Visual Basic.NET How to Program (PDF)

100 Control Structures: Part 1 Chapter 4
add grade to total total = total + grade
add 1 to counter counter = counter + 1
Fig. Fig. 4.1 4.1 Sequence structure flowchart.
Perhaps the most important flowcharting symbol is the diamond symbol, alternatively
referred to as the decision symbol, which indicates that a decision is to be made. We discuss
the diamond symbol in Section 4.5.
Visual Basic provides three types of selection structures, which we discuss in this
chapter and the next. The If/Then selection structure performs (selects) an action (or
sequence of actions) if a condition is true or skips the action (or sequence of actions) if the
condition is false. The If/Then/Else selection structure performs an action (or sequence
of actions) if a condition is true and performs a different action (or sequence of actions) if
the condition is false. The Select Case structure, discussed in Chapter 5, Control Structures:
Part 2, performs one of many actions (or sequences of actions), depending on the
value of an expression.
The If/Then structure is called a single-selection structure because it selects or
ignores a single action (or a sequence of actions). The If/Then/Else structure is called a
double-selection structure because it selects between two different actions (or sequences of
actions). The Select Case structure is called a multiple-selection structure because it
selects among many different actions or sequences of actions.
Visual Basic provides seven types of repetition structures—While, Do While/
Loop, Do/Loop While, Do Until/Loop, Do/Loop Until, For/Next and For
Each/Next. (Repetition structures While, Do While/Loop and Do Until/Loop are
covered in this chapter; Do/Loop While, Do/Loop Until, and For/Next are covered
in Chapter 5, Control Structures: Part 2; and For Each/Next is covered in Chapter 7,
Arrays.) The words If, Then, Else, End, Select, Case, While, Do, Until, Loop,
For, Next and Each are all Visual Basic keywords (Fig. 4.2). We discuss many of Visual
Basic’s keywords and their respective purposes throughout this book. Visual Basic has a
much larger set of keywords than most other popular programming languages.
Visual Basic has 11 control structures—sequence, three types of selection and seven
types of repetition. Each program is formed by combining as many of each type of control
structure as is necessary. As with the sequence structure in Fig. 4.1, each control structure
is flowcharted with two small circle symbols—one at the entry point to the control structure
and one at the exit point.
Single-entry/single-exit control structures (i.e., control structures that each have one
entry point and one exit point) make it easy to build programs—the control structures are
attached to one another by connecting the exit point of one control structure to the entry
point of the next. This is similar to stacking building blocks, so, we call it control-structure