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CMSI 186-03 Programming Laboratory, Spring 2017

Assignment 0220

This assignment is partway between the transition from Java and the deeper computational characteristics of a “full 186” program. For Java, we focus on introducing you to object-oriented programming. For computation, you get a taste of what it means to run “experiments” on a computer.

Background Reading

We remain within the Java tutorial here but are now particularly focused on classes and objects—features that, perhaps more than any other feature, give Java its particular personality (for better or worse). First, read about objects and classes here:

http://docs.oracle.com/javase/tutorial/java/concepts/index.html

Then, you can get into further detail here, except for nested classes:

http://docs.oracle.com/javase/tutorial/java/javaOO/index.html

Make sure that, between these tutorial sections and the provided starter code, you come out of this assignment with a solid understanding of basic object-oriented programming in Java.

Automated Feedback Setup

In order to connect your repository to our automated code review and feedback system, once you are up and running please send your repository’s URL to Ed Seim. You can either find him in person in the lab, tweet him at https://twitter.com/SirSeim, or create a GitHub issue on your repository and mention him in it (@SirSeim). Once he has you hooked up, the system will provide feedback on code formatting and quality whenever you commit a new version to GitHub.

To assist you in development and as a source of further feedback, test programs are provided with this repository. The test programs test the supporting methods for each program but not the full command line invocations. You will be shown how to use these programs in class. The test programs will also be run automatically whenever you update your work on GitHub. Make sure that the test program passes. I have my own additional test program for making sure that all cases are covered.

Points will be deducted if issues reported by the automated feedback system here linger in the final submission.

Classes and Objects: Game of Coins

OK, let’s get this out right away: this isn’t quite a game, but we hope you’ll still find it fun. For this program, you are asked to simulate a bag of coins. Further, the coins in this bag can be biased, meaning that they can be rigged so that the chances of producing heads vs. tails can be made to be other than 50-50.

Once you have a working simulation for a bag of coins, you will be asked to explore different bags of different sizes and different bias values by “throwing” the bags into the air and tracking whether the coins land as heads or tails. The results of these experiments shall be displayed as “ASCII art” bar graphs indicating how frequently each coin landed as heads vs. tails.

Building Blocks

You are given a fully implemented Coin class, to serve as both a building block for your code to use and as an example for how a typical Java class can be programmed. A CoinTestHarness is also provided, serving not only as a way to test the correctness of the Coin code but as an example for how Java classes are used.

You are asked to write BagOfCoins from scratch. This is to be a new Java class, to be saved under the name BagOfCoins.java, with the constructors and methods specified in the Javadoc pages available at the course website. BagOfCoins should use Coin to help implement its functionality. A BagOfCoinsTestHarness class is provided, against which you can verify the correctness of your implementation.

End-User Application

Once you have BagOfCoins written, it’s time to let command-line users play with it. Write a class called AlexanderHamilton whose main method creates a BagOfCoins, “throws” the coins in the bag multiple times, then displays the results of these throws as an “ASCII art” bar graph. The bar graph should be a histogram showing how many coins flipped as heads at a certain frequency. To save space, the histogram may skip partitions that have zero coins. For example:

$ java AlexanderHamilton
450-459  * 3 coins
460-469  * 19 coins
470-479  **** 59 coins
480-489  ************* 171 coins
490-499  ******************* 248 coins
500-509  ****************** 228 coins
510-519  ************* 166 coins
520-529  ***** 73 coins
530-539  ** 27 coins
540-549  * 5 coins
550-559  * 1 coins

The default invocation of AlexanderHamilton should throw a bag of 1000 fair coins (50-50 heads vs. tails) 1000 times, producing a histogram of 100 partitions. In the example above, this particular run yielded three (3) coins that came up as heads between 450-459 times out of the 1000 throws. 248 coins came up as heads between 490-499 times out of 1000 throws—generally expected given that these coins are unbiased.

Contrast that with this run, where we have the exact same experiment but use biased coins of 0.25. In other words, tails will generally come up at only a 25-75 ratio:

$ java AlexanderHamilton - - - 0.25
710-719  * 18 coins
720-729  **** 51 coins
730-739  *********** 145 coins
740-749  ******************** 259 coins
750-759  *********************** 292 coins
760-769  ************ 158 coins
770-779  **** 59 coins
780-789  * 14 coins
790-799  * 3 coins
800-809  * 1 coins

Notice that here, all of the coins came up as heads between 710 and 809 times out of the 1000 throws—quite a difference from the fair coins. The most common frequency is 750-759 occurrences of heads—292 coins behaved this way.

Additional Specifications

AlexanderHamilton should accept either zero (0) or four (4) command-line arguments. When 0 arguments are given, AlexanderHamilton should perform a default run of 1000 fair coins thrown into air 1000 times, displaying the results in a histogram consisting of 100 partitions.

When four (4) arguments are given, the user can customize the number of coins, the number of throws, the number of partitions, and the coin bias. The first three arguments should be positive integers and the fourth argument may have a decimal value. In addition, AlexanderHamilton should allow a dash (-) in an argument slot, indicating that the user would like the default value for that argument. For example:

$ java AlexanderHamilton 10000 - - 0.25

…means that the user would like to have a bag of 10,000 coins, all with a bias of 0.25. However the bag is to be thrown by the default number of times (1000) with the results displayed in the default number of histogram partitions (100).

Erroneous arguments in any form should be met with an appropriate error message, and not a Java exception. AlexanderHamilton must also display an error message in the special case where the number of partitions does not evenly divide the number of throws.

You might notice that we can write a lot more programs that use Coin and BagOfCoins than just AlexanderHamilton. But that’s the point: in a world of classes and objects, one can now imagine a codebase as being made of parts that a programmer can put together in different ways. What other interesting programs can you envision writing with Coin and BagOfCoins?

Bag-Throwing Experiments

As a non-code deliverable, once AlexanderHamilton is finished, invoke it using at least ten (10) different combinations of coin count, throw count, partition count, and bias. Copy-paste the results in a file called experiments.md. .md stands for Markdown—an easy-to-use text format for producing clean-looking web documents. You can examine the source code of these very instructions to learn some Markdown; you can also consult GitHub’s Markdown page, among others that can be found on the web.

Accompany the copy-pasted histograms with commentary on what the different experiments demonstrate. Use any knowledge that you already have about probability and statistics to design the experiments and discuss the results.

Summary of Deliverables

Phew! That seems like a lot to do but it’s mostly explanation. Here’s a summary:

  • Write BagOfCoins according to the specifications given on the course website
  • Write AlexanderHamilton according to the specifications given on these instructions
  • Document ten (10) or more experiments with AlexanderHamilton in a file called experiments.md.

Save them all in the GitHub repository that is given to you upon accepting the assignment.