Re: Fence post error (was: CONLANG Digest -)

FFlores wrote:

> What's the fence post error?
From the Jargon File:

fencepost error n.

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 is that?  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.

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