Loops versus recursion: when to use each?

A strong answer

A strong candidate frames Loops versus recursion: when to use each? around the exact memory layout, compiler semantics, and hardware execution model covered in Repeating Yourself: for and while Loops. Explain the difference between iterative loops and recursive function calls, with trade-offs in simplicity and stack usage.

Suppose you need to print the numbers from 1 to 5, or add up every integer from 1 to 100. With only variables and printf, you would have to write the same statement over and over:

Writing five additions by hand is tedious; writing 10,000 additions is impossible. Worse, what if the upper limit n is typed by the user at runtime via scanf? You cannot write n lines of code when you do not even know what n is until the program runs.

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: Write for loops to repeat code a known number of times.
• Mastery of core invariant: Write while loops to repeat while a condition is true.
• Mastery of core invariant: Trace loop iterations through variables and output.
• 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.