Loop Control and Common Loop Patterns

Loop Control and Common Loop Patterns

📘 Overview Loops repeat a block of code according to a condition or a sequence of values. For-loops commonly express counted repetition or sequence traversal, while-loops test a condition before each iteration, and do-while loops test after the body.

🧠 Key Idea A correct loop initializes its state, performs the intended repeated operation, and updates state toward termination. Condition placement and control transfers determine which statements execute and how often.

⚔️ Core Details: - Counter-controlled repetition uses a counter with an initial value, stopping boundary, and step size. - Sentinel-controlled repetition reads values until a designated termination value appears. - The accumulator pattern combines successive values into a running result initialized before the loop. - Sequence traversal processes each element of a collection through an index or an iterator. - A break statement immediately exits the nearest enclosing loop in common imperative languages. - A continue statement skips the remaining statements in the current iteration and transfers control to the loop's next-iteration step.

🎯 Why It Matters: - Boundary errors can omit required elements, process elements twice, or access an invalid index. - A sentinel must be checked before ordinary processing to prevent its accidental inclusion in a result. - A continue statement can cause nontermination when it bypasses a necessary manual update in a while-loop. - Nested loops can multiply execution costs; traversing an entire collection inside another full traversal can produce quadratic running time.

🧠 Quick Recall: - Pre-test loop - Checks its continuation condition before executing the body; the body may execute zero times. - Post-test loop - Checks its continuation condition after executing the body; the body executes at least once. - Loop invariant - A property that holds before and after every iteration, supporting reasoning about correctness. - Off-by-one error - A boundary mistake that causes one extra or one missing iteration. - Accumulator initialization - Addition typically starts from zero; multiplication typically starts from one.