About this lab

Your task is to explore and develop world programs. Use the examples we used in class (in “Files -> In-class examples” on Canvas) for reference; the documentation on the Universe Teachpack is also likely to be useful. is also a link to the book chapter there. Make sure to ask questions when you are stuck.

Lab tasks: a world with a falling object

For this task you start with the following world example: humpty-dumpty.rkt. This provides a skeleton for a simple world/big-bang program that you need to complete.

Your tasks are as follows:

  1. Read the description of the world above main to see what the world program is supposed to do and what the world state is.
  2. Change render to place the marble ball at the x coordinate specified in the constant ball-x-coord and the y coordinate given by the world state (it starts at the top of the canvas). Note that the check-expect tests indicate the correct behavior and provide a significant hint for the expression in render; make sure you uncomment them so you get the benefit of those tests when you run your file. You don’t need to implement the ball breaking as it reaches the ground.
  3. Once you added rendering of the ball, you need to change the fall function to make the ball falling at the number of pixels per clock tick that’s given in the variable speed. Uncomment the check-expect tests, fill in the right expression, and run the program to test. The ball doesn’t (for now) need to stop at the bottom of the canvas.
  4. Change the fall function so that once the ball reaches the ground (the ground-size constant might be useful), it stops falling and remains at the same position. That requires making fall into an if or a cond. Add one or two new check-expect tests for fall that check for appropriate behavior in this case.
  5. Now you need to make it so that when one clicks on the ball, it returns back to the top. In order to implement it, write two helper functions, distance and within-radius?. Make sure to read the signatures, the descriptions and the check-expects. Note that the within-radius? helper function uses the distance. You might want to look up the formula for distance between two points. See the figure below for examples.
  6. Write the move-to-start function using within-radius? as a helper function. Use the comments and tests for move-to-start as a guide for what needs to be done. Make sure you uncomment the check-expect tests. Also make sure you uncomment the line in the definition of main (where big-bang is called) that uses registers move-to-start as a mouse handler. Test your program.
  7. Change the rendering so that once the ball is on the ground, it displays the broken-ball image instead of the ball image; this will also require a conditional. You should also add at least one new check-expect test to render that tests this new functionality.

The following figure illustrates the ideas behind both distance and withing-radius?:

Illustration of `distance` and `within-radius?`

Originally written by @elenam, with subsequent modifications by @NicMcPhee