my_bool = True\n",
    "another_bool = False"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "type(another_bool)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## None"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "<code>None</code> is a special type that stores <code>None</code>, used to denote a null or empty value.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "the_concept_of_nothing = None"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "type(the_concept_of_nothing)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Poll Question #3\n",
    "\n",
    "After executing the following code, what will the type of `var_a` be?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "var_a = -17.5"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) String\n",
    "- B) Int\n",
    "- C) Float\n",
    "- D) Boolean\n",
    "- E) None"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Poll Question #4\n",
    "\n",
    "After executing the following code, what will the type of `var_b` be?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
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   "outputs": [],
   "source": [
    "var_b = '-17.5'"
   ]
  },
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   "source": [
    "- A) String\n",
    "- B) Int\n",
    "- C) Float\n",
    "- D) Boolean\n",
    "- E) None"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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   },
   "source": [
    "#### Poll Question #5\n",
    "\n",
    "After executing the following code, what will the type of the variable `m` be?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "n = 1\n",
    "a = 'm'\n",
    "m = n\n",
    "type(m)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) String\n",
    "- B) Int\n",
    "- C) Float\n",
    "- D) Boolean\n",
    "- E) None"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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   "source": [
    "## Aliases"
   ]
  },
  {
   "cell_type": "markdown",
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   "source": [
    "<div class=\"alert alert-success\">\n",
    "Variables are names assigned to a value. Values can have more than one name. \n",
    "</div>"
   ]
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   "cell_type": "code",
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   "metadata": {
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   "source": [
    "# Make a variable, and an alias\n",
    "a = 1\n",
    "b = a\n",
    "print(b)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
    }
   },
   "source": [
    "Here, the value 1 is assigned to the variable `a`.  \n",
    "\n",
    "We then make an **alias** of `a` and store that in the variable `b`. \n",
    "\n",
    "Now, the same value (1) is stored in both `a` (the original) and `b` (the alias)."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "### Reminders"
   ]
  },
  {
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   "metadata": {
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     "slide_type": "fragment"
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   },
   "source": [
    "- Multiple variables can relate to the same value(s)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Mutable vs Immutable\n",
    "\n",
    "The variable types we've talked about today are all **immutable**. This means they cannot be altered after they're created. "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   },
   "outputs": [],
   "source": [
    "immutable_string = 'COGS18 is the best!'\n",
    "immutable_string[4]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": false,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "# cannot change part of the string after creation\n",
    "immutable_string[4] = '0'"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Python does have **mutable** types. We'll talk about these later in the course, and these are where aliasing shines!"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Indentation\n",
    "\n",
    "Just a *brief* word on indentation.\n",
    "\n",
    "Python *does* care about whitespace. \n",
    "\n",
    "You will get an error if Python runs into unanticipated whitespace."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "a = 1\n",
    "    b = a\n",
    "    \n",
    "    print(b) "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "There *are* times when indentation will be required and expected. We'll discuss these in future lectures."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "\n",
    "<center><img src=\"img/doing_it_right.png\" width=\"900px\"></center>\n"
   ]
  }
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