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. 5.2 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.

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. 5.2. 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 the next to show the flow of control in control statements.

Consider the sequence-structure activity diagram in Fig. 5.2. 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 portion of the app that implements the activities illustrated in Fig. 5.2 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 app 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 app performs its actions.

Figure 5.2 also includes rectangles with the upper-right corners folded over. These are UML notes (like comments in C#) that describe the purpose of symbols in the diagram. Figure 5.2 uses UML notes to show the C# code associated with each action state. 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 do this here to illustrate how the diagram relates to C# code.

C# has three types of selection structures, which from this point forward we shall refer to as selection statements. The if statement performs (selects) an action if a condition is true or skips the action if the condition is false. The if…else statement performs an action if a condition is true or performs a different action if the condition is false. The switch statement (Chapter 6) performs one of many different 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, as we’ll soon see, a single 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 foreach statements. (Chapter 6 presents the do…while and for statements. Chapter 8 discusses the foreach statement.) The while, for and foreach 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. The words if, else, switch, while, do, for and foreach are C# keywords.

C# has only three kinds of control statements: the sequence, selection (three types) and iteration (four types). Every app is formed by combining as many of these statements as is appropriate for the algorithm the app implements. We can model each control statement as an activity diagram. Like Fig. 5.2, 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# apps are constructed from only three kinds of control statements, combined in only two ways. This is the essence of simplicity.