Iteration

From the syllabus

AQA: Use, understand and know how the following statement types can be combined in programs; Use definite and indefinite iteration, including indefinite iteration with the condition(s) at the start or the end of the iterative structure. A theoretical understanding of condition(s) at either end of an iterative structure is required (3.1.1.2/4.1.1.2):}

• Often in an algorithm, a group of statements needs to be executed again and again until a certain condition is met, this is iteration.

Programming often requires a series of statements to be repeated. These statements may be repeated a fixed number of times or will execute until a particular condition is reached (or not execute at all if the condition fails). These loop constructs are one of the most important constructs to master and a key feature of algorithm design.

Many program errors are caused by poorly constructed loops. If the loop never ends we enter an infinite loop, thus the key feature to remember when designing a block of statements that repeat, or loop, is the exit strategy: when should the loop terminate?

There are two types of loop:

• for loops: when the number of iterations (repetitions) is pre-determined, also known as counted loops, or count controlled loops
• while loops: when the number of repetitions is not predetermined, or conditional loops, or condition controlled loops

For example, consider the following problem:

Design an algorithm and write a program to find the average of a series of numbers.

We'll illustrate solutions to this problem with each of the three different types of loop structure available in C# in the following sections.