.. _variables:

Variables
=========

What is a variable? 
^^^^^^^^^^^^^^^^^^^

A variable describes a place to store information. This information can be a
*number*, a *character*, a *list of characters*, etc. 

A way of picturing variables, is to imagine a box. This box has a label,
for instance **ping** in the example below. The content of the box is the
information we want to store, in the example **the number 5**.

.. image:: _static/img/variables_ping5.png
    :align: center
    
How to use variables in Python? 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

How can we store the **number 5** in a box called **ping** in Python? 

In :any:`pythonShell` it is a simple as this: 

.. code-block:: python  

   >>> ping = 5 

But... what can we do with this variable, what for do we want to store a value
behind a label? 

We could try to **read** the value of the variable by using its label, and then print it, like so: 

.. code-block:: python  

   >>> print(ping) 

The whole idea of a variable is that its value varies, which means that **we put
something different in the box**. The label that is written on the box however stays the
same. 

Have a look at this **example** and try it yourself! Which value appears when
you type **print(ping)**?

+-----------------------------------------------------+---------------------------------------------------------+
|   **Step 1**                                                                                                  |
+-----------------------------------------------------+---------------------------------------------------------+
|         >>> ping = 5                                |  .. image:: _static/img/variables_ping5.png             |
|                                		      |       :align: center			      	        |
+-----------------------------------------------------+---------------------------------------------------------+
|   **Step 2**                                                                                                  |
+-----------------------------------------------------+---------------------------------------------------------+
|         >>> ping = 3                                |  .. image:: _static/img/variables_ping3.png             |
|             		                              |       :align: center			      	        |
+-----------------------------------------------------+---------------------------------------------------------+

.. image:: _static/img/variables_ping5ping3.gif 
    :align: center 




Use variables to do calculations
--------------------------------

Now that you now how to use variables, there is no limit to what you can do
with them. For instance, calculations! 


Let us first create two variables with different values: **ping** and **pong**. 

+--------------------+----------------------------------------------+-------------------+----------------------------------------------+
|    >>> ping = 5    | .. image:: _static/img/variables_ping5.png   |   >>> pong = 7    | .. image:: _static/img/variables_pong7.png   | 
|                    |      :align: center                          |                   |   :align: center                             |
+--------------------+----------------------------------------------+-------------------+----------------------------------------------+

Have a look at the example below that shows what we can do with theses
variables, and try it out yourself! For instance:  

    - Add, multiply or substract the variables **ping** and **pong**
    - Store the result in a new variable called **pingpong**
    - Compare **ping** and **pong** with *smaller than (<)*, bigger than (>)*
      or *equals (==)* 
    - ... 

.. image:: _static/img/variables_pingpong.gif 
    :align: center 


Use variable to write programs 
------------------------------

Variables are a central concept when writing computer programs. An example that
introduces variables in a program is given here: :any:`introducingVariables`.