2.4 Repetition statements, arrays and structured programming

Basic loop structures

The real power of a program is realized when the same type of operation must be made over and over.

Constructing a repetitive section of code requires that four elements be present. The first necessary element is a repetition statement. This repetition statement defines the boundaries containing the repeating section of code and also controls whether the code is executed or not. C++ provides three different forms of repetition statements:

1. while structure
2. for structure
3. do-while structure

Each of these statements requires a condition that must be evaluated, which is the second required element for constructing repeating sections of code. Valid conditions are similar to those used in selection statements. If the condition is true, the code is executed; otherwise, it is not.

The third required element is a statement that initially sets the condition. This statement must always be placed before the condition is first evaluated to ensure correct loop execution the first time the condition is evaluated.

Finally, there must be a statement within the repeating section of code that allows the condition to become false. This is necessary to ensure that, at some point, the repetition stop.

The condition being tested can be evaluated at either (1) the beginning or (2) the end of the repeating section of code.

If the test occurs at the beginning of the loop, the type of loop is called a pre-test loop or entrance-controlled loop. If the test occurs at the end of the loop, the type of loop is called a post-test loop or exit-controlled-loop.

In addition to where the condition is tested (pretest or posttest), repeating sections of code are also classified. In a fixed count loop, the condition is used to keep track of how many repetitions have occurred. In this kind of loops, a fixed number of repetitions are performed, at which point the repeating section of code is exited.

In many situations, the exact number of repetitions are not known in advance or the items are too numerous to count beforehand. In such cases, a variable condition loop is used. In a variable condition loop, the tested condition does not depend on a count being achieved, but rather on a variable that can change interactively with each pass through the loop. When a specified value is encountered, regardless of how many iterations have occurred, repetitions stop.

While loops

The while statement is used for repeating a statement or series of statements as long as a given conditional expression is evaluated to true.

The syntax for the while statement:

while( condition expression){