CS 2120: Topic 4
=================

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

    <iframe width="560" height="315" src="https://www.youtube.com/embed/dC-K3zr9vF8" frameborder="0" allowfullscreen></iframe>

.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/yBccG9CwwE8" frameborder="0" allowfullscreen></iframe>

.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/FPZMqu2JIrE" frameborder="0" allowfullscreen></iframe>

Conditionals
^^^^^^^^^^^^

.. image:: ../img/conditionals.png

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

   Using only the Python statements, try to write a program that will divide a number in half *only if* that number is a multiple of 2.
   
* **Conditionals** (or *conditional execution*) will make these types of tasks easier.

Logic
^^^^^
* To make parts of the program *conditionally* executed, we need a *formal* way to describe conditions.
* We need logic.
* Let's try some comparison:
	>>> 19 == 87
	False
	>>> 5==5
	True
* Note that ``==`` is *comparison* while ``=`` is *assignment*. They are not the same. 

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

  Figure out what the other comparison operators in Python are. Hint: ``3`` doesn't equal ``5``.

* These operators can be applied to any two expressions (could be simply a value or variable, but can be more complex):
	>>> a=15
	>>> b=37
	>>> (a+b)*9 > (b-a)*3 + 2
	True

.. admonition:: Question
  :class: Warning

  What is the *type* of the result of applying a comparison operator?

Conditional execution
^^^^^^^^^^^^^^^^^^^^^
* Comparison statements can be ``True`` or ``False``, but how do we use this to control our program?
* ``if`` some condition is ``True``, do something::

	if grade < 50:
		print('Ya I don't think I passed..')

* If the condition following the keyword ``if`` is ``True``, the *block of code* after the ``:`` gets executed.
* If the condition is ``False``, the *block* inside gets skipped over. 
* The blocks can be as long as you want (but the minimum size is 1)

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

  Using only the Python features/statements we've seen so far, can you write a program that will divide a number in half *only if* that number is a multiple of 2?

Compound conditions
^^^^^^^^^^^^^^^^^^^
* We can use the logical operators ``and``, ``or`` and ``not`` to combine conditions::

	if (Bob_has_cineplex_membership == True and Is_Friday == True) or (Bob_complains):
		print('Bob gets to go into the movie for free.')

.. admonition:: For more on logical operators..
  :class: Warning

  Look at the truth tables for the logical operations **AND**, **OR** and **NOT** for help here.Wikipedia link: `Wikipedia link <https://en.wikipedia.org/wiki/Truth_table#Truth_table_for_all_binary_logical_operators>`_.

Alternative execution
^^^^^^^^^^^^^^^^^^^^^
* ``if`` statements can have a special ``else`` statement to go with them::

	if x > 10:
		do_something()
	else:
		do_something_else()
		
Chaining alternative execution
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* Sometimes just having ``if``/``else`` isn't enough.
* What if I want several, *mutually exclusive*, alternatives?::

    if year < 1960:
    	print('Jazz')
    elif year < 1980:
    	print('Rock')
    elif year < 1990:
    	print('Synthpop')
    elif year < 2003:
    	print('Alternative')
    elif year < 2012:
    	print('Hip Hop')
    else:
        print("The Lils")

* ``elif`` means ``else if``
* **NOTE**: Once *one* of the ``elifs`` gets executed, that's *it*. The remaining ones are ignored.
* You can have as many elifs as you want
* Always end with a plain ``else`` as a "catch-all".

Nested conditionals
^^^^^^^^^^^^^^^^^^^
.. image:: ../img/russian-doll.jpg
* It's also possible to *nest conditionals* inside other conditionals::

    if x > 0:
        if y > 0:
            print('First Quadrant')
        else:
            print('Fourth Quadrant')
    else:
        if y > 0:
            print('Second Quadrant')
        else
            print('Third Quadrant')
			
* There's no limit to how deep you can nest.



Libraries
^^^^^^^^^
* Python has a *huge* variety of existing **modules** and **libraries**/**packages**.
* *Modules* are just files which contains python code which you can import for use in, for example, a python script. Modules have definitions (i.e. function definitions) and statements
* *Packages* are collections of modules
* *Libraries* are a collection of modules and/or packages.
* The Python docs have more on this (`here <https://docs.python.org/3/tutorial/modules.html>`_)
* No matter what you want to do, there's probably a library that can help you. We've already used *pandas* in assignment 1. Let's look at **NumPy**

NumPy
^^^^^
* *Numerical Python* or "NumPy"
* included with Anaconda
* Like pandas, though, it isn't 'built in' to Python, so we have to tell the interpreter that we want to use NumPy::
   >>> import numpy

NumPy Types
^^^^^^^^^^^^
* Recall that Python values have types.
* NumPy has `its own types <http://docs.scipy.org/doc/numpy/user/basics.types.html>`_.
* We can specify these::
	>>> x = numpy.float32(7.3)
	>>> print(x)
	7.3
	>>> type(x)
	<type 'numpy.float32'>

* You can convert between regular Python types and NumPy types.

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

* Read `chapter 5 of the text <http://openbookproject.net/thinkcs/python/english3e/conditionals.html>`_