Conditions and loops
====================


.. admonition:: Precap

   The default order of execution is sequentially (= from top to bottom, one statement after the other).

   Sometimes it is required to skip statements under certain conditions or to repeat statements a number of times. This can be done by using the built-in keywords

     * ``if``, ``elif``, ``else`` for `conditional <#conditional-execution>`__ and
     * ``for``, ``while``, ``else`` for `repeated <#repeated-execution>`__ execution.



Sequential order of execution
-----------------------------


By default code will be executed sequentially, i.e. one statement after the other.

.. interactive_code_block::
   :caption: Statements are executed one after the other (top-down)

   question = 'Life, The Universe, and Everything'
   answer = 42
   print('The Answer to the Ultimate Question of {} is {}.'.format(question, answer))


Conditional execution
---------------------

``if`` statement
^^^^^^^^^^^^^^^^ 

Sometimes it is necessary that not all but only selected statements have to be executed based on certain conditions. This type of control can be realized with the built-in keywords ``if``, ``elif``, and ``else``. The basic structure of conditional code is:

.. interactive_code_block::
   :caption: Pseudo code showing the basic structure of conditional code

   if <condition 1>:
       # Statement 1
       # Statement 2
       # Statement 3
   elif <condition 2>:
       # Statement 4
       # Statement 5
   elif <condition 3>:
       # Statement 6
       # Statement 7
       # Statement 8
       # Statement 9
   else:
       # Statement 10

Conditional code always starts with the ``if`` keyword, followed by a condition (= expression that resolves to ``True`` or ``False``) and a colon ``:``. Note the **mandatory** indentation (= preceeded with usually 4 spaces ot 1 tab) of the following *code block* (= one ore more statements). Optionally alternative conditions can be considered using one or more ``elif`` keywords, or, at the end and without any condition an ``else`` keyword.
|br| NOTE: The pseudo code above will **throw an error** and syntax coloring also fails.

.. interactive_code_block::
   :caption: Example: Test if a whole number is even or not

   whole_number = 123
   if whole_number % 2 == 0:
       result = "even"
   else:
       result = "odd"
   print("{} is an {} number.".format(whole_number, result))

If the condition ``whole_number % 2 == 0`` in line 2 evaluates to ``True``, line 3 will be executed. If it evaluates to ``False`` line 5 will be executed.
|br| NOTE: The modulo ``%`` operator in this exampe has the remainder as the result of a division between two numbers. E.g. ``123 % 2`` has ``1`` as a result, because 123 = 61 * 2 + 1.

..
   Match
   ^^^^^


Repeated execution
------------------

Sometimes it is necessary that selected statements are executed multiple times. This type of control can be realized with teh built-in keywords ``for``, ``break``, ``continue``, ``while``, and ``else``. There are two options for repeated code: using a sequence with ``for`` or a condition with ``while``. 


``for`` loops
^^^^^^^^^^^^^

The basic structure of a ``for`` loop is:

.. interactive_code_block::
   :caption: Pseudo code showing the basic structure of a for loop

   for <variable name> in <sequence>:
       # Statement 1
       # Statement 2
       # Statement 3
      if <condition 1>:
          break
       # Statement 4
       # Statement 5
      if <condition 2>:
          continue
   else:
       # Statement 6
       # Statement 7

Any sequence data type can be used as sequence (e.g. ``str``, ``tuple``, ``list``).

.. interactive_code_block::
   :caption: Iterating over a squence of characters in a string

   horizontal_text_printed = 'Vertically'
   for character in horizontal_text_printed:
       print(character)

.. interactive_code_block::
   :caption: Iterating over a squence of items in a tuple

   cat_parts = ('  ^-^', ' (\\'Y\\')', ' /   \\\\ (', '(\\|||/)_)')
   for cat_part in cat_parts:
       print(cat_part) 


A sequence of numbers can be created with the built-in ``range()`` function.

.. interactive_code_block::
   :caption: Iterating over a squence of numbers using the range() function

   # Counting from 0 to 4 (!)
   for i in range(5):
       print(i)

Note that the numbers start at 0 and **stop before** 5. The ``range()`` function can be used in three ways:

  * Only a stop value: ``range(stop)``
  * A start and a stop value: ``range(start, stop)``
  * A start, stop and a step value: ``range(start, stop, step)``

.. interactive_code_block::
   :caption: Example: A very basic loop using the range() function with start and stop values

   for i in range(2, 5):
       print(i)

.. interactive_code_block::
   :caption: Example: A very basic loop using the range() function with start, stop, and step values

   for i in range(1, 5, 2):
       print(i)

Depending on the number of function arguments (= the values within the round brackets ``()``) the ``range()`` function can be controlled in different ways.

If you have already have a data type that is a sequence, You can directly use that:

.. interactive_code_block::
   :caption: Example: Iterating (= looping) over a list

   weekdays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
   for w in weekdays:
       print(w)
   else:
       print('...the week is over!')

Note that the indentation of the code decides which statements are part of the loop and which are not.


``while`` loops
^^^^^^^^^^^^^^^

.. interactive_code_block::
   :caption: Pseudo code showing the basic structure of a while loop

   while <condition 1>:
       # Statement 1
       # Statement 2
       # Statement 3
   else:
        # Statement 4
        # Statement 5

.. interactive_code_block::

    weekdays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
    while weekdays:  # As long as the list is not empty
        w = weekdays.pop(0)  # Remove the first item from the list
        print(w)
    else:
        print('...the week is over!')


References
----------
https://docs.python.org/3/tutorial/controlflow.html