Listen to me ... or else ....
=============================

.. index:: ! else

While learning how to program may be fun, you should not spend all your
time in front of the computer. ``if`` it rains, keep reading, otherwise,
go outside and play! (Yes, even you grandpa!)

Two choices...
--------------

Let's rewrite the sentence that starts with ``if`` above::

    if it rains,
        keep reading,
    otherwise,
        go outside and play

If this were Python, we might have written it like this instead:

.. code-block:: python

    if it_rains():
        keep_reading()
    else:
        go_outside_and_play()

Yes, Python includes the possibility of more than one choice with
the keyword ``else``. Let's use it with another example. Reeborg can see
if there's a wall right in front him. Consider world **Around 1**. You will
use a new condition, ``front_is_clear()`` which Reeborg uses to
determine if there's a wall in front of him or not, together with the
``if/else`` pair to write a program that will guide Reeborg around the
world. Something like the following:

.. code-block:: python


    def move_or_turn ():
        if front_is_clear():
            # something
        else:
            # something else

    repeat 40:
        move_or_turn()

.. topic:: Try it!

    Write a program using the code above so that Reeborg goes around world
    **Around 1**.
    After you are done, can you modify the program (by adding one line) so
    that Reeborg deposits a token at each corner?

How to think about ``if/else``
------------------------------

We have seen how ``def``\ s and ``if`` statements could be thought
of as being (sometimes) equivalent to inserting a code block; the
exception was when the condition of the ``if`` statement was ``False``,
in which case we ignored the code block which is equivalent to deleting
it. ``if/else`` statements can be thought as inserting one or the other
code block. Thus

.. code-block:: python

    move()
    if True:
        turn_right()
    else:
        turn_left()
    move()

is equivalent to

.. code-block:: python

    move()
    turn_right()
    move()

whereas

.. code-block:: python

    move()
    if False:
        turn_right()
    else:
        turn_left()
    move()

is equivalent to

.. code-block:: python

    move()
    turn_left()
    move()

We can represent this as a flowchart:

.. figure:: ../../flowcharts/else.jpg
   :align: center