As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs.
- Maurice Wilkes discovers debugging, 1949
By now if you have been messing around with the programs you have probably found that sometimes the program does something you didn't want it to do. This is fairly common. Debugging is the process of figuring out what the computer is doing and then getting it to do what you want it to do. This can be tricky. I once spent nearly a week tracking down and fixing a bug that was caused by someone putting an x where a y should have been.
This chapter will be more abstract than previous chapters. Please tell me if it is useful.