Off-by-one error
type of numerical or counting error
An off-by-one error or off-by-one bug (known by acronyms OBOE, OBOB, OBO, and OB1) is a logic error that involves a number that differs from its intended value by 1. An off-by-one error can sometimes appear in a mathematical context. It also often occurs in computer programming when a loop iterates one time too many or too few, usually caused by the use of non-strict inequality (≤) as the terminating condition where strict inequality (<) should have been used, or vice versa. Off-by-one errors also stem from confusion over zero-based numbering.
Cases
Fencepost error
A fencepost error (occasionally called a telegraph pole, lamp-post, or picket fence error) is a specific type of off-by-one error. An early description of this error appears in the works of Vitruvius. The following problem illustrates the error:
If you build a straight fence 30 feet long with posts spaced 3 feet apart, how many posts do you need?
The common answer of 10 posts is wrong. This response comes from dividing the length of the fence by the spacing apart from each post, with the quotient being erroneously classified as the number of posts. In actuality, the fence has 10 sections and 11 posts.
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