Visual Basic.NET How to Program (PDF)

Chapter 4 Control Structures: Part 1 125
requires that a value be obtained from the command window. The pseudocode statement
Validate that the side is less than or equal to 20
can be refined as
If side is less than or equal to 20
which explicitly tests whether side is less than or equal to 20. If the condition (i.e., side is
less than or equal to 20) is true, the first statement in the body of the If is executed. If the
condition is false, the body of the If is not executed. These two control structures are said
to be nested—meaning that one is inside the body of the other.
The pseudocode statement
Draw the square
can be implemented by using nested loops to draw the square. In this example, it is known
in advance that there are precisely n rows of n * characters each, so counter-controlled repetition
is appropriate. One loop controls the row in which each * is printed. Inside this loop
(i.e., nested within this loop), a second loop prints each individual *. The refinement of the
preceding pseudocode statement is, then,
Set column to one
While column is less than or equal to side
Print *
Increment column by one
Print a line feed/carriage return
Increment row by one
After column is set to one, the inner loop executes to completion (i.e., until column
exceeds side). Each iteration of the inner loop prints a single *. A line feed/carriage
return is then printed to move the cursor to the beginning of the next line, to prepare to
print the next row of the square. Variable row is incremented by one. If the outer loop
condition allows the body of the loop to be executed, column is reset to one, because we
want the inner loop to execute again and print another row of * characters. If column is
not initialized to 1 before each iteration of the inner loop, the repetition condition of the
inner loop will fail for all but the first row of output. Variable row is incremented by one.
This process is repeated until the value of row exceeds side at which point the square of
*’s has been printed.
The complete second refinement appears in Fig. 4.19. Notice that blank lines are used
to separate the nested control structures for program readability. Also notice that we added
an Else clause that prints a message if the value input for side is too large.
Good Programming Practice 4.10
Too many levels of nesting can make a program difficult to understand. If possible, try to
avoid using more than three levels of nesting. 4.10
The pseudocode now is refined sufficiently for conversion to Visual Basic. The Visual
Basic program and sample executions are shown in Fig. 4.20.