CMPSCI 187: Programming With Data Structures

David Mix Barrington

Fall, 2011

Q&A for Programming Project #1: Mazes and Cells

Question text in black, answers in blue.

• Question P1.1, posted 8 September: On the Project 1 assignment page, it says that a Maze consists of three fields: height, width, and an array of Cell objects. Later it says that the third field is an array of String that represent the openness of the Cell objects in the Maze. Which is right?

  The Maze's third field is the array of Cell objects. What it says further down is that the third parameter of the constructor is an array of Strings. The constructor is going to have code to create the array of Cell objects, and it will look at the array of Strings to decide whether to make each cell open or closed. Most of the constructors you have seen are very boring, but in general a constructor may contain whatever code it needs to make the new object according to the specifications in the parameters.

• Question P1.2, posted 10 September: I have written the program for Project 1 but I don't understand how to submit it.

  You need to remotely access one of the edlab machines using your account. Exactly how you do this depends on what kind of machine you are using -- the simplest method is from a Unix machine, Linux machine, or the "terminal" of a Mac, using "ssh". Go here, and scroll down to the section on "remote access to edlab machines". Once you are on an edlab machine with your account, place your .java files in the directory called "cs187". And don't mess with the Unix permissions for that directory, or you may prevent us from reading your submission.

  Also, how can I check whether my program is right?

  I or the TA's can look at your program during office hours. More importantly, if you understand what the program is supposed to do you should be able to write a main methold that will test it on some examples. Your program should be able to create Maze objects using the constructors, display them with the toString method, and create the correct arrays of Cell objects with the moves method.

• Question P1.3, posted 10 September: I had some problems getting on to the edlab. I eventually solved them, but I hope that if I tell you how it may help others. First, I have both a WWW and an email ID, and it was my email ID that worked.

  Yes, for nearly everyone the edlab account has the same username as their spire email -- so a student with email dlmoody@student.umass.edu has username "dlmoody". Since you're a staff member, there were complications -- I'm glad they are resolved.

  I wasn't able to connect to the edlab machine from my Windows machine with WinSCP. But when I connected with PuTTY, then changed my password at the prompt, WinSCP worked thereafter. I think the mandatory change password prompt causes some problem for WinSCP.

  Thanks very much, that may very well help some others.

• Question P1.4, posted 10 September: I tried to test my program using your code on the assignment page and got a strange problem. When I said:

  Maze m = new Maze (3, 4, {"1111", "1001", "1111"});
  

  I got an Invalid variable declaration message from Dr. Java. But when I said:

  String [ ] s =  {"1111", "1001", "1111"});
  Maze m = new Maze (3, 4, s);
  

  it worked fine. Did I do something wrong?

  No, I did. I forgot that there is no such thing as an "array constant", only an "array initializer". (See Java Precisely, page 12.) To use a fixed array as a parameter, you need to first put it in a variable, then use the variable, as you discovered. The reason is that an array has to be created, either with new or an assignment statement that sets a variable. I will correct the assignment page.

  It's very helpful when a student spots an error like this, especially early in the assignment period. You get an extra point on the final exam -- thanks!

• Question P1.5, posted 10 September: Do you think we should use Hungarian Notation? If so, can you suggest a convention to follow?

  First off, I had to look it up, so I clearly wasn't expecting you to use it. It's a convention for naming your variables to catch type confusion. As I will say on Monday, our prime mechanism for keeping our variables straight is to break our programs into short methods, with only a few variables each to keep track of at one time.

  You should think a bit about your variable names, and how they convey information to you and to the reader of your program, but I don't have any fixed convention to suggest. (There are Java conventions about where to use capital letters and so forth, which I hope you saw in your previous course -- please use them.)

• Question P1.6, posted 11 September: I'm a little fuzzy on exceptions -- could you give me a hint about the exception that we are supposed to throw if someone asks for a Cell that isn't in the Maze?

  There are two purposes to an Exception: (1) give the user or programmer some useful information when a run-time error crashes the program, and (2) allow the programmer to include code to be run in the case of an anticipated abnormal condition. I don't think there is much on exception handling in 121, so we're only doing a very simple version of (1) in this project. The ArrayIndexOutOfBoundsException (and, by the way, I got the name of this wrong on the first version of the assignment -- see the green correction) is an arithmetic exception generated by Java's own code whenever a method tries to access an entry of an array that doesn't exist because it is out of bounds. So you don't have to generate this exception, and I'm not asking you to handle it -- it will crash the program with a run-time error, but the user will get the information as to what kind of error occurred and what the call stack contained when it did.

  My point in mentioning this exception is that only bad input to the call is supposed to cause it. If you are not careful about the boundary of the Maze, your own method could cause one of these expections with valid input, and that would be bad.

• Question P1.7, posted 12 September: Is there some reason why you list the fields of Cell as first x, then y, and then later talk about the positions of Cells as first row, then column? In your example, you talk about position (2, 3) of an array with width 4 and height 3, but the position x=2 and y=3 is not in that array at all.

  No, I just screwed this up. I've gone back over the assignment so that I always list the x (or horizontal, or east-west) direction first and then the y (or vertical, or north-south) direction. If you've written code already, you'll need to change it because the signature is now different.

  You get an extra point on the final exam for catching this mistake -- thanks!

• Question P1.8, posted 12 September: Should our classes have main methods, or should there be a driver class? You told us to define the classes but then not to do anything with them.

  That's right -- you are writing code not knowing necessarily how it is going to be used, and that is deliberate on my part. The TA's will write some driver class to run with your classes to test whether your classes have the right functionality. You, of course, are going to want to write some kind of driver to test whether you are confident that your code works. Note that the "type" Cell [ ], the output of moves, has no toString method, so you will have to write code that runs over the elements of the array and prints something to tell what they are. But this is not part of what you need to submit.

• Question 1.9, posted 12 September: The size of the output array for the method moves depends on the Maze -- how can I create this array with a new statement before I know how big it is?

  The easiest thing is to look at the neighbors of the given Cell twice, once to count the open neighbors and once to put them into the array. One student suggested another valid solution -- put the neighbor Cells into an ArrayList, which expands to include as many elements as are put into it, then use the toArray method of that class to create the array of the right size at the end.

• Question P1.10, posted 12 September: I've figured out how to remotely log in to an Edlab machine. But how to I move a file from my own machine to an Edlab machine, so that I can submit my project?

  The way to do this is actually described on the Edlab home page here, but it's not that easy to figure out until you know what you want to do. If you have a Unix machine (which you do in effect with a Mac, with the Terminal application) you can use a utility called scp to move files to another Unix machine. Here is Navin's description of how to use scp:

  • Open your terminal on your own machine and go to the folder containing the file you want to move.
  • To copy the file foo.java, you would then say scp foo.java nsharma@elnux1.cs.umass.edu:/nfs/elsrv4/users3/grad/nsharma/TA/hw1, or at least that's what you would do if you were Navin and wanted to copy the file to the folder /nfs/elsrv4/users3/grad/nsharma/TA/hw1. (To find the Unix name of the directory you want on the Edlab machine, say "pwd" when you are there.)
  • CSCF, the Computer Science Computing Facility, has a brief guide to Unix on their site. You don't have to know very much Unix to put your .java and .class files in the correct directory, but a little is good.

  Another way to move a file from a Mac, in a way more intuitive for Mac users, is to download a free utility called Fugu.

  To move a file from a Windows machine to a Unix machine, you can use a free utility called WinSCP.

• Question P1.10, posted 15 September: What are "the usual get and set methods"? Is it important that we spell them exactly the way you expect us to?

  The Cell class, for example, has three fields named x, y, and open. Normally we make these fields private and read and write them only through public methods, traditionally called getX, setX, getY, setY, getOpen, and setOpen. You should really spell these exactly as I do -- you put "get" or "set" at the beginning, and capitalize the first letter of the field name.

  But for this assignment, the TA's will only test the functionality of the constructors, the toString methods, and moves. So you just need to give those methods names that your own code will use directly. (You may not need to use all those methods -- my solution used only one of the six methods in the Cell class.)

• Question P1.11, posted 15 September: In our cs187 folder on the EdLab, should we put our .class and .java files into a subfolder called project1 or something?

  That's a good idea, especially after this assignment when we have different projects to keep apart. For this assignment, as long as we can find your classes somewhere in your cs187 folder, it's fine.

• Question P1.12, posted 15 September: In my Java environment, the escape sequence "\r" did not work as a carriage return, as you said it should. But "\n" worked fine -- may I use that instead?

  Yes, we are happy with either "\r" or "\n". In theory, Java is a totally portable language, in practice not so much...

• Question P1.13, posted 15 September: I tried to move my files from my own computer to the EdLab using scp as you describe above in your answer to Question P1.10. It didn't work, and I figured out that in your sample scp command, you have a typo -- you wrote "nfs.eslrv4" where you should have written "nfs/elsrv4".

  Thanks, I've fixed the typo above. Fortunately you were knowledgable enough to use the Unix command "pwd" to get the full name of each of the two folders involved, rather than trusting my ability to type them correctly.

• Question P1.14, posted 15 September: I think I can help further with Question 1.12 above. In my Java environment, a "\r" is a "carriage return", which moves the cursor to the beginning of the line but doesn't advance to the next line. A "\n" or "newline" has the usual behavior of the "return" key on the keyboard, which is what you want here.

  You're right, and this was my mistake. We will still accept solutions that insert either \r's or \n's, but it should be "\n" and I will correct the program assignment page.

  You get an extra point on the final exam for pointing out and correcting the error.