I've been thinking about how some code I'm maintaining could have been written to make maintenance easier.

These ideas will be very familiar, even obvious, to most experienced programmers. They're not new ideas by any means. (I was a little mortified when I saw that 6 had read this before I added this disclaimer; I must have sounded rather conceited.

• Don't write code that never gets run. People will assume that it is correct if it's in a program that behaves correctly, but it is likely that it will not be correct. Also, it's a waste of time, because it doesn't contribute to the usefulness of the code, but it does contribute to its difficulty.
• By the same token, don't write code to produce some results no one ever sees. The results will probably be wrong; if they're not wrong at first, they'll get wrong later on when someone (maybe you) changes the program. What's worse, though, is that when someone comes along to maintain the program, they will assume that the wrong results are in some sense ``right'' -- that is, the program works when they are there -- and may unknowingly add the first reference to them. Or, worse, they may duplicate your error.
• When you have to convert between two representations of something, and there are several steps between the two domains, try to do the conversion in as few places as possible.

  For example, if you have an array of some information about days of some month, in C, your array will begin with the 0th element. If all of your functions related to this array expect zero-based day numbers, it's likely that there will be many places where people will subtract one from a date that a user entered, or add one to a number returned from your array. It would be much less error-prone and simpler to use if your access functions did it -- or if element 0 of the array was unused.

• The preceding point can be greatly generalized. Don't cut and paste code if you can help it; especially don't cut and paste it from memory, because then you are likely to make mistakes. Create functions. In C++, sometimes you should create private member functions.

  I'm ambivalent about creating private member functions, though, because it often turns out that your code could be effectively used in other places too, outside the class you're working on at the moment. It creates a temptation to cut and paste.

Body of text last updated 1997-08-16. If you want my more-recent thoughts on programming (as well as other topics), you can subscribe to my thinking-out-loud mailing list. (This paragraph was added 1999-02-26.)