What is an Off-by-one Error and How to Fix It?

by Carson

Off-by-one errors are one of the most common mistakes you can make in programming. What is this error, and how to fix it? Let’s find out in this article.

Examples of Off-by-one Errors

This type of bug is a bug that renders the value of a variable wrong by one in at least some way. For instance, one of the most common manifestations of this error is that a loop is iterated once more or once less than the desired value. Thus, it results in an incorrect value of a variable or a wrong action taken by the program.

There are two common types of off-by-one errors. One is mistaking an operator for another. For instance, if you are not exactly aware of what you’re doing, you might mistake a “<” for a “<=”, for example. This works fine in normal circumstances, but it will produce the wrong value for a specific value. For instance, 3 < 3 returns false, while 3 <= 3 returns true. While you might think this kind of mistake is rare, it’s worth examining this possibility if you encounter weird errors.

Another example is overlooking zero-indexing. In real life, we usually count objects from 1. However, if you don’t know that some programming languages count items from zero, it might be very easy to make such a mistake. For instance, in Python, list[1] is not the first item in a list, but it’s the second item of the list. It can also happen when selecting items from a list. For instance, in Python, list[1:-1] returns the second to the second-last item in an array, but one might wrongly assume that it also includes the last item in the list.

How to Identify Off-by-one Errors?

Off-by-one errors could be an error that is overlooked while debugging because you may believe that it cannot occur. Well, here are some telltale signs that, when present, possibly points to this kind of bug. Before you detect these bugs, however, make sure you print out values of all potentially problematic variables to see what is going on in your program.

Make sure you check the loops as well. A common tactic to identify whether the loop is being correctly executed is to print out how many times the loop has been run after it finishes looping. It works by defining an initial value of the counter variable to zero and then increasing it by one as the loop goes on. Finally, print out the value of the counter variable at the end of the loop. Chances are that the value of the counter variable is not the one desired because you might have made an off-by-one error.

Moreover, if you have located the error to be the wrong selection from an existing list, make sure you look for off-by-one errors. In fact, these errors are one of the most common causes of selecting the wrong things in an array. Also, when you test a function, make sure you check values that are at the boundary of conditionals. For instance, if the function behaves differently when the input is less than 3 than when the input is equal to or greater than 3, test this function with the input value of 3. If you got an operator wrong, you’ll be able to fix the error at this stage.

Fixing Off-by-one Errors

Fixing these errors is straightforward once you’ve identified the offending statement. Check your assumptions about how an operator works in that programming language, and correct it if necessary. For example, if you intended to include all but the last item of a list in Python and you wrote list[1:], you should correct that to list[1:-1].


In this article, we’ve mentioned the definition of an off-by-one error, how it manifests itself, how to see if there is a similar error in your program, and how you can fix it in seconds after identifying the issue. If we missed some important symptoms of these errors or some crucial steps in solving this problem, please leave them in the comments below to improve our article.

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