CS 2120: Topic 2
================

Videos for this week:
^^^^^^^^^^^^^^^^^^^^
Part 1: Background
------------------
.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/CY9mTS4R8W0" frameborder="0" allowfullscreen></iframe>

Part 2: Coding
------------------
.. raw:: html

    <iframe width="560" height="315" src="https://www.youtube.com/embed/OURclwAn2mk" frameborder="0" allowfullscreen></iframe>

Welcome back! This week we'll cover a bit of background theory and get into variables, operations, printing to the console, and a few other things.

.. admonition:: Before we start..
	:class: Warning

	Do you have Python running on your computer? If not, see ":doc:`Getting set up for CS 2120 </gettingset>`"

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Background (Theory)
-------------------

What's a program?
^^^^^^^^^^^^^^^^^
* You can think of it as a *recipe* for the computer to do something that you want it to do
* **More formal**: A sequence of instructions that specifies *exactly* how to perform a computation

What's debugging?
^^^^^^^^^^^^^^^^^
* Fixing your errors: syntax, runtime, or semantic

Languages
^^^^^^^^^^
* What's the difference between a "natural language" and a "formal language"?
* Why is ambiguity **a problem** for a formal language?

.. admonition:: Remark
	:class: Note

	Is the following statement true or false? 
	``This statement is false.``

Background (Python)
-------------------

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. admonition:: Sanity Check
	:class: Note
	
	Write a Python program that prints a message to the console.

Values
^^^^^^
* Values are things that a program manipulates. 
   * *Strings*: "abcdef"
   * *Integers*: 7, 42, 97
   * *Floating-point numbers*: 3.792, 0.00005
* To a computer, the integer 1 is not necessarily the same thing as the floating point number 1.0... because they have different *types*
* Python will guess at the *type* of your value if you don't tell it.
* Can I ask Python to tell me its guess for the type of a value?
	>>> type(12)
	<type 'int'>
	>>> type('Toronto Blue Jays')
	<type 'str'>
	>>> type(3.75)
	<type 'float'>

Variables
^^^^^^^^^^^^
* Variables let you store values in a labelled (named) location
* You store *values* into *variables* by using the *assignment operator* **=**	
	>>> a=5
	>>> m='I like variables'
* The '=' symbol here corresponds to variable assignment... it doesn't really mean the same thing as the '=' sign in math.
* In math when we write 'a = 5' we mean that '5' and 'a' are equivalent.
* In Python when we write
	>>> a=5
* ... we're telling the Python interpreter to create a variable named ``a`` and store the value ``5`` in it.

What can you do with variables?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* Anything you can do with values.
* For example, we can add variables:
	>>> a = 5
	>>> b = 7
	>>> a+b
	12
	>>> b=5
	>>> a+b
	10

Choosing variable names
^^^^^^^^^^^^^^^^^^^^^^^^
* Some rules for Python..
	* start the name with a letter of the alphabet or an underscore ("_") 
	* a variable name **can't** start with a number
	* a variable name can only be made up of alphanumeric characters and underscores
	* variable names are case-sensitive 
	
.. admonition:: Try this
	:class: Note
	
	Create two variables, named ``maple`` and ``leafs``, and set them to ``19`` and ``67``, respectively. Try adding them. What happened? 


Statements
^^^^^^^^^^^
* A **statement** tells Python to "*do something*".
	* for example, printing to the screen (i.e. ``print("a")``)
* it is an *instruction* that can be *executed* by the interpreter.
	
Expressions
^^^^^^^^^^^^^
* An **expression** is a combination of:
	* values (e.g., ``5``)
	* variables (e.g., ``whale``)
	* operators (e.g., ``+``)
	* Example:
	>>> leafs * 2 + 7
	141

Operators
^^^^^^^^^^
* **Operators** are symbols that tell Python to perform computations on expressions.
   * ``+``, ``-``, ``\*``, ``/``    

Order of Operations
^^^^^^^^^^^^^^^^^^^^^
* Python follows the usual order of operations (``B-E-D-M-A-S``).
* You can use ``()`` to customize your expressions:
	>>> 2 + 5 * 2
	12
	>>> (2 + 5) * 2
	14

Are operators just for numbers?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
* No
* they work on *strings*, for example

.. admonition:: Try this
	:class: Note
	
	Try mixing strings and integers with various operators.

Ways to run Python
^^^^^^^^^^^^^^^^^^
* The engine which runs Python (the *interpreter*) can run in 2 ways:
	* **immediate mode**: you type a line of code and the interpreter responds with an output (like the code samples on this page)
	* **script mode**: you type all of the code out (i.e. in a PyCharm script), *run* it, the interpreter responds with all of its output in the console window   
  
For next class
^^^^^^^^^^^^^^^

* Read the rest of `chapter 2 of the text <http://openbookproject.net/thinkcs/python/english3e/variables_expressions_statements.html>`_
* Read `chapter 3 of the text <http://openbookproject.net/thinkcs/python/english3e/hello_little_turtles.html>`_