Loops

Loops are control flow constructs that repeat a block of instructions. They allow a program to perform repetitive tasks efficiently and reliably by continuing to execute code while a condition remains true or until a goal is reached. Properly designed loops can simplify code and make algorithms clearer.

Common loop forms include for loops, while loops, and do-while loops, with variations such as foreach in many languages. A for loop typically initializes a counter, checks a condition, and updates the counter each iteration. A while loop evaluates the condition before executing the body, while a do-while loop executes the body at least once and then tests the condition. Additional controls like break and continue modify flow.

Key concepts include termination conditions, iteration variables, and progress toward termination. A loop is correct when it terminates after a finite number of iterations for expected inputs. Common pitfalls include infinite loops, off-by-one errors, and unintended side effects. Strategies such as defining loop invariants and ensuring that the loop variable advances help reduce errors.

Loops are used to traverse data structures, implement fundamental algorithms (searches, sorts, iterative numerical methods), and drive simulations or repetitive tasks in software. Performance depends on the work done per iteration and the total number of iterations. Optimizations include moving invariant calculations outside the loop, reducing work inside the loop, and careful use of break and continue statements.