Visual Basic.NET How to Program (PDF)

Chapter 5 Control Structures: Part 2 159
Again, note that (besides small circles and flowlines) the flowchart contains only rectangle
and diamond symbols. Imagine, as we did in the previous chapter, that the programmer
has access to a deep bin of empty structures. This time, the bin contains Select
Case structures, and the programmer can stack and nest as many as are necessary with
other control structures to form a structured implementation of an algorithm’s flow of control.
The programmer fills the rectangles and diamonds with actions and decisions appropriate
to the algorithm. Although nested control structures are common, it is rare to find
nested Select Case structures in a program.
In Chapter 10, Object-Oriented Programming: Part 2, we present a more elegant
method of implementing multiple selection logic. We use a technique called polymorphism
to create programs that are often clearer, more manageable, and easier to extend than programs
that use Select Case logic.
5.6 Do/Loop While Repetition Structure
The Do/Loop While repetition structure is similar to the While structure and Do
While/Loop structure. In the While and Do While/Loop structures, the loop-continuation
condition is tested at the beginning of the loop, before the body of the loop is performed.
The Do/Loop While structure tests the loop-continuation condition after the loop
body is performed. Therefore, in a Do/Loop While structure, the loop body is always executed
at least once. When a Do/Loop While structure terminates, execution continues
with the statement after the Loop While clause. The program in Fig. 5.12 uses a Do/Loop
While structure to output the values 1–5.
Testing and Debugging Tip 5.4
Infinite loops occur when the loop-continuation condition in a While, Do While/Loop or
Do/Loop While structure never becomes false. 5.4
1 ' Fig. 5.12: DoWhile.vb
2 ' Demonstrating the Do/Loop While repetition structure.
3
4 Module modDoWhile
5
6 Sub Main()
7 Dim counter As Integer = 1
8
9 ' print values 1 to 5
10 Do
11 Console.Write(counter & " ")
12 counter += 1
13 Loop While counter