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 in Fig. 4.2 illustrates a typical sequence structure in which two calculations are performed in order. C++ lets you have as many actions as you want in a sequence structure. As we’ll soon see, anywhere a single action may be placed, we may place several actions in sequence.

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, like the sequence structure in Fig. 4.2. Activity diagrams are composed of symbols, such as action-state symbols (rectangles with their left and right sides replaced with outward arcs), diamonds and small circles. These symbols are connected by transition arrows, which represent the flow of the activity—that is, the order in which the actions should occur.

Like pseudocode, activity diagrams help you develop and represent algorithms. Activity diagrams clearly show how control structures operate. We use the UML in this chapter and Chapter 5 to show the flow of control in control statements. Online Chapters 25–26 use the UML in a real-world ATM (automated-teller-machine) case study.

Consider the sequence-structure activity diagram in Fig. 4.2. It contains two action states, each containing an action expression—for example, “add grade to total” or “add 1 to counter”—that specifies a particular action to perform. The arrows in the activity diagram represent transitions, which indicate the order in which the actions represented by the action states occur. The program that implements the activities illustrated in Fig. 4.2 first adds grade to total, then adds 1 to counter.

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

Figure 4.2 also includes rectangles with the upper-right corners folded over. These are UML notes (like comments in C++)—explanatory remarks that describe the purpose of symbols in the diagram. Figure 4.2 uses UML notes to show the C++ code associated with each action state. A dotted line connects each note with the element it describes. Activity diagrams normally do not show the C++ code that implements the activity. We do this here to illustrate how the diagram relates to C++ code. For more information on the UML, see our optional online object-oriented design case study (Chapters 25–26) or visit www.uml.org.

C++ has three types of selection statements. The if statement performs (selects) an action (or group of actions), if a condition is true, or skips it, if the condition is false. The if…else statement performs an action (or group of actions) if a condition is true and performs a different action (or group of actions) if the condition is false. The switch statement (Chapter 5) performs one of many different actions (or group of actions), depending on the value of an expression.

The if statement is called a single-selection statement because it selects or ignores a single action (or group of actions). The if…else statement is called a double-selection statement because it selects between two different actions (or groups of actions). The switch statement is called a multiple-selection statement because it selects among many different actions (or groups of actions).

C++ provides four iteration statements (sometimes called repetition statements or looping statements) that enable programs to perform statements repeatedly as long as a condition (called the loop-continuation condition) remains true. The iteration statements are the while, do…while, for and range-based for statements. (Chapter 5 presents the do…while and for statements and Chapter 7 presents the range-based for statement.) The while and for statements perform the action (or group of actions) in their bodies zero or more times—if the loop-continuation condition is initially false, the action (or group of actions) will not execute. The do…while statement performs the action (or group of actions) in its body one or more times.

Each of the words if, else, switch, while, do and for are C++ keywords. Keywords cannot be used as identifiers, such as variable names, and must be spelled with only lowercase letters. Figure 4.3 provides a complete list of C++ keywords.

C++ has only three kinds of control structures, which from this point forward we refer to as control statements: the sequence statement, selection statements (three types) and iteration statements (four types). Every program is formed by combining as many of these statements as is appropriate for the algorithm the program implements. We can model each control statement as an activity diagram. Like Fig. 4.2, each diagram contains an initial state and a final state that represent a control statement’s entry point and exit point, respectively. Single-entry/single-exit control statements make it easy to build programs— we simply connect the exit point of one to the entry point of the next. We call this control-statement stacking. We’ll learn that there’s only one other way in which control statements may be connected—control-statement nesting—in which one control statement appears inside another. Thus, algorithms in C++ programs are constructed from only three kinds of control statements, combined in only two ways. This is the essence of simplicity.