Computer Programming Concepts and Visual Basic David I. Schneider

86 Computer Programming Concepts and Visual Basic
EXAMPLE 7
The following program demonstrates the use of Form_Load.
Dim pi As Single ‘In (Declarations) section of (General)
Private Sub Form_Load()
‘Assign a value to pi
pi = 3.14159
End Sub
Private Sub cmdCompute_Click()
‘Display the area of a circle of radius 5
picArea.Cls
picArea.Print “The area of a circle of radius 5 is”; pi * 5 * 5
End Sub
[Run, and then click the command button. The following is displayed in the picture box.]
The area of a circle of radius 5 is 78.53975
COMMENTS
1. In addition to the reasons presented earlier, some other reasons for using Sub
procedures follow:
(a) Programs with Sub procedures are easier to debug. Each Sub procedure can
be checked individually before being placed into the program.
(b) The task performed by a Sub procedure might be needed in another program.
The Sub procedure can be reused with no changes. Programmers refer
to the collection of their most universal Sub procedures as a library of Sub
procedures. (The fact that variables appearing in Sub procedures are local to
the Sub procedures is quite helpful when reusing Sub procedures in other
programs. There is no need to worry if a variable name in the Sub procedure
is used for a different purpose in another part of the program.)
(c) Often, programs are written by a team of programmers. After a problem has
been broken into distinct and manageable tasks, each programmer is
assigned a single Sub procedure to write.
(d) Sub procedures make large programs easier to understand. Some programming
standards insist that each Sub procedure be at most two pages long.
(e) Sub procedures permit the following program design, which provides a
built-in outline of an event procedure. A reader can focus on the main flow
first, and then go into the specifics of accomplishing the secondary tasks.
Private Sub Object_Event()
‘An event procedure written entirely as Sub procudures
Call FirstSubprocedure ‘Perform first task
Call SecondSubprocedure ‘Perform second task
Call ThirdSubprocedure ‘Perform third task
End Sub
2. Sub procedures can call other Sub procedures. In such cases, the calling Sub
procedure plays the role of the event procedure with respect to the called Sub
procedure. Complex problems are thereby broken into simpler tasks, which are
then broken into still more elementary tasks. This approach to problem solving
is called top-down design.