Programming Assignment Checklist: Loops

Programming Assignment Checklist: Conditionals and Loops

Frequently Asked Questions

What are the goals of this assignment? To write several small Java programs so that you get accustomed to: declaring and initializing variables, using expressions, using conditionals and loops, arrays, and debugging your code.

What preparation do I need before beginning this assignment? Read Sections 1.2 and 1.3 of the textbook. Read Sections 1.3 and 1.4 of the textbook. You may also find it instructive to work through some of the other exercises and look at the solutions on the booksite afterwards.

How should I format my Java code? How much do I need to comment my code? Follow the style guidelines. Don't forget to put your name, login and precept in the header of every file, as well as a brief description (and/or example run) of each program.

I have the constants 6, 10, 51, 60, and 61 sprinkled through my TenDice program. Is there a better way? Yes, give your constants meaningful symbolic names, such as SIDES or NUMBER_OF_DICE. This will make your code easier to read, maintain, and debug. Beginning on this assignment, you will lose points for not doing so.

What does this error mean?
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0 at TenDice.main(TenDice.java:6)
This means that you are trying to access an element in the array that doesn't exist. The number at the end of the first line is the index into the array.
When the number is 0, it is possible the error refers to args[0]. Since that doesn't have a value, you probably aren't passing in any command line arguments to your program even though your program expects it.
Use the index number and the line number, in this case, 6, to determine what is wrong with your code.

Do I have to use command-line arguments to read the inputs? Yes, or you will lose a substantial number of points.

My output from RGBtoCMYK is almost the same as the sample on the assignment page. Only the last digit or two is different. Why is there this tiny discrepancy, and is my answer wrong? Computers work with limited precision, and so different algebraically equivalent solutions can give slightly different answers. In grading we ignore such tiny discrepancies (except for certain assignments later where we explicitly tell you otherwise).

Submission

Submission. When you submit, be sure to click the Check All Submitted Files button. Make sure that you submitted the right files and that they compile cleanly. You may resubmit a file after making corrections. The newly submitted file will overwrite the old one.

Testing and Debugging

Here are our results for sample runs of Time.

% java Time 725 
Start time: 12:00pm
End time: 12:05am

% java Time 1420
Start time: 12:00pm
End time: 11:40am

Possible Progress Steps

These are purely suggestions for how you might make progress. You do not have to follow these steps. The key to writing correct programs is to develop them incrementally, testing after each step.

Time.java program.

What variables do you need? You certainly need to input the number of minutes that have elapsed. What else would be useful? Starting hour? Ending hour? Starting minute? Ending minute?
How will you compute the number of full hours? How will you translate that to an appropriate ending time? How will you compute the number of minutes left over? (Hint: Integer division and remainder may be useful.)
Finally, how will you determine whether to print "am" or "pm" after the end time?

A drunkard's walk. This is similar in many ways to the gambler's ruin example from lecture and the textbook. The key to building a larger program is developing it incrementally.

Random walker. First, think about what variables you need to maintain. You certainly need to read in and store the command-line argument N. You also need to store the current location (x, y) of the random walker. What should be the type of the variables x and y? What should be their initial values? To choose a random direction, consider using the idiom from Section 1.2 to generate a random integer in a given range.
Random walkers. To get started, copy RandomWalker.java to a file RandomWalkers.java. Now, think about what additional variables you need to maintain. You certainly need to read in and store the command-line arguments N and T. In addition to the current location (x, y) of the random walker, you need an accumulator variable, say totalSquaredDist, that stores the total sum of squared distances so far. Nest the loop inside an outer loop that repeats T times and add code to update totalSquaredDist after each time through the outer loop.

Dice and the Gaussian distribution.

Write a program that prints the result of rolling one fair die. To generate a random die roll, use the idiom from Section 1.2 to generate a random integer between 0 and N-1, and modify it slightly to get one in the desired range.
Add a loop to print the sum of rolling 10 fair dice.
Add a second loop to repeat this N times, printing out the sum after each trial. (This printing of the sum is for testing purposes only. Remember to remove it from your final version.)
Maintain an array tally[] so that tally[k] stores the number of times the sum is exactly k.

Reviewing your Programs

Needless to say, you should continue to follow the guidelines from earlier in the course. Here are some new conventions to keep in mind.

Efficient and clear code: This will be more important as we write more complicated programs. This week it includes ideas like:

Assign a command-line input to a well-named variable. Do not repeatedly use Double.parseDouble() or Integer.parseInt() inside mathematical expressions.
Don't use Math.pow() if all you need to do is square or cube a number. Multiplication is much faster.
Think about ways to re-organize your code. Can you make it clearer or more concise? Here are two examples dealing with if and if-else logic:
1. if (condition == true) should usually be written as if (condition)
2. if (x>y) { if (y>x)... is badly written since the inner condition can never be true.
Avoid hard-wired, unexplained constants.
- 0, 1 or 2 used in an obvious manner is OK, but anything else should either be named, or have a comment explaining where it comes from.
- If you do not understand the use of a constant, then a comment stating that the equation comes from the assignment page will suffice.
Use descriptive variable names.
- Each variable's purpose should be obvious from its name, if possible. Use condition instead of c or oldValue instead of ov. (Only exception: simple variables like i are commonly used for loop indices.)
Follow directions.
- If an input is listed as an integer, read it into an integer data type.
- Follow output formats exactly.
- Did the directions specify a while or a for loop?
- Are your if-else conditions or loop boundaries missing any special extreme possibilities (corner cases)?

Enrichment

Here are some famous and not-so-famous quotations about learning to program.