Why does adding one to a pointer move by a whole element?

A strong answer

A strong candidate frames Why does adding one to a pointer move by a whole element? around the exact memory layout, compiler semantics, and hardware execution model covered in Pointer Arithmetic and the Link to Arrays. Explain scaled arithmetic, array decay, and the one past the end rule for loops.

Two earlier lessons fit together in a way that is not obvious yet. Lesson 1 said that a[i] is found by adding i times the element size to the start of the array. Lesson 6 gave you variables that hold addresses. So could you walk through an array by moving a pointer along it?

Two loose ends are waiting as well. The last check of lesson 1 showed an array parameter reporting the size of a pointer, and lesson 5 passed a string as a const char*. Both come from one rule, which this lesson explains.

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 address produced by adding an integer to a pointer.
• Mastery of core invariant: Explain that a[i] means *(a + i) and what decay does to an array.
• Mastery of core invariant: Use one-past-the-end pointers and pointer subtraction correctly.
• 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.