Studio : Gambler's Ruin and Pi

Studio activities should not be started before class! Come to the session and work on the activity with other students!

Studio Setup and Procedure

  • Form a group of 2-3 students and find a TA or instructor to work with.
  • All but one member of your group should have this web page open so you can follow along and see the instructions as you work.
  • Plan to work on one computer (using Eclipse).
    • Initially, one of you will be in charge of typing at that computer.
    • Throughout the studio you should trade who is in charge of the keyboard.


  1. Have one person in your group create a new team by clicking here and going to the OR Create a new team box at the bottom of the page. The team name should include the last names of all your group members. For example, if Xia and Smith are working together, the team name should be something like “XiaSmith”.
  2. After the team is created, all other members of your team should click on the same link and follow the instructions to join the team.
    1. Be careful to join the right team! You won’t be able to change teams yourself and will have to contact instructors if there’s a problem.
    2. Be sure everyone else joins the team! If grades don’t get entered correctly we will use the team to help verify credit for a particular studio.
  3. Finally, one person should import the studio repository into Eclipse, as described in Assignment 0’s Add the assignment to Eclipse
    • All team members will have access to the work pushed to GitHub. Be sure to Commit and Push at the end of the day so everyone can refer back to the work later as needed.

Problem 1: Gambler’s Ruin


In this assignment you’ll simulate a version of Gambler’s Ruin problem. The problem is:

  • You’re a gambler and you’ll be visiting a casino.
  • You start the day with some money (a start amount).
  • Over the course of the day you repeatedly play a game, which has known odds of winning called the “win chance”.
    • Each time you play you either win or lose $1 based on the “win chance”. So the total amount of money you have on-hand either increases or decreases by 1 after each game.
  • If the total amount of money you have ever hits a “win limit” you stop gambling and leave the casino for the day. It was a successful day!
  • If you lose all of your money you also stop gambling. The day was a ruin!

As a professional gambler, you’ll actually think about doing this every day. You know that some days will be a success and some days you’ll ruin. You also have observed a lot of games at a lot of casinos and will know the “win chance” of each. You still have some difficult questions you eventually want to answer:

  • What “win limit” should you pick?
  • What percentage of time will you “ruin”?

There are two major ways to solve these questions:

  1. By simulating many gambling sessions and using the results to estimate the probability of “ruin” for a particular combination of “win limit” and “win chance”
  2. By using a mathematical formula that computes the “expected” answer. This is the value you should find if you ran your simulation an infinite number of times.

In this assignment you’ll do both (and compare the simulated value to the expected one)


  1. With your repository open create a new class in the studio2 package:
    • Right click Right-click on the package name in which you want to define the new class. In this case, use studio2.
    • Click New...
    • Choose Class
    • Pick the name Ruin for this class. Java style dictates that its classes should begin with a capital letter!
    • Click the check box for public static void main(String[] args) to have Eclipse automatically create the main() method for you.
  2. Have your program accept the following inputs (If you don’t remember how to do this look at code from a previous studio):

    Variable Name Concept
    startAmount The amount of money that you start with
    winChance The win probability, or the probability that you win a single play
    winLimit If you reach this amount of money you had a successful day and leave
  3. Start by simulating a single day. Write code that will repeatedly play until “success” or “ruin”
    • Include println statements showing each win or loss. You’ll remove these soon, but they help you verify that your code is working (Remember this trick!)
  4. Test your work! Incremental testing is vital to your success in this course. Coming up with creative ways to test your work will be immensely helpful.
    • Brainstorm ways to ensure successful days or ruined days. For example, can you pick values that ensure you win all plays and leave the casino after 5 plays? What about values that ensure you lose after 8 plays? If your code doesn’t behave as expected, carefully review it and ask for help if you can’t find the cause of the problem.
    • How would you expect it to behave if you started with $500, will stop at when you have $1000 (or ruin), and the game has a 50% (0.5) chance of winning?
  5. Now we want to answer the question posed earlier: If you play every day, what percentage of time will you “ruin”? Start in adding and prompting for a new variable:

    Variable Name Concept
    totalSimulations The number of days you’ll simulate
  6. Then use this value to repeatedly play an entire day’s casino visit (the work you’ve already done) until you succeed or ruin (totalSimulations times).

  7. Remove any println() statements for individual plays and instead print:
    1. The simulation (day) number
    2. the number of plays that took place that day, and
    3. whether the day ended in success or ruin

    (Get this part working before proceeding: See the examples values and corresponding sample output below. It should look somewhat like this, but since you’re using random values the results may not be identical.)

  8. Compute an estimate of the “ruin rate” (percentage of simulations resulting in ruin) and print it after all the rounds of simulation.

  9. Finally, calculate and print the “expected ruin rate” in your program using the following formula:

, where:

Example Run

Here’s an example run. You should try to match this format. Given:

Variable Name Value
startAmount 2
winChance 0.7
winLimit 10
totalSimulations 500

Example Output

Simulation 1: 16 LOSE
Simulation 2: 10 WIN
Simulation 3: 6 LOSE
Simulation 4: 8 WIN
Simulation 499: 28 WIN
Simulation 500: 18 WIN
Losses: 90 Simulations: 500
Ruin Rate from Simulation: 0.18 Expected Ruin Rate: 0.18350278772959913

Final Testing & Review

  • Come up with creative ways to test your code until you’re confident it’s working as expected. Talk to your TA/instructor and try to justify that all you calculations are correct.
  • Review the development process. The order of your work was chosen carefully to increase the likelihood you’d be successful. Review the steps and try to identify strategies that may help you be successful in other course work. Again, share with your TA/instructor.

Problem 2: Computing Pi by throwing darts

Computer scientists often use simulation as a means of modeling, understanding, and predicting real-world phenomena.

Your group is auditioning for survivor by proving your group’s ability to compute Pi using only the materials at hand, as follows:

  • A unit-square dart board (1 meter by 1 meter). Unit-square dart boards are astoundingly resilient in plane crashes and yours is nicely intact.
  • Some darts, suitable for throwing at the dart board.
  • A 1 meter string and a stylus, suitable for inscribing an arc of a circle in your unit-square dartboard. That is, you can pin the string to a corner of the dartboard and use the stylus with the other end to draw an arc of a circle on the dartboard.
  • A dart-throwing expert. However, since the plane crash, the expert is left with the (uncanny) ability to throw darts that always land somewhere, uniformly and randomly, within the unit-square dart board. While the thrower never misses the unit square, the darts sometimes land within the inscribed circle, sometimes not.

As a group, develop an approach for computing Pi based on the above materials.

Hint (click triangle to reveal)

Think about the chances of where a dart lands. Can you develop a forumla for it? How could you estimate this chance via a simulation?

  1. Create a new class called Pi in your studio2 package
  2. Implement your approach using iteration:
    • You will need to simulate a random dart thrower. Think carefully about how you can use Math.random() to generate a random dart throw.
    • You’ll need to know where the dart hit. You may find the Math.sqrt() function helpful.
  3. Investigate and discuss how well your technique computes Pi.
  4. As before, discuss ways to test and validate your work.

Further investigations

If you have time, pick one or both of the following:

  1. Investigate the fairness of the Math.random() method.
    1. What properties should a random number possess?
    2. How can you measure the fairness of a random number generator?
    3. Implement some tests and discuss your results amongst yourselves and other groups.
  2. There are other ways of computing Pi. Try some of these and study their effectiveness in terms of the number of iterations you use.

Demo (get credit for your) your work

Commit and Push your work. Be sure that any file you worked on is updated on GitHub.

To get participation credit for your work talk to the TA you’ve been working with and complete the demo/review process. Be prepared to show them the work that you have done and answer their questions about it!

Before leaving check that everyone in your group has a grade recorded in Canvas!