Debugging is what you do to fix your program when it doesn't work. [As opposed to what you do when it does not compile. -- 20 May 1996] The term originated with the first tube computers. Sadly debugging has always been a part of computer programming. Modern programming techniques try to reduce or eliminate the need for extensive debugging of code, by preparing logically perfect programs.

The Best Cure is Prevention

When you come to debug your program, you already know it doesn't work. If you knew before that it didn't work then you could correct it before you began. If you understand your program you will be less likely to make mistakes and be better at correcting them. Write the program so you can understand it. Use comments and simple code.

Is Clarity More Important Than Correctness?

If your program is clear you can make it correct. Can you be sure your program is correct if it isn't clear?

Slide 2: How to Debug

There's never time to do it right, but there's always time to do it again.

Plan ahead
Plan to have your program complete three days early. You can use the extra time to debug or carefully test your program.
Plan your program before you type it
- It will be less likely to be wrong
- If it is wrong you will know where to look
Program in modules (encapsulation!) [Encapsulation was a major focus of the lab that term -- 21 May 1996]
- Write programs using stubs
  - [test the stubs before incorporating them into the rest of the program! -- 20 May 1996]
  - [then test the logic that connects the stubs to the rest of the program. We briefly discussed what makes for a managable size code chunks in class. -- 28 May 1996]
- Comment as you write
  - Use comments as an outline
  - It would be nice if you'd tidy up the comments before handing-in your assignments
- Use PRE and POST conditions
  - What does each module do?
    - What must be true before the function is called?
    - What will be true after the function runs?
      - e.g., what value will changed variables have?
      - e.g., what will show up on the screen?
  - This will be easier to check

Slide 3: How to Debug

To Solve a Problem You Must Understand It

Find the code that generates the problem

Print values of variables to trace program execution
- Always print to stderr
- Always print the procedure name (see example immediately below)
Include debugging code
```
   #ifdef DEBUG
     fprintf(stderr, "top of main loop k= %d\n",k);
   #endif
```
- This code will only be run if the .c file has a #define DEBUG before the #ifdef
- Easy to turn on and off
- Can also use DEBUG levels (e.g., #if DEBUG > 1)