.. _pythonPrimerPart2:

Python Primer - Part 2
======================

Talking to your Program
^^^^^^^^^^^^^^^^^^^^^^^

As usual, we will start with a small example. Run the following code bellow and
type in different values.

.. literalinclude:: ../examples/turtle/turtle_loop_io.py
    :linenos:
    :language: python	


**From the above code, can you deduce which functions allow you to interract
with your program?** 

    1. **print()** 
           Allows you to display a message on the Terminal. The
           message can be composed by text or a value, or a combination. 
    2. **input()** 
           Not only displays a message to the Terminal, but also reads
           what the user types in! In our simple example, the number of steps
           is read and stored in the variable *steps*.  


Challenge 1
^^^^^^^^^^^

**Play with the input and output function and develop a small program, which
requires interaction with the user!**

Here is a small example: 

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

As inspiration, here is the code as well: 

.. literalinclude:: ../examples/turtle/age.py
    :linenos:
    :language: python	
 
.. tip:: No programmer knows all functions and how to use them by heart.
         
         Therefore, keep in mind that **Google is your friend!**


.. note:: In both examples above, an **endless loop** was used: **while(1)**.   
          
        **'1'** in programming means *true*, as opposed to **'0'** which means *false*. 

        Setting the loop condition to 1, means that it is always true and will
        go on... **forever**! 


Not very clear yet? 

Let us learn more about conditions in the next section, and 
everything will make more sense. 


Making Decisions
^^^^^^^^^^^^^^^^

Yes or No?
----------

Part of our human intelligence is based on our capacity to **take decisions**,
given certain **conditions** that surround us, and **act accordingly**.  

Does the example bellow look familiar? 

.. image:: _static/img/condition_food.png 
     :align: center


The idea is to give part of this intelligence to the computer program, so that
it can make decisions on its own! 

.. note:: In programming languages, this is done with **conditional statements**: 
            **IF - THEN - ELSE** 


==================== ==============================================================================
Block                Description
==================== ==============================================================================
IF                   Checks if the condition is **true**. 

THEN                 If the condition is **true**, this action is executed. 

ELSE                 Otherwise, if the condition is **not true**, this other this other action is executed.   
==================== ==============================================================================

.. image:: _static/img/condition_food_if.png
    :align: center

Let us illustrate this with our Turtle example from above, we will make it more
**intelligent**.

Do you remember how the turtle would always go left? Annoying, huh? 

**Why not ask the user which direction he wants to go, left or right?** 


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


.. note:: Our turtle is now intelligent enough to go left OR right, depending
        on the user's input! 

        Have a look at how this is done in Python...

.. literalinclude:: ../examples/turtle/turtle_loop_io_conditions.py
    :linenos:
    :language: python			        


In the above example...
    - What is condition? 
    - What happens if the condition is fulfilled? 
    - What happens if the condition is not fulfilled? 


Else, if...
-----------


Alright, you type in "r", turlte goes right. You type in "l", turlte goes left.
That is what we wanted. 

**Have you tried typing in any other letter?**

**What happens then?**

**Looking back at the code, can you figure out why?**