• What is a debugger?
• A tool for programmer to
• Trace the execution path of a program
• Step through the code line by line and investigate the value of variables at each step
• When you need a debugger?
• When a program does not act as the programmer expected, the debugger can help to find out the logical bug

Outline

• In this lab you will learn how to
• Trace a program
• Display the value of variable
• Finish exercises

Lab Exercise

• Debug a sample program (debug1.cpp)
• Steps:
  1. Create a new project in Visual Studio (VS)
  2. Download the debug1.cpp from Canvas and save it to the project folder
  3. In Visual Studio, add an existing item to the project

Add the sample program to the project

Tip: Display Line Numbers in Visual Studio

• To display line numbers in code
  1. On the menu bar, choose Tools, Options. Expand the Text Editor node, and then select either the node for the language you are using, or All Languages to turn on line numbers in all languages. Or, you can type line number in the Quick Launch

Tip: Display Line Numbers in Visual Studio

• To display line numbers in code

2. Select the Line numbers checkbox.

Set a breakpoint

Steps to invoke the debugger

• Build/Rebuild the solution
• Start the debugger
• Start a debugging session using F5 (Debug > Start Debugging). This command starts your app with the debugger attached.
• The green arrow on the tool bar also starts the debugger (same as F5).
• The program execution should stop at the line x=3

Start the debugger (menu bar)

Start the debugger (tool bar)

Execution stop at x=3

Step through a program

• Step over (F10):
• Execute the current statement and stop at the next statement.
• If the current statement is a function call, the function will be called.
• Step Into (F11):
• If the current statement is a function call, go inside the function body and stop at the first statement.
• Step out (Shift+F11):
• Finish the execution of current function and stop at the point where the function is called.

Step into code, line by line

Step through code, skipping functions

Step through code, skipping functions

Summary of short-cut key

Display the value of a variable

• Value of variable can be found in
• Autos: display related variables (selected by VS)
• Locals: display local variable only
• .N: display user selected variable

Display the value of a variable

Example: x = -858993460

Example: x = 3

Example: input new x

Example: x = 5

If you cant find the watch window

Add a variable to Watch

Add a variable to Watch

Add a variable to Watch

Exercise 1

• Download the program cToF.cpp
• The program accepts a real number in Fahrenheit (F) and output the temperature in Celsius, where C = (F 32)

* 5 / 9.

• Compile and execute the program.
• Type 100 as the input
• Notice that the output of the program is not correct and the program contains logical errors.
• Try to locate and debug the program using the VS debugger
• Hint: what is the value of 5/9?

Exercise 1: Wrong Output

Exercise 2

• Do this question after you have learnt the topic on arrays
• Download sumArray.c, correct the logical errors in the program with the use of the debugger