Fence Post Errors

It is so common to write to one element past the end of an array that this bug has its own name. It is called a fence post error. This refers to the problem of counting how many fence posts you need for a 10-foot fence if you need one post for every foot. Most people answer 10, but of course you need 11. Figure 15.2 makes this clear.

This sort of “off by one” counting can be the bane of any programmer's life. Over time, however, you'll get used to the idea that a 25-element array counts only to element 24, and that everything counts from 0. (Programmers are often confused why office buildings don't have a floor zero. Indeed, some have been known to push the 4 elevator button when they want to get to the fifth floor.)