What is an off-by-one error and how do I fix it?

Question

What is an off-by-one error? If I have one, how do I fix it?

Answer 1

An off-by-one error is when you expect something to be of value N, but in reality it ends up being N-1 or N+1. For example, you were expecting the program to perform an operation 10 times, but it ends up performing 9 or 11 times (one too few or one too many times). In programming this is most commonly seen happening when dealing with "for" loops.

This error happens due to a misjudgement where you do not realize that the number you are using to keep track of your counting may not be the same as the number of things you are counting. In other words, the number you are using to count may not be the same as the total of things you are counting. There is nothing that obligates both things to be the same. Try to count out loud from 0 to 10 and you end up saying 11 numbers in total, but the final number that you say is 10.

One way to prevent the problem is to realize that our brain has a tendency (maybe a cognitive bias) to make that error. Keeping that in mind may help you identify and prevent future situations. But I guess that the best thing you can do to prevent this error is to write unit tests. The tests will help you make sure that your code is running as it should.