Not ... true!
=============

.. index:: ! not

Reeborg is upset. It's **not** raining; it's **not** snowing. Yet, he
cannot go outside and practise for his hurdles race.

Why, do you ask? It's because Reeborg is waiting for you to learn about
Python's **not**.

Time to be negative.
--------------------

Python, we can indicate that something is not true by writing ``not True``
which is synonymous of ``False``. Likewise, ``not False`` is
equivalent to ``True``.

Please, make Reeborg happy
--------------------------

You have already written a program that enables Reeborg to jump hurdles;
parts of it went something like this:

.. code-block:: python

   def run_jump_or_finish ():
        if at_goal():
            # something
        elif front_is_clear():
            # something
        else:
            # something

.. topic:: Try it!

    Make Reeborg happy by re-writing this program in three other versions,
    by choosing different combinations of the negation keyword ``not`` **and**
    different combinations of ``if/elif/else``.

You should use the three code samples below but pay close
attention to where the ``not`` keyword occur **and** to what is actually
included in each code block.

.. code-block:: python

   # first choice:

   def run_jump_or_finish ():
        if at_goal():
            # something
        elif not front_is_clear():
            # something
        else:
            # something

   # second choice ... trickier

   def run_jump_or_finish ():
        if not at_goal():
            if front_is_clear():
                # something
            else:
                # something
        else:
            # something

   # third choice:

   def run_jump_or_finish ():
        if not at_goal():
            if not front_is_clear():
                # something
            else:
                # something
        else:
            # something

Another option
--------------

.. index:: wall_in_front()

You have just seen how it is possible to change the order
in which the conditions appear in an ``if/elif/else`` code block while
still accomplishing the same goal.  Two different programmers will often
use different strategies to get the same final result.  There are other
ways in which different programmers will write different but equivalent
programs: by using different functions.

The function ``front_is_clear()`` will tell Reeborg whether or not a
wall is blocking its way.  It will do the same for **water**, **brick walls**,
**fences**, etc., which we have not seen yet but likely will in future worlds.
There is a function that is more specific to wall; it is called
``wall_in_front()``; I leave it up to you to guess what it does.

.. topic:: Try it!

    Write a program using ``wall_in_front()`` instead of the equivalent
    ``not front_is_clear()``.