Lab: Histograms

Last week, two of the discussion questions were:⁺

• One can simulate the movement of a fish in an aquarium by repeatedly flipping a coin. When "heads" is flipped, the fish moves one foot to the right; when "tails" is flipped, it moves one foot to the left. (We are assuming a long, narrow aquarium.) Assume that the fish's initial location is labeled 0 and that locations to the right are positive while locations to the left are negative. For example, the fish in its initial location has left and right neighbors at locations -1 and +1, respectively. What are the possible positions of the fish, relative to its starting position, at the end of a simulation with six coin flips? Explain. (Don't worry about the tank being too short.)
• Conduct several such simulations, each with six coin flips as described in the exercise above, and keep track of where the fish ends up after each simulation. Is the fish more likely to end up in one position than another? Explain why or why not.

In this lab you will write a program that will simulate a fish (or other object) moving randomly back and forth six times, starting at location 0. Initially your program will print the final location of the object (an integer between -6 and 6). You will then modify your program to run the simulation 1000 times, keeping track of how many times the object ends up in each of the possible final locations. Finally, you will enhance your program to draw a histogram (bar graph) of the various final locations. For example, a text-based histogram might look like the following:

     -6   xxxxx
     -4   xxxxxxxxx
     -2   xxxxxxxxxxxxxx
      0   xxxxxxxxxxxxxxxxx
      2   xxxxxxxxxxxxxx
      4   xxxxxxxxx
      6   xxxxx

Simulate an object moving six times.

• Download and unzip the HistogramUsingBlueJ zip file. Open the project.
• All of your modifications for this lab will be in HistogramLab.java. In fact, for the first part of the lab, all of your modifications will be where there is a comment saying "some code is missing here!" For now, you can ignore the commented-out code to construct a table and plot values. As you modify the file, remember to write appropriate comments that describe the purpose of the code you are about to write (what you are trying to accomplish) before writing it.
• Research the class documentation (not the code!) for the Coin class to learn how to construct a coin using the default Coin constructor. Also create a local integer variable to represent the object's location (-6 through 6). Initialize it to 0.
• Research the Coin class documentation to learn how to toss a coin and how to determine whether the tossed coin is showing heads or tails. Modify your program to toss the coin six times. Each time you toss the coin, update the location variable to reflect a move to the right if the coin comes up heads, or a move to the left if the coin comes up tails.
• After moving the object six times, print its final location. Run your program several times to test it.

Add multiple runs.

• In order to keep track of how many times the object ends up in each location over the course of many runs of the simulation, create a local integer variable to represent each of the possible final locations, e.g., minusSixCount, zeroCount, etc. Be sure to initialize each of them to 0. (Question: how many integer variables will you need to represent all of the possible final locations?)
• Embed the code you wrote earlier in a loop, in order to run the simulation 1000 times. Remember that the object needs to start at location 0 each time. Rather than printing out 1000 final location values, just increment the appropriate location counter after each run. (Note that there's a constant called NUM_ITERATIONS that you should use instead of "hard-coding" the number 1000 throughout your code.)
• After running the simulation NUM_ITERATIONS times, print the number of times the fish (or other object) ended up in each of the possible final locations. Run your program several times to test it. Do your results seem to make sense? You may wish to double-check that the various counts add up to 1000.

Draw a histogram.

• Examine the commented-out block of code that constructs a DisplayableTable object to display a histogram object. The first statement creates a table of cells that can be accessed by row and column. The next two statements put the text "-6" in text cell (0, 2) and colors the cell (0, 5) red. The last statement displays the histogram table. Uncomment this code, run the program, and see what happens.
• Research the DisplayableTable class documentation to learn how to call the add method. Note that there are two methods named add, both of which expect three parameters. The first method expects a Color to paint the cell, the cell's row, and the cell's column. The second add method expects a String of text to put in the cell and the cell's row and column. The code you uncommented contained an example of each add method.
• Now you are ready to make a histogram of your results. To do this, you will create rows of color blocks, where each row will indicate the number of times the fish ended up in a particular final location. First, create just a single row indicating how many times the fish ended up in the -6 location. Create a loop around the second add call to add the appropriate number of red blocks for that row of the histogram. Each red block will go in a new column. Test your program.
• Once you have the first row of your histrogram displaying correctly, add in the code to fill in the remaining rows of the histogram.
• Run your program several times to test it. You may also want to experiment with running it different numbers of times; for example, run the program several times with NUM_ITERATIONS set to 10, then several times with NUM_ITERATIONS set to 20 and 100. How does the behavior change as the number of iterations changes? Why?
• When you're sure that your program runs correctly, you may remove the System.out.println statements that printed the various final location counters. Also remove the statement at the beginning of the main method that said what the program would do once it was written.

Print and save your modifications.

• Update the class documentation for HistogramLab.java to accurately describe the purpose and behavior of the class from a user's perspective.  Focus on what the program does, rather than how it does it. Include your name and the date as well as the names of anyone from whom you received help. Providing proper documentation is an important step towards writing well-structured and reusable programs.
• When you are done and your program works correctly, print out HistogramLab.java to turn in.

After you have completed this lab and the More Fish! mini-lab, you can start work on Programming Project #3 (due at the beginning of Lab 4).

⁺These questions came from the Advanced Placement Computer Science Marine Biology Simulation Case Study, available from the College Board.