CS 1711 Lab 4 - October 19 and 21, 1999
Debugging decision and loop structures
Introduction
The purpose of this lab is to work with if-statements
and while-statements, the basic decision and repetition structures of
all imperative programming languages in addition to C++.
To help with the design of decision and looping structures, many integrated
development environments (IDEs) include a debugger which allows the programmer
to step through their program one line at a time and inspect variables to see
if they are changing the way they are expected to.
Debugger
The Visual C++ debugger is operated by choosing Build | Start Debug.
The debugger makes use of some of the files in the Debug folder that is created
in each project when you compile your program. Note that the debugger
will only work on a program which has compiled successfully.
You can use the choices that appear when you select the debugger. They
include Step over and Step into selections.
The first will
execute the program a line at a time, including the execution of any functions
called by the action of that line. The second will also follow execution
through the steps of function calls.
They are also available on a button bar at the top right of the file window
after you start debugger. Note that there is a Stop Debugging
button as well.
If you use Step into
for library functions such as cout you will arrive
in a rather complicated situation. If you do this inadvertently you may
simply use Step Out.
A useful feature of the debugger is the Run to cursor choice.
Perhaps the most useful feature of the debugger is the objects window
that opens at the bottom left. It keeps track of the values of all of the
program objects while the debugger is operating.
You will be tested on the use of the debugger before leaving the
lab! Try it out on the programs you will be modifying in the lab.
Designing and Testing a Payroll Program
In this section, you will extend an existing program. The program will
include the use of a new if-statement and will require the selection of
appropriate test data to ensure the new modification works correctly.
- Create a project on your M: drive called Payroll.
Add the file named payroll.cpp (this will be located in the same
area as the previous labs).
- Read through the program to get an understanding of what it does. This
program contains a free function. The function is named `instructpay()'; here is how it works. When the computer executes the instruction
instructpay(); in the main() function, it jumps down to the
statements of the function instructpay() at the bottom of the program.
Take a peek at how this function looks. It turns out that functions are
nearly the most important thing learned in C++.
- Also notice the location of the two objects TAX_BRACKET and
TAX. Both of these are special objects called constant (because
of the word const in front of them). They are given a value when
declared that you can never change. This is where you will usually put
constant declarations. You must NEVER, NEVER, NEVER
put regular variable declarations here, you will lose marks on your
assignments.
- Modify the program so that it will also pay 50% overtime for all
hours worked over 40 hours. The overtime should be added to the gross
salary before taxes are evaluated.
- The lab assistant will ask to see your modified version of payroll.cpp before
you leave.
- Now pick some appropriate test data. What is appropriate? You must
pick a collection of input data that tests every branch of the program.
Select the appropriate data and run the program to complete the table:
| Test |
Hours |
Rate |
Overtime? |
Taxes? |
Gross |
Net |
| 1 |
|
|
No |
No |
|
|
| 2 |
|
|
Yes |
No |
|
|
| 3 |
|
|
No |
Yes |
|
|
| 4 |
|
|
Yes |
Yes |
|
|
Testing Loops
In this section we investigate a collection of programs named labloop1.cpp,
..., labloop5.cpp. The program allows the
user to enter a list of nonnegative integers then prints out the count
(number of elements in the list), the sum, and the average of the integers.
Each of the programs contain at least one logic error. Run each of the
programs and determine the error.
Before looking at the actual program, you should run the program a few times
to determine what types of input cause the errors to appear. This will
help you determine the type of error to look for. If required, you may use
the debugger to step through the program and inspect the variables at
each step to see what has gone wrong.
Keep a list of the programs you ran and what errors you managed to fix.
Running Stand-Alone Executables
The Visual C++ compiler always produces an executable file (.exe) that
can be run without the need for the compiler.
The executable is always placed in the Debug folder of your project.
You are to open up a DOS window and run a copy of the payroll program.
- Select Start|Programs|Command prompt.
- Type the letter of the drive where your projects are stored,
followed by a colon, and press Enter. For example M:.
- Type the command cd \Myprojects\CS1711\Lab4\Payroll
(if that is where your Payroll project is stored!)
Type the command cd \Debug
- Type the command dir and see if you can find the file
payroll.exe.
- Type the command payroll to run the program.
End of Lab
Make sure you have reported to the lab assistant
so that you receive credit for the lab.