In-Class Activity #1: Drawing Pictures

Objectives

The goal of today's activity is to get comfortable working with basic features of the Python programming language including the Python interpreter, variables, user defined functions and loops.

Introduction

The following example illustrates the process of importing and executing turtle commands.

Python 3.4.0 (default, Apr 11 2014, 13:05:11) 
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from turtle import *
>>> forward(100)
>>> left(90)
>>> forward(100)

Complete documentation for the turtle module can be found in the Python Standard Library Documentation. The following functions should be enough to get you started for today's activities:

forward(distance) backward(distance)	Move the turtle forward/backward the indicated distance (in pixels).
left(angle) right(angle)	Turn the turtle left/right by the indicated angle (in degrees).
penup() pendown()	After penup() is called, the turtle will not draw until pendown() is called.
goto(x, y)	Move the turtle to the indicated position. The orientation is not changed.
setheading(to_angle)	Set the orientation of the turtle to the indicated angle. Zero degrees points to the right, and ninety degrees points up.
exitonclick()	This command prevents the turtle screen from closing until it is clicked by the user. This should be the last command in your main.

Note that you must be using a version of Python that has Tk support, because the turtle module uses the tkinter module for graphics output. The labs in ISAT/CS (143, 248 and 250) should have this software installed.

Exercises

1. Getting Started
   • If you haven't already, open a terminal window and start the Python interpreter by typing "python3" at the prompt.
   • Spend a few minutes experimenting with the turtle commands from the table above.
2. Rectangles
   • Save the file shapes.py to your Desktop.
   • Start Idle3 and open shapes.py.
   • Take note of the import statement at the top of the file. We will learn about this statement later, but for now you should know that it allows you to call turtle module functions without prefacing them with the module name. In other words, you may simply write "forward(100)" instead of "turtle.forward(100)". In general, this is a bad idea (for reasons we will discuss later), but it will simplify our code for the purposes of this lab.
   • Add a new function with the following signature:

     rectangle(x, y, width, height)

     This function should draw a rectangle with the indicated width and height. The lower-left corner of the rectangle will be at position (x, y).

   • Modify your main so that it calls your newly created function. Test your program by pressing F5 or clicking on "Run Module" under the Run menu.

     You can also execute your code by typing "python3 shapes.py" at the terminal prompt. Try out both methods.

3. Polygons
   • Add a new function to your program with the signature

     polygon(x, y, sides, size)

     This function should draw a polygon with the indicated number of sides at position (x, y). The length of each side is determined by the size parameter.
     
     Hints:
     • This could be accomplished using either a for loop or a while loop. The best approach is to use a for loop along with the range function to control the number of loop iterations.
     • Notice that the turn angles required for drawing a square were 90° and that 90 = 360.0 / 4.
   • Update your main so that it includes several calls to your newly defined polygon function.
4. Rows
   • Add a new function to your program with the signature

     row(x, y, count, size)

     This function should draw a horizontal row of squares with the lower-left corner of the row at position (x, y). The left edge of each square should be aligned with the right edge of the previous square. The count and size parameters indicate the number of squares in the row and the size of each square respectively.

     Don't copy/paste code from the existing square function! Your row function should invoke square with appropriate parameter values.

   • Update your main so that it includes a call to your completed function.
5. If you Have Extra Time
   • Create and test a new function with the following signature:

     grid(x, y, rows, columns, size).

     This function should create a rows x columns grid of squares. The lower-left corner of the grid will be at position (x,y). The size parameter indicates the width of the individual squares.
   • Create two new functions: filled_square and checkerboard. The filled_square function should create a single colored square, and the checkerboard function should create a checkerboard-style grid of alternating filled and un-filled squares. You will need to look through the documentation for the begin_fill() and end_fill() functions.
   • If you want some practice with a more complex algorithm, try writing a function that draws a Sierpinski triangle. You should be able to reuse the polygon function you wrote earlier, and you may wish to use the speed() or tracer()/update() functions to speed up your code development. Here is an example of what the end result should look like:
     
     
     
     Hint: use recursion!