if Single-Selection Statement

Applications use selection statements to choose among alternative courses of action. For example, suppose that the passing grade on an exam is 60. The pseudocode statement

if grade is greater than or equal to 60
       print "Passed"

determines whether the condition "grade is greater than or equal to 60" is true or false. If the condition is true, "Passed" is printed, and the next pseudocode statement in order is "performed." (Remember that pseudocode is not a real programming language.) If the condition is false, the Print statement is ignored, and the next pseudocode statement in order is performed. The indentation of the second line of this selection statement is optional, but recommended, because it emphasizes the inherent structure of structured applications.

The preceding pseudocode if statement may be written in C# as

if ( grade >= 60 )
 Console.WriteLine( "Passed" );

Note that the C# code corresponds closely to the pseudocode. This is one of the properties of pseudocode that makes it such a useful application development tool.

Figure 5.2 illustrates the single-selection if statement. This activity diagram contains what is perhaps the most important symbol in an activity diagramthe diamond, or decision symbol, which indicates that a decision is to be made. The workflow will continue 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 transition arrow). If a guard condition is true, the workflow enters the action state to which the transition arrow points. In Fig. 5.2, if the grade is greater than or equal to 60, the application prints "Passed" then transitions to the final state of this activity. If the grade is less than 60, the application immediately transitions to the final state without displaying a message.

Figure 5.2. if single-selection statement UML activity diagram.

The if statement is a single-entry/single-exit control statement. You will see that the activity diagrams for the remaining control statements also contain initial states, transition arrows, action states that indicate actions to perform and decision symbols (with associated guard conditions) that indicate decisions to be made, and final states. This is consistent with the action/decision model of programming we have been emphasizing.

Envision seven bins, each containing only one type of C# control statement. The control statements are all empty. Your task is to assemble an application from as many of each type of control statement as the algorithm demands, combining the control statements in only two possible ways (stacking or nesting), then filling in the action states and decisions with action expressions and guard conditions appropriate for the algorithm. We will discuss in detail the variety of ways in which actions and decisions can be written.