Off-by-one error
An off-by-one error is a boundary or counting mistake in which an index, range, iteration, or computed value is exactly one too high or low.
100 Java Mistakes and How to Avoid Them
An off-by-one error is a boundary error in which an iteration or indexing operation starts or stops one position or value too early or too late.
- How it is used
- Diagnoses boundary failures in Java indexing, range validation, and forward or reverse iteration.
- How deep it goes
- Explains failure modes, corrected loop forms, overflow-safe range checks, and analyzer limits without developing complete programs.
- Examples
- Uses empty strings, arrays, lists, contiguous ranges, descending for loops, and Java range-checking methods.
- What you can do after
- Diagnose and prevent boundary bugs with explicit checks, corrected loop bounds, Java validation methods, and targeted analyzers.
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Software Design for Python Programmers
An off-by-one error is a computational error in which a value or operation count is exactly one greater or one less than intended.
- How it is used
- Prevents surprising runtime logic errors by making loop counts and index conventions explicit.
- How deep it goes
- Explains loop bounds and zero-based indexing through worked examples, then stops at two simple corrective designs.
- Examples
- Uses a six-iteration while loop and a Python Date class whose month lookup initially prints MAR for February.
- What you can do after
- Diagnose incorrect counts or values by checking loop boundaries and translating human-facing values before zero-based indexing.
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100 C++ Mistakes and How to Avoid Them
An off-by-one error is a boundary error in which an iteration or index range includes one element too many or too few.
- How it is used
- Prevents classic loop-boundary mistakes by favoring range-based iteration over manual index mechanics.
- How deep it goes
- Defines the boundary error, shows its misleading continuation test, and explains the safer range-based alternative.
- Examples
- Uses indexed and range-based vector summation examples, including the problematic <= continuation test.
- What you can do after
- Apply it via range-based for loops to iterate over containers without manually writing boundary tests.
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Go in Practice, Second Edition
An off-by-one error is a boundary mistake in which a loop or index begins, ends, or counts one position incorrectly.
- How it is used
- Uses table-driven tests to find incorrect loop starts and exclusive endpoints in generated output.
- How deep it goes
- Explains two boundary mistakes and demonstrates diagnosis through failing and passing Go test output.
- Examples
- Uses a Fizz Buzz loop, exact expected strings, table cases, and go test output.
- What you can do after
- Diagnose boundary mistakes with table-driven tests and correct loop starts, endpoints, and expected output.
Read how Go in Practice, Second Edition explains off-by-one error →
Related concepts
- runtime logic errorproduces · A runtime logic error is a defect in a program’s computation or control logic that causes incorrect behavior during execution, even though the program runs.
- range-based for loopaddressed by · A range-based for loop is an iteration statement that traverses the elements of a range or container without explicit index or iterator management.
- iteratoraddressed by · An iterator is an object or abstraction that provides successive values from a sequence or data source through a traversal protocol.
- indexingused by · Indexing is the organization or use of positions, keys, or lookup structures to identify and retrieve selected data or elements.
- loopuses · A loop is a programming control-flow construct that repeatedly executes a block of instructions according to an iteration rule or continuation condition.
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