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So far, the statements in our programs have been executed in their physical order. The first statement is executed, then the second, and so on until all of the statements have been executed. But what if we want the computer to execute the statements in some other order? Suppose we want to check the validity of input data and then perform a calculation or print an error message, not both. To do so, we must be able to ask a question and then, based on the answer, choose one or another course of action. |
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The If statement allows us to execute statements in an order that is different from their physical order. We can ask a question with it and do one thing if the answer is yes (true) or another if the answer is no (false). In the first part of this chapter, we deal with asking questions; in the second part, we deal with the If statement itself. |
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The order in which statements are executed in a program is called the flow of control. In a sense, the computer is under the control of one statement at a time. When a statement has been executed, control is turned over to the next statement (like a baton being passed in a relay race). |
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Flow of Control The order in which the computer executes statements in a program. |
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Flow of control is normally sequential (see Figure 5-1). That is, when one statement is finished executing, control passes to the next statement in the program. Where we want the flow of control to be nonsequential, we use control structures, special statements that transfer control to a statement other than the one that physically comes next. Control structures are so important that we focus on them in the remainder of this chapter and in the next four chapters. |
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Control Structure A statement used to alter the normally sequential flow of control. |
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