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.)
[glossary]
[Reference(s) to this entry by made by: {banana problem}{off-by-one error}{slop}{zeroth}]