CS 2005 Techniques of Programming WPI, B Term 1996
Craig E. Wills Lab 6
Assigned: Wednesday, December 11, 1996

Turtle Graphics and Recursion

Turtle graphics is a popular way to write graphics programs. You may have seen turtle graphics before if you used LOGO. The idea is that there is a ``turtle'' (it really just looks like a triangle), that you can move around the screen. As the turtle moves it draws a track, so you can make just about any line drawing by giving turtle commands. Even though there are just a few turtle graphics commands, they are perfect for playing around with recursion, which is when a function in a program calls itself. Here are the commands in our version of turtle graphics:

right( int angle )       - turn (in one place) "angle" degrees clockwise
left( int angle )        - turn (in one place) "angle" degrees counter-clockwise
forward( int steps )     - move "steps" distance in the direction you're heading
back( int steps )        - move "step" opposite the direction you're heading
hide()                   - make turtle invisible (this makes drawing faster)
show()                   - make turtle visible (this makes drawing slower)
clear()                  - clear window
repeat( int num )        - this is like a 'C' for loop
init_turtle()            - set up turtle graphics
wait_button()            - wait for user to click right mouse button

Here's an example of a simple turtle graphics function to draw a triangle:

tri( int length, int level )
        if( level == 0 ) return;

        repeat( 3 )
                forward( length );
                right( 120 );


  1. First to set stuff up, make a lab6 directory somewhere in your home directory and copy files to it, as follows.
    > mkdir lab6
    > cd lab6
    > cp /cs/cs2005/pub/labs/lab6/* .

  2. Edit the file squares.c. Note that in this assignment the source files are written in C rather than C++, hence the lower case ``c'' file extension. Write the code for the function square() in the section where it says /* ADD COMMANDS HERE */ , and make it draw a square. Use the triangle function as a guide. To compile and run your program type:

    > make squares
    > squares

  3. Add the following line as the last line in your square() function, and say what gets drawn when you run square? Explain why.
    	square( length/2, level-1 );

  4. Move the line you just added, so that it is right after the ``forward'' command in the square() function. What gets drawn when you run squares now? Explain why.

  5. What does the level parameter control; in other words, what happens when you change the line in main to:
            square( 100, 6 );

  6. Edit the file tree.c The function vee() draws a 'V', like a branch of a tree. Try running tree to see what it does. To compile and run the tree program type:

    > make tree
    > tree

  7. Change the vee() function (you only have to add 2 lines!), so that a smaller 'v' is drawn at the end of every branch.

  8. Turn in squares.c, tree.c and lab6.questions for today's lab, using the following command line

       /cs/cs2005/bin/turnin lab6 squares.c tree.c lab6.questions

  9. If you would like you can experiment with other recursive functions to draw other fun designs. As an example there is also a snow.c file in your directory that you can compile and run.