Off-by-one error facts for kids

An Off-by-one error is a common mistake in Computer programming. It happens when a computer program is off by exactly one number. This often occurs when checking the edges or limits of something, like the start or end of a list.

A straight fence with n sections has n+1 posts.

What is an Off-by-One Error?

An off-by-one error is a type of logic error. This means the program runs without crashing, but it doesn't do what it's supposed to do because of a small mistake in its thinking. Imagine you need to count items, but you end up with one too many or one too few. That's an off-by-one error!

The Fencepost Problem

A classic example of an off-by-one error is called the fencepost error. It's also sometimes called a telegraph pole or lamp-post error.

If you build a straight fence 100 meters long with posts 10 meters apart, how many posts do you need?

Many people might quickly guess 10 posts. But let's think about it:

• The first 10-meter section needs 2 posts (one at the start, one at the end).
• The second section needs another post.
• If you have 10 sections, you actually need 11 posts! (One post for each section, plus the very first one).

So, the intuitive answer of 10 is wrong. The fence has 10 sections, but it needs 11 posts.

The opposite mistake can also happen. If you know the number of posts, you might wrongly assume there are the same number of sections. But the actual number of sections is always one less than the number of posts.

More generally, think about this:

If you have n telegraph poles, how many gaps are there between them?

The correct answer depends on the situation. It could be n − 1, n, or n + 1. You have to think carefully about the exact problem. Fencepost errors happen when you count the items instead of the spaces between them, or vice versa. They also happen if you forget to count both ends of a row of items.

Off-by-One Errors in Programming

Off-by-one errors often happen when programmers use loops. A loop is a part of a program that repeats a set of instructions. For example, a loop might go through a list of items, like numbers in an array. An array is like a numbered list or a row of boxes where you can store information.

If there's an off-by-one error in a loop, it means:

• The loop runs one time too few, missing the last item in the list.
• The loop runs one time too many, trying to access an item that doesn't exist.

If the loop runs one time too few, one item in the array might be left out. This can be hard to spot because the program doesn't crash. The only clue might be that one item isn't changed or doesn't have the right value.

If the loop runs one time too many, the program tries to look for an item outside the array's size. This usually causes a runtime error. A runtime error means the program stops working while it's running. This happens because the program checks the size of the array and sees that you're trying to reach a spot that isn't there.

See also

In Spanish: Off-by-one error para niños