4.5 Developing complex programs

To develop and use more complex procedures it will be useful to learn some helpful techniques for understanding what is going on when procedures are evaluated. It is very rare for a first version of a program to be completely correct, even for an expert programmer. Wise programmers build programs incrementally, by writing and testing small components one at a time.

The process of fixing broken programs is known as debugging. The key to debugging effectively is to be systematic and thoughtful. It is a good idea to take notes to keep track of what you have learned and what you have tried. Thoughtless debugging can be very frustrating, and is unlikely to lead to a correct program.

A good strategy for debugging is to:

  1. Ensure you understand the intended behavior of your procedure. Think of a few representative inputs, and what the expected output should be.

  2. Do experiments to observe the actual behavior of your procedure. Try your program on simple inputs first. What is the relationship between the actual outputs and the desired outputs? Does it work correctly for some inputs but not others?

  3. Make changes to your procedure and retest it. If you are not sure what to do, make changes in small steps and carefully observe the impact of each change.

First actual bug. Grace Hopper’s notebook, 1947

For more complex programs, follow this strategy at the level of sub-com­po­nents. For example, you can try debugging at the level of one expression before trying the whole procedure. Break your program into several procedures so you can test and debug each procedure independently. The smaller the unit you test at one time, the easier it is to understand and fix problems.

DrRacket provides many useful and powerful features to aid debugging, but the most important tool for debugging is using your brain to think carefully about what your program should be doing and how its observed behavior differs from the desired behavior. Next, we describe two simple ways to observe program behavior.