CS 2120: Topic 7
================

.. image:: ../img/nap.jpg
   :width: 350pt

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

    <iframe width="560" height="315" src="https://www.youtube.com/embed/65H1UCwEFwQ" frameborder="0" allowfullscreen></iframe>

.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/3II7RQR-bEs" frameborder="0" allowfullscreen></iframe>

A note on comments
^^^^^^^^^^^^^^^^^^^

* Helpful for *others* who are reading your code (they make it easier to read and understand)
* Helpful for *you* when you revisit your own code (i.e. when you return to a project a month later)
* You can add *comments* to your code in Python with ``#``::

   # we can comment above
   my_car = 5 #or to the right
   
* Once Python sees ``#`` it ignores the rest of the current line
* Don't just repeat what's obvious from reading the code... provide a *higher level* description of what's happening.

Python docstrings
^^^^^^^^^^^^^^^^^

* These are comments for functions (also other things like methods, classes, etc.)
* Used immediately after the function definition::

   def set_up_cities(names):
      """
      Set up a collection of cities (world) for our simulator.
      Each city is a 3 element list, and our world will be a list of cities.
        
      :param names: A list with the names of the cities in the world.
        
      :return: a list of cities
      """

      function code here

* Explain what the function is going to do, in plain English. 
* Also explain *every* parameter and *what the  function returns*

More on strings
^^^^^^^^^^^^^^^

* **Question**: How is a string different from the other data types we've seen (``int``, ``float``, ``bool``, etc.)?
* **Answer:** We can print individual *characters* of a string by *indexing* the string:
	
	>>> a='Boomboombobo'
	>>> print(a[0])
	B
	>>> print(a[1])
	o

* Note that in Python, the first index is ``0``, not ``1``

* What does ``print(a[0:4])`` do?

	>>> print(a[0:4])
	Boom

* This process is called *slicing*.

More on loops
^^^^^^^^^^^^^
* ``while`` loops are great, but they rely on a boolean condition returning false (i.e. while condition is true, do something)::
	
	while(mouse_not_yet_clicked = True):
		print("Please click the mouse")

* ``for`` loops allow for iteration through a specified range or over elements in a list (among others)::

   # one way you may use a for loop
   for i in range(len(a)):
      print(a[i])

* for i from 0 until the length of the string a, print the ith element of a::

   # another way you may use a for loop
   for char in a:
      print(char)
      
* *for* each character in the string ``a``, we run the indented code block.
* Yes, you could create a ``while`` loop which does exactly the same thing... the ``for`` loop is just cleaner here (less typing).

Mutability
^^^^^^^^^^^
* We can *access* an individual character in a string with an index (i.e. a[0])... can we *set* an individual character by index as well?
* Let's try::

   >>> a[7]='x'
   Traceback (most recent call last): 
   line 4, in <module> 
   a[7] = 'x'
   TypeError: 'str' object does not support item assignment

* Variables of some types are *mutable*... i.e. they can be changed.
* Strings are immutable --- you can't change a string, you have to make a *new one*::

   >>> new_a = a[:7] + 'x' + a[8:]
   >>> print(new_a)
   Boombooxbobo

Python Lists
^^^^^^^^^^^^
* You've seen these in Assignment 1, and maybe you made your own in Activity 2
* What is it? An *ordered and changeable collection of values or elements* 
* Lists are like strings, except that the elements can be any type (not just characters)::

>>> myList = []
>>> myList.append(5)
>>> print(myList)
[5]
>>> myList.append(10)
>>> print(myList)
[5,10]
>>> print(myList[0])
5

* We can add to the end of lists with ``append``
* Like strings, we can access the elements of a list with myList[i], where ``i`` is the list's index (starts from 0 for the first element)

For next class
^^^^^^^^^^^^^^
* Read `chapter 9 of the text <http://openbookproject.net/thinkcs/python/english3e/tuples.html>`_
* Take a look at Assignment 2 (due October 30th)