HOMEWORK 2, VARIABLE-SIZE BAG OF ITEMS Solution

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Purposes:  Extend an existing class to have new methods and a different implementation. Manage the array by making it larger or smaller as needed, using the C++ commands new and delete, and learn how to write code that neither leaks memory nor dangles references.

 

As with HW01, we’re going to frame it in terms of an interface (what an outsider can ask the class to do) and an implementation (how you, the programmer, choose to make the code do this).  You will be given the interface file, which does not need to be changed; the implementation file, which needs to be completed; and a small test file to get you started (which is different from the complete version of the test file we’ll use).

 

You are making a variable-size Bag, which has 4 variables. Only one of them is new:

 

  1. 1. An integer called DEFAULT_CAPACITY, telling you the Bag’s default starting size (the size of a new Bag).   This number is a constant (we usually write these variable names in ALL CAPITALS).  It is defined as 4.
  2. 2. An integer called myCapacity, which tells you the Bag’s current siz This number

is subject to change when we make the Bag larger or smaller.

  1. 3. An array of items called itemArray, which has a number of slots equalling

myCapacity.

  1. 4. An integer called numberOfItems, which tells you how many slots you have filled, counting from the first slot, itemArray[0].

 

DEFAULT_CAPACITY doesn’t change, unless you change 4 to another number.  So the variables itemArray, myCapacity, and numberOfItems are all that you need to manage to get all the behaviors we want the Bag to have, for now.  (In the readings, these variables

have different names, but I wanted to use the same names for the C++ and Java Bag variables.)

 

Be sure you understand the slides from Wednesday (posted on the moodle site) and the mechanics of adding and removing items from the array of items.

On your VM, please make a directory for homework 2 and copy ALL the files for the C++ Bag from the moodle site into that directory.  You only need to change the ArrayBag.cxx file for this assignment, unless you want to add more tests to the test code.

 

Your code (as supplied in the ArrayBag.cxx file) consists of stubs. You are free to use your own Bag functions for the stubs that do not change. We’ll keep a lot of the same interface (the public Bag functions) but inside the implementation, we’ll change the ArrayBag constructor you had and add two more constructors, add a destructor, and include code to resize the Bag’s

array on the fly.  Resizing only increases the size of the bag; we won’t worry about resizing

down.  Write these functions first.  You should be sure to read pp.152-154 on resize() if

you’re attacking the C++ version.

 

When you have those new functions working, we’ll make the other Bag functions adapt to the resizing. Some functions, like getCapacity(), will need little changes, and others, like getCurrentSize(), contains(), and getFrequency(), will still work the same.  The main change you need to make is in add(), so that the Bag can resize up if it needs to.

 

The heart of this assignment is a loop that copies items from a small itemArray to a bigger one, without losing track of them.  In C++, the library algorithm includes a function to do this automatically, using a function called copy; it’s not in your text but you can look to (http://www.cplusplus.com/reference/algorithm/copy/?kw=copy) for a description. You can also accomplish this objective with a while loop, a do loop, or a for loop; I am completely agnostic about which method you choose, as long as the copying works perfectly at all times and doesn’t copy unnecessary junk items between arrays.

 

I’m assuming that you will use Geany for this (other editors are allowed but not described in this set of directions).  Assuming that you will be using Geany, open ArrayBag.cxx in Geany.

 

Again, please tackle the constructors ArrayBag(), ArrayBag(int capacity), and ArrayBag(const ArrayBag &anotherBag) first.  Unlike the HW01 bag, these constructors actually make the itemArray, using the new command, so this array is now being maintained by you on the heap. Next, write the destructor, which uses the delete command to clean up the ArrayBag’s memory. Because these functions involve pointers and memory, they can be tricky and errors in them may not appear till a segmentation fault shows up when you run your test code.  It’s important to get them debugged first.  Next, write resize(int new_capacity).  Be sure not to downsize the Bag; if you are asked to downsize below your current capacity, do nothing.  As you get each function written, please make sure the code you have added is not full of typos and errors by compiling it. For this, open the ArrayBagTester2.cxx file and make sure that it is the active tab. You should be able to see the code for ArrayBagTester2.cxx (not your ArrayBag.cxx code), as below:

 

 

Click on the down arrow next to the bookshelf icon (see below for this) to add a parameter to the Build commands:

 

Now, clicking on the Build button () runs the command

 

g++ -Wall -o “%e” “%f”            (you can see or change this in Set Build Commands)

 

which in turn compiles the file by replacing %f with the filename for the active-tab file, and replacing %e with the compiled executable, producing the command

 

g++ -Wall -o “ArrayBagTester2” “ArrayBagTester2.cxx”

 

As you fix errors and add code, eventually you will stop getting error messages; this means that the compilation has been successful and your ArrayBagTester2.cxx test code now understands the ArrayBag.cxx code you wrote. At that point, you will also see a new file in the directory, called ArrayBagTester2; this is now a file you can run to test your bag.  Clicking the gears icon (

 

) will run this ArrayBagTester2 code you made. (It’s worth mentioning that you can only make executable files from C++ programs with a main() function, like ArrayBagTester2.cxx; other things, like your ArrayBag.cxx class, get used by programs with main() functions but can’t run on their own.

 

 

 

ADDENDUM for people who just don’t like Geany’s shapes, bookshelves, and gears:

 

If Geany won’t cooperate and you want to build and run your code in the terminal, start a terminal window by clicking () in the sidebar menu.  You will need to use cd, the change directory command, to navigate to the directory with your HW02 files.  (In the screen capture below, you will see that mine are in Dropbox, in a directory called CSCI2270Fall2013/Homework/HW02/Cxx; you can use a different name here.)  Once I’ve changed to the proper directory (by using the first 2 commands that start with cd), you can see the g++ command that compiles the code, and the ./ArrayBagTester2 command that runs it.

Note that with the terminal window, you need to add ./ before the command-line function (it’s a concession to prevent a potential security hole; read  http://www.linfo.org/dot_slash.html if you would like to know why, but only if you want to).

 


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