while Repetition Statement

A repetition statement allows you to specify that an application should repeat an action while some condition remains true. The pseudocode statement

while there are more items on my shopping list
    purchase next item and cross it off my list

describes the repetition that occurs during a shopping trip. The condition "there are more items on my shopping list" may be true or false. If it is true, then the action "Purchase next item and cross it off my list" is performed. This action will be performed repeatedly while the condition remains true. The statement(s) contained in the while repetition statement constitute the body of the while repetition statement, which may be a single statement or a block. Eventually, the condition will become false (when the last item on the shopping list has been purchased and crossed off the list). At this point, the repetition terminates, and the first statement after the repetition statement executes.

As an example of C#'s while repetition statement, consider a code segment designed to find the first power of 3 larger than 100. Suppose int variable product is initialized to 3. When the following while statement finishes executing, product contains the result:

int product = 3;

while ( product <= 100 )
 product = 3 * product;

When this while statement begins execution, the value of variable product is 3. Each repetition of the while statement multiplies product by 3, so product takes on the subsequent values 9, 27, 81 and 243 successively. When variable product becomes 243, the while statement conditionproduct <= 100becomes false. This terminates the repetition, so the final value of product is 243. At this point, application execution continues with the next statement after the while statement.

Common Programming Error 5 3

Not providing in the body of a while statement an action that eventually causes the condition in the while to become false normally results in a logic error called an infinite loop, in which the loop never terminates.

The UML activity diagram in Fig. 5.4 illustrates the flow of control that corresponds to the preceding while statement. Once again, the symbols in the diagram (besides the initial state, transition arrows, the final state and three notes) represent an action state and a decision. This diagram also introduces the UML's merge symbol. The UML represents both the merge symbol and the decision symbol as diamonds. The merge symbol joins two flows of activity into one. In this diagram, the merge symbol joins the transitions from the initial state and the action state, so they both flow into the decision that determines whether the loop should begin (or continue) executing. The decision and merge symbols can be distinguished by the number of "incoming" and "outgoing" transition arrows. A decision symbol has one transition arrow pointing to the diamond and two or more transition arrows pointing out from the diamond to indicate possible transitions from that point. Each transition arrow pointing out of a decision symbol has a guard condition next to it. A merge symbol has two or more transition arrows pointing to the diamond and only one transition arrow pointing from the diamond, to indicate multiple activity flows merging to continue the activity. None of the transition arrows associated with a merge symbol have guard conditions.

Figure 5.4. while repetition statement UML activity diagram.

Figure 5.4 clearly shows the repetition of the while statement discussed earlier in this section. The transition arrow emerging from the action state points back to the merge, from which program flow transitions back to the decision that is tested at the beginning of each repetition of the loop. The loop continues to execute until the guard condition product > 100 becomes true. Then the while statement exits (reaches its final state), and control passes to the next statement in sequence in the application.

Formulating Algorithms Counter Controlled Repetition