11-writing-functions.ipynb

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   "source": [
    "# Writing Functions"
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    "## Break programs down into functions to make them easier to understand.\n",
    "\n",
    "- Human beings can only keep a few items in working memory at a time.\n",
    "- Understand larger/more complicated ideas by understanding and combining pieces.\n",
    "  - Components in a machine.\n",
    "  - Lemmas when proving theorems.\n",
    "- Functions serve the same purpose in programs.\n",
    "  - _Encapsulate_ complexity so that we can treat it as a single “thing”.\n",
    "- Also enables re-use.\n",
    "  - Write one time, use many times."
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    "## Define a function using `def` with a name, parameters, and a block of code.\n",
    "\n",
    "- Begin the definition of a new function with def.\n",
    "- Followed by the name of the function.\n",
    "  - Must obey the same rules as variable names.\n",
    "- Then _parameters_ in parentheses.\n",
    "  - Empty parentheses if the function doesn’t take any inputs.\n",
    "  - We will discuss this in detail in a moment.\n",
    "- Then a colon.\n",
    "- Then an indented block of code."
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    "def print_greeting():\n",
    "    print('Hello!')\n",
    "    print('The weather is nice today.')\n",
    "    print('Right?')"
   ]
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   "source": [
    "## Defining a function does not run it."
   ]
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    "- Defining a function does not run it.\n",
    "  - Like assigning a value to a variable.\n",
    "- Must call the function to execute the code it contains."
   ]
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   "source": [
    "print_greeting()"
   ]
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    "## Arguments in a function call are matched to its defined parameters.\n",
    "\n",
    "- Functions are most useful when they can operate on different data.\n",
    "- Specify _parameters_ when defining a function.\n",
    "  - These become variables when the function is executed.\n",
    "  - Are assigned the arguments in the call (i.e., the values passed to the function).\n",
    "  - If you don’t name the arguments when using them in the call, the arguments will be matched to parameters in the order the parameters are defined in the function."
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    "def print_date(year, month, day):\n",
    "    joined = str(year) + '/' + str(month) + '/' + str(day)\n",
    "    print(joined)\n",
    "\n",
    "print_date(1871, 3, 19)"
   ]
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    "Or, we can name the arguments when we call the function, which allows us to specify them in any order and adds clarity to the call site; otherwise as one is reading the code they might forget if the second argument is the month or the day for example."
   ]
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   "source": [
    "print_date(month=3, day=19, year=1871)"
   ]
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    "## Functions may return a result to their caller using `return`.\n",
    "\n",
    "- Use `return ...` to give a value back to the caller.\n",
    "- May occur anywhere in the function.\n",
    "- But functions are easier to understand if `return` occurs:\n",
    "  - At the start to handle special cases.\n",
    "  - At the very end, with a final result."
   ]
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    "def average(values):\n",
    "    if len(values) == 0:\n",
    "        return None\n",
    "    return sum(values) / len(values)"
   ]
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    "a = average([1, 3, 4])\n",
    "print('average of actual values:', a)"
   ]
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    "print('average of empty list:', average([]))"
   ]
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    "- Remember: every function returns something.\n",
    "- A function that doesn’t explicitly `return` a value automatically returns `None`."
   ]
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   "source": [
    "result = print_date(1871, 3, 19)\n",
    "print('result of call is:', result)"
   ]
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    "## Key Points\n",
    "\n",
    "- Break programs down into functions to make them easier to understand.\n",
    "- Define a function using `def` with a name, parameters, and a block of code.\n",
    "- Defining a function does not run it.\n",
    "- Arguments in a function call are matched to its defined parameters.\n",
    "- Functions may return a result to their caller using `return`."
   ]
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