Programming Fundamentals › Programming Basics

Off-by-One Error

A loop or index that runs one step too many or too few.

Also known as: OBOE, fencepost error, off by one

An off-by-one error is a loop or index that runs one step too many or one too few. It’s among the most common bugs in programming.

items = ["a", "b", "c"]          # indexes 0, 1, 2

for i in range(1, len(items)):   # skips items[0]
    ...
items[len(items)]                # IndexError: the last index is len - 1

Where they hide

• Zero-based vs one-based. A list of length 3 has indexes 0 to 2.
• Inclusive vs exclusive ends. range(0, 3) and items[0:3] stop before 3. SQL BETWEEN and some APIs include the end.
• < vs <= in loop conditions.
• Counting items vs gaps (the fencepost problem): 10 fence segments need 11 posts.
• Date ranges. “From June 1 to June 30” is 30 days if both ends count, 29 if you subtract.
• Pagination. Page 1 starts at offset 0, not 1.

How to prevent them

• Loop over items, not indexes, when you can: for item in items.
• Check the edges by hand: run through the empty list, one item, and the first and last items.
• Write tests for boundaries, such as an empty input, a single element, and exactly at the limit.
• Be consistent about whether ranges are half-open (start included, end excluded). That convention makes end - start equal the length and adjacent ranges not overlap.

When a result is “almost right but one off”, suspect a boundary (slicing, indexing).