Loops (for / while / do-while)

C++ has three counted/conditional loop forms — for, while, do-while — plus break/continue to alter flow; they differ only in where the condition is tested.

Why it matters

The loop is where programs spend their time, so its shape drives performance: a tight for over contiguous memory is what auto-vectorizers and the cache love (see time-and-space-complexity). The pre- vs post-test distinction is a real correctness choice — do-while guarantees one execution (retry loops, menus), while while may run zero times. Knowing when a hand loop should instead be a standard algorithm is a senior judgment call.

How it works

• for (init; cond; step) — pre-tested; the canonical counted loop. All three clauses are optional (for(;;) is an idiomatic infinite loop).
• while (cond) — pre-tested; runs zero or more times.
• do { } while (cond) — post-tested; runs at least once.
• break exits the nearest loop; continue jumps to the next iteration (the step clause still runs in a for).

Form	Test timing	Min iterations	Best for
for	before	0	known count / index
while	before	0	condition-driven
do-while	after	1	run-then-check

Prefer the range-based-for-loop when you just iterate a container, and a named algorithm when intent is find/accumulate/transform.

Example

for (std::size_t i = 0; i < n; ++i) sum += a[i];   // counted, vectorizable
 
int tries = 0;
do {                                  // runs at least once
    result = attempt();
} while (!result && ++tries < 3);     // retry up to 3x
 
while (auto* node = next()) process(node);   // condition with side effect

Pitfalls

• Signedness in the bound: for (int i = 0; i < vec.size(); ++i) compares signed int to unsigned size_t (-Wsign-compare); use std::size_t.
• Iterator invalidation: erasing from a std::vector inside a loop invalidates iterators/indices past the point — use the erase-return idiom or std::erase_if.
• Off-by-one from <= vs < on n elements walks one past the end — UB.
• continue in a while skips your manual increment, causing an infinite loop; for’s step still runs.

See also

• range-based-for-loop
• algorithms-algorithm