Programming Assignment Checklist: Hamming Codes

Frequently Asked Questions

What are the goals of this assignment? To learn about error-correcting codes; to learn to program in a machine language; and to appreciate the programming and debugging advantages of a structured programming language like Java over machine language!

What is the format of a .toy file? To initialize a location in memory to have a value, write (on its own line) the index of the memory location (two hex digits), followed by a colon, followed by a space, followed by the value (four hex digits). Anything else on the line is ignored by the TOY simulator and treated as a comment. As well, lines not of this format are ignored. Read the sample TOY program multiply.toy. isn't following the TOY program that I gave it! What is going on? It's indeed doing what you asked, but you might not have asked for what you meant to.

How much do I need to comment my TOY code? At a minimum, you should:

How do I format the TOY output? You can't. Just print one integer per line (0000 or 0001).

When I run my TOY code in the simulator I get the error java.lang.NumberFormatException: null. What does this mean? This is the error that produces when you try to read from standard input when there is no input left. Be sure that your TOY program terminates when it encounters FFFF.

I can't launch the Java Web Start version of the Visual X-TOY simulator. Any ideas? The program requires something called Web Start. When you installed Java initially, this was included. However, the last Windows security patch I applied broke this. If this happens to you, go to and click the Get It Now button.

Running TOY Programs

To simulate the execution of your program on a TOY machine, use either the text-based or the Visual X-TOY simulator — both are available on the assignment page.

Command-line TOY simulator. The command-line TOY simulator lets you simulate the code, using Java's standard input/output to simulate TOY's standard input/output. takes one command-line argument, which is the name of a TOY program file.

Visual X-TOY simulator. The Visual X-TOY simulator from lecture provides lots of useful development features. There are three modes (edit, debug, and machine).


Input, Output and Testing.  Once you have written your first version of encode.toy, run

% java-introcs TOY encode.toy
to type your inputs interactively, or
% java-introcs TOY encode.toy < encode1.txt
to read your inputs from a file instead. The file encode1.txt consists of a single test case for your encode.toy
0000 0001 0000 0001
and if your program is working correctly, testing on this input should give the following, surrounded by initial and final core dumps.

This correct answer also appears in the file encode1-answer.txt. We provide several input test files of different sizes for both encode.toy and decode.toy, as well as the answers for each. Note that for convenience, the answer files list each input line and just a shortened version of the corresponding outputs.

The most complete way to test your TOY programs is to encode and decode all possible inputs. Use the sample inputs encode16.txt and decode128.txt to do this.

Reference solutions.   As a special bonus, we provide and, which are two programs that encode or decode numbers in the same format as the assignment. You are welcome to examine or execute this code to help you in any way you see fit.