Sequence Structure in C++

The sequence structure is built into C++. Unless directed otherwise, the computer executes C++ statements one after the other in the order in which they’re written—that is, in sequence. The UML activity diagram of Fig. 4.1 illustrates a typical sequence structure in which two calculations are performed in order. C++ allows you to have as many actions as you want in a sequence structure. As you’ll soon see, anywhere a single action may be placed, you may place several actions in sequence.

Fig. 4.1. Sequence-structure activity diagram.

In this figure, the two statements add a grade to a total variable and add the value 1 to a counter variable. Such statements might appear in a program that averages several student grades. To calculate an average, the total of the grades being averaged is divided by the number of grades. A counter variable would be used to keep track of the number of values being averaged. You’ll see similar statements in the program of Section 4.6.

An activity diagram models the workflow (also called the activity) of a portion of a software system. Such workflows may include a portion of an algorithm, such as the sequence structure in Fig. 4.1. Activity diagrams are composed of special-purpose symbols, such as action state symbols (a rectangle with its left and right sides replaced with arcs curving outward), diamonds and small circles; these symbols are connected by transition arrows, which represent the flow of the activity.

Activity diagrams clearly show how control structures operate. Consider the sequence-structure activity diagram of Fig. 4.1. It contains two action states that represent actions to perform. Each action state contains an action expression—e.g., “add grade to total” or “add 1 to counter”—that specifies a particular action to perform. Other actions might include calculations or input/output operations. The arrows in the activity diagram are called transition arrows. These arrows represent transitions, which indicate the order in which the actions represented by the action states occur—the program that implements the activities illustrated by the activity diagram in Fig. 4.1 first adds grade to total, then adds 1 to counter.

The solid circle at the top of the diagram represents the activity’s initial state—the beginning of the workflow before the program performs the modeled activities. The solid circle surrounded by a hollow circle that appears at the bottom of the activity diagram represents the final state—the end of the workflow after the program performs its activities.

Figure 4.1 also includes rectangles with the upper-right corners folded over. These are called notes in the UML—explanatory remarks that describe the purpose of symbols in the diagram. Figure 4.1 uses UML notes to show the C++ code associated with each action state in the activity diagram. A dotted line connects each note with the element that the note describes. Activity diagrams normally do not show the C++ code that implements the activity. We use notes for this purpose here to illustrate how the diagram relates to C++ code. For more information on the UML, see the ATM case study in Chapters 22–23.