05-Functions.ipynb

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    "# Functions\n",
    "\n",
    "- defining a function\n",
    "    - `def`\n",
    "    - `return`\n",
    "    - default values\n",
    "    - keyword vs. positional arguments\n",
    "- executing a function\n",
    "    - parameters\n",
    "    - separate namespace"
   ]
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   "source": [
    "## Vocab\n",
    "\n",
    "- **Define/create/make** a function: To set up and write the instructions for a function.\n",
    "- **Execute/call/use** a function: To actually use the pre-defined function.\n",
    "\n",
    "- **Input/parameter/argument**: A variable defined by the user that is put/passed in between the parantheses `()` that comes after the function name.  \n",
    "- **Output**: The variable that is `return`ed to the user after the function is executed."
   ]
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   "source": [
    "## Functions\n",
    "\n",
    "<div class=\"alert alert-success\">\n",
    "A function is a re-usable piece of code that performs operations on a specified set of variables, and returns the result.\n",
    "</div>"
   ]
  },
  {
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   "source": [
    "![cheeseburger](img/cheeseburger.png)"
   ]
  },
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   "source": [
    "## Modular Programming\n",
    "\n",
    "<div class=\"alert alert-success\">\n",
    "Modular programming is an approach to programming that focuses on building programs from independent modules ('pieces'). \n",
    "</div>"
   ]
  },
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   "source": [
    "Copy + Pasting the same/similar bit of code is to be avoided.\n",
    "\n",
    "**Functions** are one way to avoid this.\n",
    "\n",
    "**Loops** are another! (we'll get to these soon...)"
   ]
  },
  {
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   "source": [
    "## Functions for Modular Programming"
   ]
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    "- Functions allow us to flexibly re-use pieces of code\n",
    "- Each function is independent of every other function, and other pieces of code\n",
    "- Functions are the building blocks of programs, and can be flexibly combined and executed in specified orders\n",
    "    - This allows us to build up arbitrarily complex, well-organized programs"
   ]
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   "source": [
    "# you've seen functions before\n",
    "# here we use the type() function\n",
    "my_var = 3\n",
    "type(my_var)"
   ]
  },
  {
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   "source": [
    "# the function print() doesn't depend on type()\n",
    "# but they can both be used on the same variable\n",
    "print(my_var)"
   ]
  },
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   "source": [
    "## Function Example I\n",
    "\n",
    "When you use `def`, **you are defining a function**.\n",
    "You are metaphorically writing the instructions for how to make the cheeseburger."
   ]
  },
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   "source": [
    "# define a function: double_value\n",
    "# Notice that the parameter `num` is not explicitly defined with an = \n",
    "# This is because it will be defined later by the user when they execute the function.\n",
    "def double_value(num):\n",
    "\n",
    "    # do some operation\n",
    "    doubled = num + num\n",
    "    \n",
    "    # return output from function\n",
    "    return doubled    "
   ]
  },
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   "source": [
    "# excecute a function by calling the function's name\n",
    "# and defining the parameter within the parentheses\n",
    "double_value(num = 2) "
   ]
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   "source": [
    "# equivalent function call\n",
    "# Here the parameter `num` is defined without \n",
    "# explicitly specifying the name of the parameter\n",
    "double_value(2) "
   ]
  },
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   "source": [
    "## Function Example II\n",
    "\n",
    "Here we are defining a function with multiple parameters"
   ]
  },
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   "source": [
    "def add_two_numbers(num1, num2):\n",
    "    \n",
    "    # Do some operations with the parameters\n",
    "    answer = num1 + num2\n",
    "    \n",
    "    # Return the variable answer\n",
    "    return answer"
   ]
  },
  {
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   "source": [
    "add_two_numbers(5, 14)"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "# Execute our function again on some other inputs to double check \n",
    "# that our function is working how we think it should\n",
    "output = add_two_numbers(-1, 4)\n",
    "print(output)"
   ]
  },
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   "source": [
    "## Function Properties"
   ]
  },
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   "source": [
    "- Functions are defined using `def` followed by the name of the function, parentheses `()`, parameters within the parentheses, and then `:` after the parentheses, which opens a code-block that comprises the function\n",
    "    - Running code with a `def` block *defines* the function (but does not *execute* it)"
   ]
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   "source": [
    "- Functions are *executed* with the name of the function and parentheses - `()` without `def` and `:`\n",
    "    - This is when the code inside a function is actually run"
   ]
  },
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   "source": [
    "- Inside a function, there is code that performs operations on the available variables"
   ]
  },
  {
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   "source": [
    "- Functions use the special operator `return` to exit the function, passing out any specified variables"
   ]
  },
  {
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   "source": [
    "- When you use a function, you can assign the output (whatever is `return`ed) to a variable"
   ]
  },
  {
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    "jupyter": {
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   },
   "source": [
    "#### Clicker Question #1\n",
    "Given the function defined below, what will the second code cell below return?\n",
    "\n",
    "A) 0   &emsp;B) 2    &emsp;C) 4    &emsp;D) '2r.2 + 1'    &emsp;E) ¯\\\\\\_(ツ)\\_/¯"
   ]
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   "source": [
    "def remainder(number, divider):\n",
    "    \n",
    "    r = number % divider\n",
    "    \n",
    "    return r"
   ]
  },
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   "source": [
    "ans_1 = remainder(12, 5)\n",
    "ans_2 = remainder(2, 2)\n",
    "\n",
    "print(ans_1 + ans_2)"
   ]
  },
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   "source": [
    "#### Clicker Question #2\n",
    "\n",
    "Write a function `greet` that takes the parameter `name`. Inside the function, concatenate 'Hello', the person's name, and 'Good morning!\". Assign this to `output` and return `output`.\n",
    "\n",
    "A) I did it! &emsp;B) I think I did it. &emsp;C) I tried but I am stuck. &emsp;D) Super duper lost."
   ]
  },
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   "outputs": [],
   "source": [
    "## YOUR CODE HERE "
   ]
  },
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   "cell_type": "code",
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   "source": [
    "# TEST YOUR FUNCTION HERE"
   ]
  },
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   "source": [
    "## Default Values"
   ]
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   "source": [
    "<div class=\"alert alert-success\">\n",
    "Function parameters can also take <b>default values</b>. This makes some parameters optional, as they take a default value if not otherwise specified.\n",
    "</div>"
   ]
  },
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   "source": [
    "#### Default Value Functions\n",
    "\n",
    "Specify a default value in a function by doing an assignment within the function definition."
   ]
  },
  {
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   "source": [
    "# Create a function, that has a default values for a parameter\n",
    "def exponentiate(number, exponent=2):  \n",
    "    return number ** exponent"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "# Use the function, using default value\n",
    "exponentiate(3)"
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {
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   "outputs": [],
   "source": [
    "# Call the function, over-riding default value with something else\n",
    "# python assumes values are in order of parameters specified in definition\n",
    "exponentiate(2, 3)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   "outputs": [],
   "source": [
    "# you can always state this explicitly\n",
    "exponentiate(number=2, exponent=3)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Positional vs. Keyword Arguments"
   ]
  },
  {
   "cell_type": "markdown",
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     "slide_type": "fragment"
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   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "Arguments to a function can be indicated by either position or keyword.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "# Positional arguments use the position to infer which argument each value relates to\n",
    "exponentiate(2, 3)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "# Keyword arguments are explicitly named as to which argument each value relates to\n",
    "exponentiate(number=2, exponent=3)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "exponentiate(exponent=3, number=2)"
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "# Note: once you have a keyword argument, you can't have other positional arguments afterwards\n",
    "# this cell will produce an error\n",
    "exponentiate(number=2, 3)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "Reminder, setting a default value for parameters is allowed during function *definition*.\n",
    "\n",
    "(This may look like what we did above, but here we are including a default value for one parameter during function definition. During function *execution*, you can't mix and match using positional vs. keywords)"
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "def exponentiate(number, exponent=2):    \n",
    "    return number ** exponent"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Clicker Question #3\n",
    "\n",
    "What will the following code snippet print?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   "source": [
    "def exponentiate(number, exponent=2):    \n",
    "    return number ** exponent\n",
    "\n",
    "exponentiate(exponent=3, number=2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
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   "source": [
    "- A) 8\n",
    "- B) 9\n",
    "- C) SyntaxError\n",
    "- D) None\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "Note: when using Keyword arguments position/order no longer matters"
   ]
  },
  {
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   "source": [
    "## Code Style: Functions\n",
    "\n",
    "- Function names should be snake_case \n",
    "- Function names should describe task accomplished by function\n",
    "- Separate out logical sections within a function with new lines\n",
    "- Arguments should be separated by a comma and a space\n",
    "- Default values do NOT need a space around the `=`"
   ]
  },
  {
   "cell_type": "markdown",
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   },
   "source": [
    "### Functions: Good Code Style"
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {
    "slideshow": {
     "slide_type": "-"
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   "outputs": [],
   "source": [
    "def remainder(number, divider=2):\n",
    "    \n",
    "    r = number % divider\n",
    "    \n",
    "    return r"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "### Functions: Code Style to Avoid"
   ]
  },
  {
   "cell_type": "code",
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    "slideshow": {
     "slide_type": "-"
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   "outputs": [],
   "source": [
    "# could be improved by adding empty lines to separate out logical chunks\n",
    "def remainder(number,divider=2): # needs space after comma\n",
    "    r=number%divider # needs spacing around operators\n",
    "    return r"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "slide"
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   },
   "source": [
    "## Function Namespace "
   ]
  },
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   },
   "source": [
    "- Each function has its own namespace\n",
    "- Functions only have access to:\n",
    "    - variables explicitly passed into them\n",
    "    - variables defined inside the function"
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "# You can check variables defined in the global namespace with `%whos`\n",
    "%whos"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "slide"
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   },
   "source": [
    "### Variables defined inside a function only exist within that function."
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "# Names used inside a function are independent of those used outside\n",
    "# variables defined outside of functions are global variables\n",
    "# global variables are always available\n",
    "my_var = 'I am a variable'\n",
    "\n",
    "print(my_var)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "# define a function that uses my_var inside the function\n",
    "def concat_self(my_var):\n",
    "    \n",
    "    my_var = my_var + ' ' + my_var\n",
    "    \n",
    "    return my_var\n",
    "\n",
    "print(concat_self(my_var))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# see that my_var in global name space remains unchanged\n",
    "print(my_var)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# only way to change my_var in global namespace\n",
    "# is to assign to variable my_var in global namespace\n",
    "my_var = concat_self(my_var)\n",
    "print(my_var)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {
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    "tags": []
   },
   "source": [
    "#### Clicker Question #4\n",
    "\n",
    "Write a function `convert_to_f` that will convert a temperature in Celsius to Farenheit, returning the temperature in Farenheit. \n",
    "\n",
    "Note: A temperature in Celsius will be multipled by 9/5 and then 32 will be added to that quantity to convert to Farenheit\n",
    "\n",
    "A) I did it! &emsp;B) I think I did it. &emsp;C) I tried but I am stuck. &emsp;D) Super duper lost."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "## YOUR CODE HERE "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# TEST YOUR FUNCTION HERE"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Summary\n",
    "- how to define a function\n",
    "- how to execute a function\n",
    "- default values\n",
    "- position vs. keyword arguments\n",
    "- global namespace vs. local namespace\n",
    "- code style"
   ]
  }
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