CS 2120: Topic 6
================

.. image:: ../img/thanksssgiving.png

Videos for this week:
^^^^^^^^^^^^^^^^^^^^^
.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/x08_BOIdZTA" frameborder="0" allowfullscreen></iframe>

.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/ldYKcNotgDc" frameborder="0" allowfullscreen></iframe>

.. admonition:: Correction
   :class: Warning

   @ 6:40 in video 2, the ``return`` statement is not needed.

Reusing variables
^^^^^^^^^^^^^^^^^

.. admonition:: Try this..
   :class: Note

   Consider this code::
       
       a = 5
       print(a)

       b = 6
       print(a)

       a = a+b

       print(a)

       a = 3
       a = a+1

       print(a)

   What is the value of the variable ``a`` at each of the print statements?

* **The point**: we can *reuse* the same variable, *reassigning* it to a new value::
    
    a = a + 1

The while loop
^^^^^^^^^^^^^^
.. image:: ../img/loops.jpeg

* So far, if we want Python to do the same thing over and over, we have to tell it explicitly by repeating those instructions over and over. 
* Question: How do we automate this?
* Answer: the ``while`` statement.

.. admonition:: The while statement
   :class: Warning

   "*While* some condition is true, keep doing the code in the indented block"::

    a = 1
    while a < 11:
        print(a)
        a = a + 1

* What is happening here?

  * Before the ``while`` statement, we *initialize* the loop variable ``a``
  
  * The ``while`` statement is followed by a condition (any Boolean expression).. if the condition is ``True``, the body of the loop gets executed, otherwise it gets skipped.

.. admonition:: Question
   :class: Note
  
   What would happen if we didn't have ``a=a+1``?

* Consider this code::

   def dostuff(n):
       answer = 1
       while n > 1:
          answer = answer * n
          n = n - 1
       return answer

.. admonition:: Tracing through code
   :class: Warning

   What does the code above do? Trace through it, using pen and paper, for a few example values of ``n``.

* To trace, we can build a table of values.
* Let's trace ``dostuff(4)``. We'll look at the values of ``n`` and ``answer`` right after the ``while`` statement.

+------------------------+------------+
|         n              | answer     | 
+========================+============+ 
|          4             | 1          |  
+------------------------+------------+ 
|          3             | 4          |  
+------------------------+------------+ 
|          2             | 12         |  
+------------------------+------------+ 




* Side note: In Python, the pattern ``a = a + 1`` can be written as ``a += 1``.

Encapsulation
^^^^^^^^^^^^^^
* In the scope of the course, when we refer to ``encapsulation`` we are talking about putting some piece of code into a function.

Generalization
^^^^^^^^^^^^^^
* In the scope of the course, when we refer to ``generalization``, we are talking about making something more general (i.e. ``n`` instead of ``5``)

.. admonition:: Example of Encapsulation
   :class: Note

   Consider this while loop::
   
       while(i<5):
           print(2*i)
           i = i + 1

   We can encapsulate it like this::
   
       def print_multiples():
           while(i<5):
               print(2*i)
               i = i + 1

.. admonition:: Example of Generalization
   :class: Note

   Let's generalize the previous function::
   
       def print_multiples(n):
           while(i<5):
               print(n*i)
               i = i + 1

For next class
^^^^^^^^^^^^^^^

* Read `chapter 7 of the text <http://openbookproject.net/thinkcs/python/english3e/iteration.html>`_
* Complete Activity 2 and submit it on OWL by the end of the week (Friday October 16 @ 11:59 PM)