How does array indexing compute an element address?

A strong answer

A strong candidate frames How does array indexing compute an element address? around the exact memory layout, compiler semantics, and hardware execution model covered in Arrays: Contiguous Elements and Indexing. Explain contiguous layout, zero based offsets, and why out of range access has no check.

A variable holds one value. To keep a thousand test scores you would need a thousand variables with a thousand names, and no loop could visit them. What you want is one name for a whole collection, with positions, so that a loop can walk through it. That is an array: a fixed number of elements of the same type, stored one after another in memory.

The interesting questions are all about memory. Where do the five numbers live, and how does scores[i] find the right one?

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: Compute the byte offset and address of an array element from its index.
• Mastery of core invariant: Explain why arrays are indexed from zero and why indexing takes constant time.
• Mastery of core invariant: Write loop bounds that visit every element of an array exactly once.
• 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.