In a loop structure, the program asks a question, and if the answer requires an action, it is performed and the original question is asked again until the answer is such that the action is no longer required. For example, a program written to compute a company’s weekly payroll for each individual employee will begin by computing the wages of one employee and continue performing that action in a loop until there are no more employee wages to be computed, and only then will the program move on to its next action. Each pass through the loop is called an iteration. Loops constitute one of the most basic and powerful programming concepts.

All logic problems in programming can be solved by forming algorithms using only the three logic structures, and they can be combined in an infinite number of ways. The more complex the computing need, the more complex the combination of structures.