fencepost error

n. 1. [common] A problem with the discrete equivalent of a boundary
condition, often exhibited in programs by iterative loops. From the
following problem: If you build a fence 100 feet long with posts 10 feet
apart, how many posts do you need? (Either 9 or 11 is a better answer than
the obvious 10.) For example, suppose you have a long list or array of
items, and want to process items m through n ; how many items are there? The
obvious answer is n - m , but that is off by one; the right answer is n - m
+ 1. A program that used the obvious formula would have a fencepost error in
it. See also zeroth and off-by-one error , and note that not all off-by-one
errors are fencepost errors. The game of Musical Chairs involves a
catastrophic off-by-one error where N people try to sit in N - 1 chairs, but
it's not a fencepost error. Fencepost errors come from counting things
rather than the spaces between them, or vice versa, or by neglecting to
consider whether one should count one or both ends of a row. 2. [rare] An
error induced by unexpected regularities in input values, which can (for
instance) completely thwart a theoretically efficient binary tree or hash
table implementation. (The error here involves the difference between
expected and worst case behaviors of an algorithm.

