Functions
=========

A function in Python is a block of code that performs a specific task. You can “call” the function whenever you need to use that task, instead of writing the same code again and again​.

It’s like a mini-program inside your main program - it keeps your code organised, reusable and easier to understand​.

Example 1 - Creating a Function
-------------------------------

.. code-block:: python
    :linenos:

    def sayHello():
        firstName = input("What is your first name? ")

        print(f"Hello, {firstName}!")
        print("How are you today?")

    sayHello()

.. dropdown:: Show Output

    | What is your first name? John
    | Hello, John!
    | How are you today?

    .. admonition:: Explanation

        The first line defines a function called ``sayHello``.

        The second line asks the user to type in their first name. Whatever the user enters is stored in a variable called ``firstName``.

        The fourth line displays a message that includes the user’s name. The f-string lets the variable ``firstName`` appear inside the message.

        The fifth line displays another friendly message on the screen.

        The final line calls the function which means Python runs all the code inside it.

Example 2 - Creating a Function with a Single Parameter
-------------------------------------------------------
.. code-block:: python
    :linenos:

    def greet(firstName):
        print(f"Greetings, {firstname}!")

    greet("John")

.. dropdown:: Show Output

    Greetings, John!

    .. admonition:: Explanation

        This code defines a function called ``greet`` that takes one parameter named ``firstName``. Inside the function, the ``print`` statement uses an f-string to combine the word “Greetings,” with the value stored in ``firstName``. When the function is called using ``greet("John")``, the word "John" is passed into the function as the value of ``firstName``.

Example 3 - Creating a Function with Multiple Parameters
--------------------------------------------------------

.. code-block:: python
    :linenos:

    def greet(firstName, lastName):
        print(f"Greetings, {firstName} {lastName}!")

    greet("John", "Doe")

.. dropdown:: Show Output
    
    Greetings, John Doe!

    .. admonition:: Explanation

        This code defines a function called ``greet`` that takes two parameters, ``firstName`` and ``lastName``. Inside the function, the ``print`` statement uses an f-string to combine the word “Greetings,” with both the first and last names. When the function is called using ``greet("John", "Doe")``, the values “John” and “Doe” are passed into the function and replace the parameters.

Example 4 - Creating a Function with a Default Parameter
--------------------------------------------------------

.. code-block:: python
    :linenos:

    def sayGoodbye(firstName="Unknown"):
        print(f"Goodbye, {firstName}!")
        print("It was nice to see you.")

    sayGoodbye()
    sayGoodbye("Bob")

.. dropdown:: Show Output

    | Goodbye, Unknown!
    | It was nice to see you.
    | Goodbye, Bob!
    | It was nice to see you.

    .. admonition:: Explanation

        This code defines a function called ``sayGoodbye`` that takes one parameter named ``firstName``. The parameter has a default value of ``"Unknown"``, which means if no name is given when the function is called, it will automatically use the word “Unknown.” Inside the function, there are two ``print`` statements. The first one uses an f-string to display a message saying goodbye to the person, and the second one prints an extra message saying “It was nice to see you.”

Example 5 - Creating a Function That Performs a Calculation
-----------------------------------------------------------

.. code-block:: python
    :linenos:

    def add(number1, number2):
        result = number1 + number2

        print(f"{number1} + {number2} = {result}")

    add(10, 5)

.. dropdown:: Show Output

    10 + 5 = 15

    .. admonition:: Explanation

        This code defines a function called ``add`` that takes two parameters, ``number1`` and ``number2``. Inside the function, a new variable called ``result`` is created to store the sum of the two numbers. The ``print`` statement then uses an f-string to display the full calculation and its result in a clear format. When the function is called using ``add(10, 5)``, the values ``10`` and ``5`` are passed into the function, so ``result`` becomes ``15``.

Return Values
-------------

When a function in Python finishes running, it can send a result back to the part of the program that called it. This is called returning a value​.

You can think of it like the function doing some work, then handing the answer back to you so you can use it somewhere else in your program​.

Example 6 - Creating a Function That Returns a Value
----------------------------------------------------

.. code-block:: python
    :linenos:

    def add(a, b):
        return a + b

    result = add(10, 5)

    print(result)

.. dropdown:: Show Output

    15

    .. admonition:: Explanation
        
        This code defines a function called ``add`` that takes two parameters, ``a`` and ``b``. Inside the function, the ``return`` statement is used to send back the sum of the two numbers. When the function is called with ``add(10, 5)``, it calculates ``10 + 5`` and returns the value ``15``. This result is then stored in a variable called ``result``.