``Good'' Programs

What distinguises a ``good program'' from a ``bad program''? Obviously, a good program has to work correctly and reliably -- a difficult goal in itself, as we'll see. But this is far from enough. In practice, very few programs are written once, used for a while, and discarded: much more often, a program is used until the need for it changes, the program is modified to handle the new requirements, the modified program is used for a while, and the cycle repeats. Thus a ``good program'' must be not only correct the first time around, but structured in such a way that it can easily be modified to accomodate likely changes in requirements. We'll study how to design such programs.