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A fencepost error is a specific type of off-by-one error that has to do with unlikely or misunderstood algorithms for development.
Fencepost errors are also known as telegraph pole errors and lamppost errors.
At the heart of the fencepost error is the idea that the number of posts used for a fence project differs based on whether the fence is long with two opposite ends, or whether it circles back to a closed loop. In the former scenario, a fencepost error can occur if a person makes an intuitive supposition that a fence only needs as many posts as dividing the length by the length between posts. For example, if the fence is 50 feet long and has posts every 5 feet, the human working on this problem might suppose that the fence needs 10 posts. However, if the fence is not a closed loop, it needs an additional post at the end — in other words, it needs a post at the starting point or zero, as well as a post at the ending point or end number. Algorithms demonstrating the fencepost principle would use a phrase like “n+1” posts. However, it must be noted that if the fence is a closed loop, the additional fencepost is not needed.
The fencepost error has multiple allegories and applications to coding and development. It represents one of many logical errors that cause programs to experience bugs or produce inaccurate results.