fencepost error

1. (Rarely "lamp-post error") 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. The "obvious" formula exhibits

a fencepost error.

See also zeroth 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 coding

implementation. The error here involves the difference

between expected and worst case behaviours of an algorithm.

[Jargon File]

(1994-12-01)