CPSC 170 Lab 1: More Arrays

Collecting Tunes

1. Compile and run Tunes.java. You should see a list of 6 CDs with a total value of $97.69 and an average cost of $16.28 followed by a menu that lets you add, search for, or remove a CD. Only the add part has been implemented, so add a few CDs to see what happens. When you add the 11th CD (total), you should get a message that says "Increasing size of array" (you may have to scroll back to see it), which comes from the increaseSize() method. I added this just to help you follow the program's execution.

2. Now you'll add the search functionality:
   1. First you need to be able to tell when two CDs are "the same." You could compare titles and/or artists, but it's cleanest to add an equals method to your CD class and use that. Add method public boolean equals(CD otherCD) to the CD class, and have it require that both the title and the artist be the same for two CDs to be equal.

   2. Add the findCD method from the prelab to the CDCollection class. Remember our discussion in class, and don't look through the array any longer than you have to. Don't forget the documentation.

   3. Now run Tunes again and choose the search option. Look for some CDs that are in the collection, some that some not, and at least one that you added to the original collection.

   4. As written, the search option in Tunes asks for the title, artist, price, and tracks because the CD constructor requires all four pieces of information. There's no need to make the user enter the price and tracks when only the title and artist are relevant to the search. You could pass dummy values to the constructor for the price and tracks, but better yet, add a new constructor to the CD class that takes just the title and artist. Now modify the Tunes class so that it prompts for just the title and artist on the search function, then creates a new CD object (using the new two-parameter constructor) and passes it to findCD.

   Test your search option thoroughly!

3. Now you'll add the remove functionality. A skeleton for a method public void removeCD(CD cd) is already in the CDCollection class. Your job is to complete it so that it removes the given CD from the collection if it is in there; if not, the collection should remain unchanged. Proceed as follows:
   • When you remove a CD, you'll need to update the collection's total cost. To find the cost of the CD you are removing you will need to add a getCost method to the CD class. Add this method.

   • To remove a CD, you first need to find it. You already have a findCD method but it just returns a boolean, not the location of the CD. Write a new method private int findCDIndex(CD cd) that takes a CD and returns the index of its location in the array, or -1 if it is not in the array. Now modify findCD so that it just calls findCDIndex and returns false if the result is -1, true otherwise. The body of findCD should now be just one line! And you can use the findCDIndex in removeCD as well.

   • Now you can start writing code for removeCD. First, find the index of the CD in the array. If it is there (index != -1) you will need to "take it out," but what does that mean? You can't leave an empty slot in the middle of the array -- if there are n CDs in the collection they should be in array locations 0..n-1, not 0..n with a hole in the middle. The solution is to shift over everything that comes after the CD you are removing in the array. For example, if there are 10 CDs in the collection and you remove the CD in array location 6, you will have to move the CD in location 7 to 6, the CD in location 8 to 7, and the CD in location 9 to 8. Think carefully about how to do this.

   • Don't forget to decrement the count.

   You'll also need to add code in Tunes to get information from the user and call removeCD when they choose the remove option.

   Test the remove functionality. Be sure to try removing more than one CD, removing a CD that is not in the collection, and mixing removes with searches and adds.

   Magic Squares

   One interesting application of two dimensional arrays is magic squares. A magic square is a square matrix in which the sum of every row, every column, and both diagonals is the same. Magic squares have been studied for many years, and there are some particularly famous magic squares. In this exercise you will write code to determine whether a square is magic.

   File Square.java contains the shell for a class that represents a square matrix. It contains headers for a constructor that takes the size of the square and a scanner to read from, plus headers for methods to read values into the square, print the square, find the sum of a given row, find the sum of a given column, find the sum of the main (or other) diagonal, and determine whether the square is magic. Note that the read method has been written for you, but you will need to write the others.

   File SquareTest.java contains the shell for a program that reads input for squares and tells whether each is a magic square. Following the comments fill in the remaining code, but take it a piece at a time as follows:

   • First fill in code for the constructor and the printSquare method in the Square class. Then add code to SquareTest just to read and print each square. (Note that the other methods in the Square class have dummy return values so it will compile.) Compile and run your program on file magicData (you'll need to redirect the input to do this: java SquareTest < magicData) to be sure you're reading all 7 squares correctly.

   • Now add code to compute and print the sums of the rows and run your program again to make sure this is working correctly.

   • Now add code to compute and print the sums of the columns and the sums of the diagonals, testing as you go.

   • Finally write the magic method and indicate whether each square is magic. You should find that the first, second, and third squares in the input are magic, and that the rest (fourth through seventh) are not. Note that the -1 at the bottom tells the test program to stop reading.