Contents page

Index (83KB)

fencepost error

fencepost error: n. 1. 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.)