Geographical-Computing

A different environment

Exit the Python console with

exit()

and start up IDLE with either idle or idle3 (if one doesn’t work, the other should). You will get another Python command prompt. This one is a primitive IDE (Integrated Development Environment) for editing code, and makes it fairly easy for us to write and save and run simple programs. We are going to use it to explore the Python turtle module.

Right. So the API documentation for turtle is available here. That is where you should look for information on things that are unclear, or if you get into it and want to know more.

For now, I am going to provide a supplement to Chapter 4 in Think Python.

The turtle module

Before we start we have to import the turtle module.

>>> import turtle

To ‘fire it up’ we make a turtle. We’ll call ours something more local.

>>> kiwi = turtle.Turtle()

You’ll get more than a turtle: you should see a blank window appear with a little black triangle at its centre. This is kiwi our first Turtle to whom we can issue commands using function calls that will cause uh… them (?!) to draw things in the window. The fd(x) function causes the Turtle object asked to execute it and move x units forward. There is also a bk(x) command to cause a Turtle to move backwards.

>>> kiwi.fd(100)

We can also ask a Turtle to change direction with the right(x) or left(x) function which causes the turtle to turn in the appropriate direction through an angle of x degrees. These functions can also be abbreviated as rt(x) or lt(x).

kiwi.right(90)

Now if we issue a forward command, the line will get drawn at an angle to the previous one.

kiwi.fd(100)

That’s enough to get the general idea. It is also useful, before continuing to be able to reset the screen and send kiwi back to their start location.

kiwi.reset()

Simple repetition

So… using what you already know: make kiwi draw a square.

No, really… before reading on, issue commands to make kiwi draw a square.

The book tells you what you most likely wrote.

It probably involved some repetition, which even allowing for copy and paste is tedious to deal with.

Thankfully, programming is all about repetition (usually called iteration), so we have commands in Python that allow us to repeat actions many times. For example

# Simple repetition
for i in range(4):
    print("Here we go again")

With that in mind, rewrite your square drawing code to use a for loop. Note that we’ll look at iteration more closely in a couple of weeks.

Exercises from section 4.3

These are taken directly from Think Python. They won’t make much sense this week (we get to functions next week), but it is worth looking at them now, and revisiting them in the next while as you learn more about Python.

The best way to do this is to open a new file in IDLE with File - New File. Type code in the file and when you are ready to try running it, save it to a file with a name ending in .py such as square.py. Once it is save, you run it with Run - Run Module (shortcut is F5). Keep in mind you’ll need to import the turtle module as part of your program. Here’s a skeleton program file (right-click Save as…) to get you started.

  1. Write a function called square that takes a parameter named t, which is a turtle. It should use the turtle to draw a square.

    Write a function call that passes a turtle as an argument to square, and then run the program again.

    Here’s the outline, to get you started:

def square(t):
    print("Replace this print statement with code to get t to draw a square")

## and here's a line of code to run the function
square(kiwi)

  1. Add another argument, named length, to square. Modify the function body so that the length of the sides is length, and then modify the function call to provide a length argument. Run the program again. Test your program with a range of values for length.

    Again here’s a function header

def square(t, length):
    print("You know what to do")

  1. Make a copy of the square function code and change the name to polygon. Add another parameter named n and modify the body so it draws an n-sided regular polygon. Hint: The interior angles of an n-sided regular polygon are 180 - 360 / n degrees.

    No clues this time…

If you get really into this turtle thing, try this

  1. Write a function called circle() that takes a turtle, t , and radius, r , as parameters and draws an approximate circle by calling polygon with an appropriate length and number of sides. Test your function with a range of values of r.

    Hint: figure out the circumference of the circle and make sure that length * n = circumference.

  2. Make a more general version of circle called arc() that takes an additional parameter angle, which determines what fraction of a circle to draw. angle is in units of degrees, so when angle=360, arc should draw a complete circle.

Read the book!

The book spells out nicely how to complete these tasks in sections 4.4 to 4.7 and I suggest that you work through those while tackling this material.

Clean up

To clear the graphics window:

turtle.done()