You saw in Chapter 2 that decisions can be based on conditions containing relational or equality operators. Actually, in C++, a decision can be based on any expression that evaluates to zero or nonzero—if the expression evaluates to zero, it’s treated as false; if the expression evaluates to nonzero, it’s treated as true. C++ also provides the data type bool for Boolean variables that can hold only the values true and false—each of these is a C++ keyword.

Figure 4.4 illustrates the single-selection if statement. This figure contains the most important symbol in an activity diagram—the diamond, or decision symbol, which indicates that a decision is to be made. The workflow continues along a path determined by the symbol’s associated guard conditions, which can be true or false. Each transition arrow emerging from a decision symbol has a guard condition (specified in square brackets next to the arrow). If a guard condition is true, the workflow enters the action state to which the transition arrow points. In Fig. 4.4, if the grade is greater than or equal to 60 (i.e., the condition is true), the program prints “Passed,” then transitions to the activity’s final state. If the grade is less than 60 (i.e., the condition is false), the program immediately transitions to the final state without displaying a message.

The if statement is a single-entry/single-exit control statement. We’ll see that the activity diagrams for the remaining control statements also contain initial states, transition arrows, action states that indicate actions to perform, decision symbols (with associated guard conditions) that indicate decisions to be made, and final states.