How do you prove a loop boundary is correct?

A strong answer

A strong candidate frames How do you prove a loop boundary is correct? around the exact memory layout, compiler semantics, and hardware execution model covered in Off-by-One Errors and Loop Invariants. State a loop invariant, check init, maintenance, and exit, and pick boundary tests that expose off-by-one errors.

An off-by-one error is a loop that runs one time too many or one time too few. It misses the last item, processes a nonexistent one, or reads a single position past the end of the data. It is the most common bug in loops, and the reason is the fencepost problem: a fence 10 meters long with a post every meter needs 11 posts, because both ends have one.

For n equal to 3 it produces 3 instead of 6. Nothing about the typing is wrong. The error is in reasoning about the boundaries, and this lesson gives you two tools for that: a convention that makes boundaries predictable, and a way of thinking, the loop invariant, that tells you exactly what a loop computes.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Count the iterations of a loop from its boundaries, for half-open and closed ranges.
• Mastery of core invariant: State a loop invariant and check its initialization, maintenance and termination.
• Mastery of core invariant: Use the invariant and the exit condition to say what a loop computes.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.