Lab 6: 99 Red Balloons With apologies to Nena.

1 March 2024

Learning objectives

The purpose of this lab is to give you practice:

  • working with lists of data,
  • using reactors to make interactive programs, and
  • playing with virtual balloons.

Contents

↑ Contents

Introduction

In this lab, you will be completing a world program simulating releasing an arbitrary number of balloons. These are represented as a list of positions, using the Posn data type.

When a balloon is released (by clicking the mouse), it floats away, and when it has floated out of our field of view, we can remove it from the list that represents the state of the world, since we no longer need to keep track of its position.

Task: To get started, save a copy of lab06.arr in CPO.

If you run the reactor by entering interact(balloons) in the Interactions Pane, you’ll see that parts of the program are complete: You can click the mouse and a balloon is added to the list of positions and displayed. However, the balloons just hang there in the sky, which isn’t very realistic.

To make the program work, you’ll need to finish designing three functions that are currently just entries in the wish list near the bottom of the program: next-posns, tick-posns, and onscreen-only.

Exercise 1: next-posns

The next-posns function is called by the reactor every time the clock ticks – 28 times per second. It’s responsible for updating the state of the world – a list of positions – by making every balloon float up SPEED pixels and removing from the list any balloons that are now off screen.

Task: Write a single test case for next-posns that will only pass if both parts work:

  • A position in the list is updated to be SPEED pixels higher (that is, its y coordinate is decreased by SPEED, since y is measured from the top of the window), and
  • A position in the list that was already at the top edge of the window (i.e., y was 0) “floats away” and is removed from the list.

Note: This test won’t pass until the very end of the lab.

This is fine.

Task: Define the body of next-posns. This function’s body can be a single line. It combines calls to tick-posns (Exercise 2) and onscreen-only (Exercise 3), which will do all the work.

That’s all there is to writing next-posns. Now to make it work, you’ll need to finish the other two functions on the wish list.

Exercise 2: tick-posns

The tick-posns function goes through a list of positions and moves each to be SPEED pixels higher.

Task: Write two test cases for tick-posns: One for an empty list and one for a list containing one or more Posns.

Task: Complete the function body to make tick-posns work. Your solution can go through the list recursively or use a higher-order function to do the recursion for you.

At this point, if you run interact(balloons) and release some balloons, they will float away. However, the balloon count will not decrease after they go off screen, and the test case for next-posns will still fail. The off-screen balloons are still cluttering up the list representing the state of the world. This is a more serious problem than it may sound like! If a user plays with the program long enough, the computer will run out memory and the program will crash.

Exercise 3: onscreen-only

The onscreen-only function goes through a list of positions and removes those that are no longer visible in our window.

Task: Write two test cases for onscreen-only: One for an empty list and one for a list containing one or more Posns.

Task: Complete the function body to make onscreen-only work. Your solution can go through the list recursively or use a higher-order function to do the recursion for you.

At this point, all of the test cases should pass, and when you run interact(balloons) and balloons float off screen, the balloon count will decrease. If this isn’t the case, click on the test cases that fail and think about why: Is the test case right? Is the code right?

Challenge Exercise: Pop!

Relaxing as it is to watch our balloons float away, it may be more entertaining to make them pop!

Task: Update the handle-mouse function so that if you click on a balloon it’s removed from the list.

Hints:

  • Since this requires significant changes to your code, you should make a new copy of your working solution before you try this!
  • Note that this requires going through the list of balloons to see if one is under the mouse. This requires a bit of work, so it should be a separate function.
  • If you pop a balloon, the same click should not release a new balloon.

Submitting the lab

  • When you’ve completed the exercises, show your code to your instructor or one of the coaches.

  • Then upload your lab06.arr file to the Lab 6 assignment on Gradescope. If you worked with a partner, you should submit a single copy of the lab with both your names – see these instructions.