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Conditional Loops

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Introduction

============

Exercise

========

Conditional statements

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Introduction

============

So far we have accomplished predefined tasks, but in all honesty we were accomplishing no better achievements than the 18th century automata, or the music boxes following one set of instruction to the end. What makes programming so much more powerful is conditional statements. This is the ability to *test* a variable (or a name) against a value and act in one way if the condition is met by the variable or an other way if not. They are also commonly called by programmers *if statements*.

To know if a condition is *True* of *False*, we need a new type of data:

the booleans. They allow logical operations.

A logic statement or operation can be evaluated to be *true* or *false*.

Our conditional statement can then be understood like this:

**if** *(evaluation of a condition returns true)* **then** *do some operation* **else** *do an other operation*.

And any operation that can be avaluated as *true* or *false* can but put to the test. All comparisons return *true* or *false*: *=,>,<*.

Examples

========

Here are some simple examples::

condition = True

if condition:

print "condition met"

if not condition:

print "condition not met"

direction = -30

if direction > 0 :

forward(direction)

else:

left(180)

forward (-direction)

Condition an action

===================

Exercise

--------

Let's try to make our turtle to stop when it reaches the end

of the window. Let's only take into account horizontal movements.

We want the turtle to go forward for a specified distance (taken as a function parameter) until it reaches this distance, or stop and turn around if it reaches the border of the screen.

.. note::

* We start from the center of the screen, which has coordinate (0;0)

* We won't turn, until we reach the boundery our turtle wants to get away as quickly as possible

* We can get the x coordinate of our turtle with the function *xcor*

* We can get the width of the screen with the function *window_width*

Solution

--------

::

def stop_at_end_of_screen(distance):

future_x_coord = xcor() + distance

diff = window_width()/2-future_x_coord

if diff > 0 :

forward(distance)

else:

forward(window_width()/2-xcor())

left(180)

Bonus exercise

--------------

Now when the turtle reaches the end of the screen, it turns around and

continue its movement in the other direction, the remaining of the distance

.. note::

* heading() gives you the current angle the turtle's direction makes with the original east facing turtle. (a turtle going straight up has a heading of 90 degrees)

Solution

--------

::

def uturn_at_border(distance):

if heading() == 0 :

future_x_coord = xcor() + distance

else:

future_x_coord = xcor() - distance

diff = window_width()/2-future_x_coord

if diff > 0 :

forward(distance)

else:

forward(window_width()/2-xcor())

left(180)

forward (-diff)

.. note:: Python uses *whitespaces to identify blocks of code* belonging together. While other languages use special characters (like curly brackets) in python a block is introduced with a colon at the end of the line and commands within a deeper identation level - ususally 4 spaces. The block ends with the first line with a lesser identation level.

Introduction

============

Now we know how to *factorise* this code a little. But functions as we have defined them so far are not flexible. the variables are defined inside the function,

and we need to rewrite a whole function to cahnge the value of an angle, or a distance in it.

That is why we need to be able to give parameters, or also called *arguments* so that *names* we use in the function can be used with different values each time we call the function:

Remember how we defined the function ``line_without_moving`` in the previous section::

def line_without_moving():

forward(50)

backward(50)

We can improve it by giving it a parameter::

def line_without_moving(length):

forward(length)

backward(length)

The parameter acts as a *name* only known inside the function's definition. We use the newly defined function by calling it with the value we want the parameter to have like this::

line_without_moving(50)

line_without_moving(40)

We have been using functions with parameters since the beginning of the tutorial with the *forward*, *left*, etc...

And we can put as many arguments (or parameters) as we want, separating them with commas and giving them different names::

def tilted_line_without_moving(length,angle):

left(angle)

forward(length)

backward(length)

A parameterised function for a variable size hexagon

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Exercise

--------

Write a function that takes allows you to draw hexagons of any size you want, each time you call the function.

A function of several parameters

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Exercise

--------

Write a function that draws a honeycomb with a variable number of hexagons, of variable sizes

Solution

--------

::

def hexagon(size):

for i in range(6):

forward(size)

left(60)

def honeycomb(size,count):

for i in range(count):

hexagon(size)

forward(size)

right(60)

Getting started

***************

Starting Python

===============

After installing Python on your system successfully, you can start the interactive python prompt by typing "python" in the command and press <enter>. It will show you some context information about python similar to this::

Python 2.7.2 (default, Feb 1 2012, 00:28:57)

[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>>

.. note:: On windows the installer is not always setting up the "path" correctly. If that is the case on your system, you probably didn't see this message before but have to follow the steps described on the `python docs <http://docs.python.org/using/windows.html#excursus-setting-environment-variables>`_ and **restart your command line**.

Those three ">>>" in the last line indicate that you are now in the interactive shell of python. Type for example::

print("Hello world")

press <enter> and see what happens. You will now see the phrase "Hello world" appear and then python will bring you back to the interactive input, where you could enter another command now::

>>> print("Hello world")

Hello world

>>>

Running python files

====================

But you don't want to type everything into the Python shell everytime but have a file with commands for python to execute instead. In order to do that you can just pass a file name to the python command in your shell and it will execute that file. Let's try that. Just open the file "hello.py" in this directory in your favourite text editor and paste the print command from above. Now save that file, go back the command line and type::

python hello.py

When pressing <enter> now, the file is executed and you see the print as before. But this time, after python executed all commands from that file, it exited instead of going back to the interactive shell. In some opearting systems (and depending on your setup) you might also be able to just double click on the hello.py file to execute it

.. note:: Wordpad, TextEdit, Notepad and Word are **not** suited text editors. If you are unsure whether you already have a usable editor, you might want to download and install `Sublime Text2 <http://www.sublimetext.com/>`_. Sophisticated editors like this also take care of identation and help you run and debug your code.

And now we are all set and can get started with turtle.

getting_started

conditionals

logical_operators

conditional_loops

Logical operators

=========================

We can also do some more complex things with

We can return the opposite of a logic operation, with **not** as a keyword in front of your expression, or combine several logic statements together with logical **and** and **or** links between them (they are usually called *operators*)

.. note:: You can not save numbers in variables, like we did here for "x", but you can actually save various kinds of things in them. A typical other thing you want to have store often is a "string" - a line of text. Strings are indicated with a starting and a leading \". You'll learn about this and other types, as those are called in python, and what you can do with them later on.