Conditional statements
======================

Sources
-------

This lesson is based on the `Software Carpentry
group's <http://software-carpentry.org/>`__ lessons on `Programming with
Python <http://swcarpentry.github.io/python-novice-inflammation/>`__.

Basics of conditional statements
--------------------------------

Conditional statements can change the code behavior based on meeting certain conditions.

1. Let's take a simple example.

   .. code:: python

       >>> num = 37
       >>> if num > 100:
       ...     print('greater')
       ... else:
       ...     print('not greater')
       ...
       not greater

   What did we do here?
   First, we used the ``if`` and ``else`` statements to determine what parts of the code to execute.
   Note that both lines containing ``if`` or ``else`` end with a ``:`` and the text beneath is indented.
   What do these tests do?
   The ``if`` test checks to see whether the variable value for ``num`` is greater than 100.
   If so, 'greater' would be written to the screen.
   Since 37 is smaller than 100, the code beneath the ``else`` is executed.
   The ``else`` statement code will run whenever the ``if`` test is false.

2. The combination of ``if`` and ``else`` is very common, but both are not strictly required.

   .. code:: python

       >>> num = 53
       >>> if num > 100:
       ...     print('53 is greater than 100')
       ...
       >>>

   Note that here we use only the ``if`` statement, and because 53 is not greater than 100, nothing is printed to the screen.

3. We can also have a second test for an ``if`` statment by using the ``elif`` (else-if) statement.

   .. code:: python

       >>> num = -3
       >>> if num > 0:
       ...     print(num, 'is positive')
       ... elif num == 0:
       ...     print(num, 'is zero')
       ... else:
       ...     print(num, 'is negative')
       ...
       -3 is negative

   Makes sense, right?
   Note here that we use the ``==`` to test if a value is equal to another.
   The complete list of these comparison operators is given in the table below.

   +------------+----------------------------+
   | Operator   | Meaning                    |
   +============+============================+
   | ``<``      | Less than                  |
   +------------+----------------------------+
   | ``<=``     | Less than or equal to      |
   +------------+----------------------------+
   | ``==``     | Equal to                   |
   +------------+----------------------------+
   | ``>=``     | Greater than or equal to   |
   +------------+----------------------------+
   | ``>``      | Greater than               |
   +------------+----------------------------+
   | ``!=``     | Not equal to               |
   +------------+----------------------------+

4. We can also use ``and`` and ``or`` to have multiple conditions.

   .. code:: python

       >>> if (1 > 0) and (-1 > 0):
       ...     print('Both parts are true')
       ... else:
       ...     print('One part is not true')
       ...
       One part is not true
       >>> if (1 < 0) or (-1 < 0):
       ...     print('At least one test is true')
       ...
       At least one test is true

   This can be quite handy.